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ABSTRACTSThe Role of the Biosphere in the Climate of Tropical RegionsWest Africa:Drought frequency analysis of annual rainfall
series in central and western Sudan Abstract: Rainfall is the most important water resource in central and western Sudan, a region affected by the recent drought in Africa. A general methodology for studying the annual rainfall process is presented and applied to data from central and western Sudan. It is assumed that certain time series models adequately describe the annual rainfall process in the region. Based on this assumption, the drought frequencies are calculated in the subregions with stationary series. The theory of runs is applied in calculating drought frequencies using a data generation method. Dynamics of wet and dry years in West Africa Abstract: This paper proposes a theoretical framework for describing interannual climatic variability over West Africa. The dynamical theory of zonally symmetrical thermally direct circulations suggests that a meridional monsoon circulation must develop over any tropical region (off the equator) when the absolute vorticity near the tropopause reaches a threshold value of zero. However, for a moist atmosphere that satisfies a quasi-equilibrium balance between moist convection and the radiative forcing, the absolute vorticity at upper-tropospheric levels is a function of both latitude and the meridional distribution of boundary-layer entropy. Hence, the onset of a monsoon circulation depends in a nonlinear fashion on these two factors. The theory predicts that a flat distribution of entropy does not drive any circulation and that a relatively large gradient of entropy should drive a strong monsoon circulation. The location of the region of West Africa, relatively close to the equator, dictates that the dynamics of a monsoon over that region are relatively sensitive to interannual fluctuations in the meridional gradient of boundary-layer entropy. Here, we present observations on entropy and wind over West Africa during the monsoon seasons of 1958 and 1960. The following observations were consistent with the proposed relationship between boundary-layer entropy and the monsoon circulation: a large meridional gradient of boundary-layer entropy, a healthy monsoon, and wet conditions over the Sahel region were observed in 1958; and a nearly flat distribution of entropy, very weak circulation, and relatively dry conditions were observed in 1960. Moreover, the proposed theoretical relationship between the meridional gradient of boundary-layer entropy and the monsoon circulation over West Africa is consistent with the empirical observations of sea surface temperature anomalies (SSTAs) in the tropical Atlantic and rainfall in the Sahel region. Theoretically, a cold (warm) SSTA in the region located south of the West African coast should favor a large ( small ) meridional gradient of entropy, a strong (weak) monsoon circulation, and wet (dry) conditions in the Sahel. A large body of observations confirms that cold (warm) SSTAs off the southern coast of West Africa are associated with wet (dry) years in the Sahel region. Role of vegetation in sustaining large-scale
atmospheric circulations in the tropics Abstract: The focus of this paper is the role of rain forests in large-scale atmospheric circulations. The significance of this role is investigated by studying the response of the tropical atmosphere to a perturbation in the state of vegetation (deforestation) over three regions: the Amazon, Congo, and Indonesia. A theory is developed to relate tropical deforestation and thc resulting changes in the large-scale atmospheric circulation. Field observations and numerical simulations support the argument that tropical deforestation reduces the total net surface radiation, including terrestrial and solar forms. However, the energy balance at the land-atmospheric boundary dictates that for equilibrium conditions, any reduction in net surface radiation has to be balanced by a similar reduction in the total flux of heat, including sensible and latent forms. Since these fluxes supply heat as well as entropy from the forest into the atmospheric boundary layer, a reduction in the total flux of heat reduces the boundary layer entropy. In a moist atmosphere, that satisfies a quasi equilibrium between moist convection and radiative forcing, the equilibrium temperature profile is uniquely related to the boundary layer entropy. Under such conditions, large-scale deforestation reduces boundary layer entropy relative to the surroundings, cools the upper troposphere, and results in subsidence, divergent flow in the boundary layer, and weakening of the large-scale circulation. These changes are simulated using a simple linear model of atmospheric flow. The comparison of the model predictions with observations of atmospheric circulations over the Amazon, Congo, and Indonesia suggests a significant role for vegetation in maintaining large-scale atmospheric circulations in the tropics. Sources of moisture for rainfall in west Africa Abstract: The objective of this study is to identify the sources of moisture for rainfall in west Africa. A model of precipitation recycling is developed and applied to the region of west Africa to obtain quantitative estimates of the moisture contributed by local evaporation as well as the moisture contributed by the zonal and meridional fluxes from the surrounding regions. We estimated the recycling ratio for the entire region by specifying three subregions where evaporation is treated as the source of moisture: west Africa, central Africa, and the tropical Atlantic Ocean. We find that evaporation from the tropical Atlantic Ocean, west Africa, and central Africa contribute about 23, 27, and 17% of rainfall in west Africa, respectively. Moisture fluxes from the tropical Atlantic are almost in phase with rainfall in west Africa. However, we find that moisture supply from central Africa is strongly regulated and limited by the westerly flow associated with the monsoon circulation. Hence the large-scale monsoon circulation is not only the main forcing of rainfall over west Africa, but the dynamics of this circulation exert significant control on where the moisture comes from. The response to deforestation and desertification in a model
of West African monsoons Abstract. Since Charney proposed his theory on the dynamics of deserts and droughts in the Sahel [Charney, 1975], there has been significant scientific interest in the interaction between vegetation and climate in this region. The essence of this interaction is that the atmospheric circulation, and therefore rainfall, over this region may be sensitive to changes in vegetation cover near the desert border. Here we describe simulations of the West African monsoons with a simple zonally-symmetric model. The results suggest that the potential impact of human induced change of land cover on regional climate depends critically on the location of the change in vegetation cover. That is, desertification along the border with the Sahara (e.g., in Chad, Niger, Mali and Mauritania) leaves a relatively minor impact on monsoon circulation and regional rainfall; deforestation along the southern coast of West Africa (e.g., in Nigeria, Ghana and Ivory Coast) may result in complete collapse of monsoon circulation, and a significant reduction of regional rainfall. The role of vegetation in the dynamics of West African monsoonsXinyu Zheng and Elfatih A.B. Eltahir Abstract: The focus of this paper is the role of meridional distribution of vegetation in the dynamics of monsoons and rainfall over West Africa. We develop a moist zonally symmetric atmospheric model coupled with a simple land surface scheme to investigate these processes. Four primary experiments have been carried out to examine the sensitivity of West African monsoons to perturbations in vegetation patterns. Each perturbation experiment is identical to the control experiment except that a change in vegetation cover is imposed for a latitudinal belt of 10° in width. The numerical experiments demonstrate that West African monsoons and therefore rainfall depend critically on the location of the vegetation perturbations. While the magnitude of local rainfall is sensitive to changes in local vegetation, the location of the Inter-Tropical Convergence Zone (ITCZ) is not sensitive to changes in the vegetation northward or southward from the location of ITCZ in the control experiment. However, the location of the ITCZ is sensitive to changes of the vegetation distribution in the immediate vicinity of the location of the ITCZ in the control experiment. The modeling results indicate that changes in vegetation cover along the border between the Sahara desert and West Africa (desertification) have a minor impact on the simulated monsoon circulation. On the other hand, coastal deforestation may cause the collapse of the monsoon circulation and have a dramatic impact on the regional rainfall. The observed deforestation in West Africa is then likely to be a significant contributor to the observed drought. A Mechanism Relating Tropical Atlantic Spring Sea Surface
Temperature and West African Rainfall Abstract: In this paper, we describe a mechanistic study on the
role of tropical Atlantic sea surface temperature (SST) variability in the dynamics of
West African monsoon. A hypothesis that warm spring (April-June) SST results in a wet
monsoon is explored using a moist, zonally-symmetric model. A positive spring rainfall
anomaly has been simulated over the ocean, in response to the warm SST, which then
propagates onto the land and persists two to three months, even after the SST anomaly
vanishes. While the ocean-atmosphere interaction is crucial for the initial development of
the rainfall anomaly over land, the interactions between the ocean, land, and atmosphere
are found to be important for relating tropical Atlantic spring SST to West African
rainfall. Furthermore, the positive feedback between rainfall and soil moisture is
responsible for some of the persistence in the rainfall anomaly. We present a case study
for the wettest (1994) and driest (1992) years of the 1990s so far. The observations show
that a warm spring SST anomaly in 1994 was associated The Role of Ecosystem Dynamics in Biosphere-Atmosphere
Interaction over the Coastal Region of West Africa In this study, we develop a one-dimensional model of the tropics which includes two-way
interaction between the biosphere and the atmosphere, including ecosystem dynamics. The
model integrates an atmospheric model, a biospheric model and a monsoon circulation model
and is applied to coastal West Africa to test the sensitivity of the coupled system to
changes in vegetation cover. We perform three sets of simulations - one with a fixed
monsoon circulation, one with an interactive monsoon circulation, and one with modified
boundary conditions at the northern edge of the domain. Our control Guiling Wang and Elfatih A. B. Eltahir Abstract: In this study we develop a zonally symmetric, synchronously coupled biosphere atmosphere model including ecosystem dynamics, and apply this model to study biosphere-atmosphere interactions in the region of West Africa. The atmospheric model is zonally symmetric, and includes representation of atmospheric dynamics, a radiation scheme, a moist convection scheme, a boundary layer scheme, and a cloud parameterization scheme. The biosphereic model is the Integrated BIosphere Simulator (IBIS), which includes representation of the water, energy, momentum, and carbon balance, vegetation phenology, and vegetation dynamics. We modified the representation fo canopy hydrology in IBIS to account for the impact of rainfall sub-grid variability. The biospheric model and atmospheric model are separately tested against observations. The synchronously coupled model is then used to simulate the biosphere atmosphere system of West Africa. A study on the role of biosphere-atmosphere interactions, including ecosystem dynamics in the climate variability over West Africa using this model will be presented in a companion paper. Biosphere-Atmosphere Interactions Over West Africa: 2. Multiple Climate EquilibriaGuiling Wang and Elfatih A. B. Eltahir Abstract: This paper presents both theoretical and numerical analyses on the multiple-equilibrium nature of the regional climate system in West Africa. Based on simple analyses on how the coupled biosphere atmosphere system responds to vegetation perturbations within the scope of a dynamic ecosystem, we propose that the regional climate system may have multiple equilibrium states coexisting under the same precessional forcing. Using a synchronously coupled biosphere-atmosphere model which includes explicit representation of ecosystem dynamics, we show that the equilibrium state of the model is sensitive to initial vegetation distribution. This modeling result supports the existence of multiple climate equilibria. Using the same model, further experiments are carried out to investigate how the coupled system responds to non-permanent vegetation perturbations. Our results demonstrate how transitions between different climate equilbria can take place when governed by the two way biosphere-atmosphere feedback. These findings advance our understanding regarding teh mechanisms of climate variability over West Africa. Modeling the Biosphere-Atmosphere System: The Impact of Sub-Grid
Variability in Rainfall Interception The Role of Vegetation Dynamics in Enhancing the Low-Frequency Variability
of the Sahel Rainfall Rainfall observations in the Sahel region of West Africa show significant variability at the time scale of decades. Here we explore the mechanisms of this low-frequency variability using a coupled biosphere-atmosphere model which includes explicit representation of vegetation dynamics. By forcing the model with the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1898-1997, numerical experiments on the climate variability of West Africa have been carried out. The results of these experiments suggest that vegetation dynamics is a significant process in shaping the natural variability of the Sahel rainfall. The response of the regional climate system to large-scale forcings is significantly regulated by vegetation dynamics. The relatively slow response of vegetation to changes in the atmosphere acts to enhance the low-frequency rainfall variability. The regional climate system over West Africa has several climate regimes coexisting under the current precessional forcing. Climate transitions between different regimes act as another mechanism contributing to the low-frequency rainfall variability. Climate persistence at one regime and climate transition towards another collectively compose a distinct type of multi-decadal variability. Ecosystem Dynamics and the Sahel Drought The Sahel region of Africa has been experiencing a persistent drought throughout the last three decades. Here, we present a new perspective on the underlying physical mechanism behind this phenomenon. We use a coupled biosphere-atmosphere model including explicit representation of ecosystem dynamics to demonstrate that, regardless of the nature of the initial forcing, the natural response of the local grass ecosystem to the dry conditions of the late 1960s played a critical role in maintaining the drought through the following decades. The onset of the drought has been marked by a forced shift from a self-sustaining wet climate equilibrium to a similarly self-sustaining but dry climate equilibrium. Role of topography in facilitating coexistence of trees and grasses within savannas The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that "variations in elevation at relatively short horizontal scales of similar to1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas.'' To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator ( IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen ( SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9degreesN the model simulates trees even when the GWT is assumed to be infinitely deep; at 13degreesN the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11degreesN, trees are simulated when the GWT is at similar to2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas. The Amazon:Estimation of the fractional coverage of rainfall in climate modelsElfatih A. B. Eltahir and Rafael L. Bras Abstract: The fraction of the grid cell area covered by rainfall, 1, is a very important parameter in the descriptions of land surface hydrology in climate models. A simple procedure is presented for estimating this fraction, based on extensive observations of storm areas and rainfall volumes. It is often observed that storm area and rainfall volume are linearly related. This relation is utilized in rainfall measurement to compute rainfall volume from radar observations of the storm area. The authors suggest that the same relation be used to compute the storm area from the volume of rainfall simulated by a climate model. A formula is developed for computing y, which describes the dependence of the fractional coverage of rainfall on the season of the year, the geographical region, rainfall volume, spatial resolution of the model, and the temporal resolution of the model. The new formula is applied in computing y over the Amazon region. Significant temporal variability in the fractional coverage of rainfall is demonstrated. The implications of this variability for the modeling of land surface hydrology in climate models are discussed. A description of rainfall interception over large
areas Abstract: A new scheme is developed for describing interception at spatial scales comparable to the typical resolution of climate models. The scheme is based on the Rutter model of interception and statistical description of the subgnd-scale spatial variability of canopy storage and rainfall. The interception loss simulated by the new scheme is significantly smaller than those simulated by other schemes that do not include considerations for spatial variability. The explanation of this result is partly in the enhancement of spatially averaged canopy drainage due to the large local drainage from the few buckets of large canopy storage. The relative reduction in interception loss simulated by the new scheme may explain the overestimation of interception loss by climate models that do not include the effects of spatial variability on interception processes. On the response of the tropical atmosphere to
large-scale deforestation Abstract: Recent studies on the Amazon deforestation problem predict that removal of the forest will result in a higher surface temperature, a significant reduction in evaporation and precipitation, and possibly significant changes in the tropical circulation. Here, we discuss the basic mechanisms contributing to the response of the tropical atmosphere to deforestation. A simple linear model of the tropical atmosphere is used in studying the effects of deforestation on climate. It is suggested that the impact of large-scale deforestation on the circulation of the tropical atmosphere consists of two components: the response of the tropical circulation to the negative change in precipitation (heating), and the response of the same circulation to the positive change in surface temperature. Owing to their different signs, the changes in predicted temperature and precipitation excite competing responses working in opposite directions. The predicted change in tropical circulation determines the change, if any, in atmospheric moisture convergence, which is equivalent to the change in run-off. The dependence of run-off predictions on the relative magnitudes of the predicted changes in precipitation and surface temperature implies that the predictions about run-off are highly sensitive, which explains, at least partly, the disagreement between the different models concerning the sign of the predicted change in Amazonian run-off. Precipitation recycling in the Amazon basin Abstract: Precipitation recycling is the contribution of evaporation within a region to precipitation in that same region. The recycling rate is a diagnostic measure of the potential for interactions between land surface hydrology and regional climate. In this paper we present a model for describing the seasonal and spatial variability of the recycling process. The precipitation recycling ratio, p, is the basic variable in describing the recycling process. p is the fraction of precipitation at a certain location and time which is contributed by evaporation within the region under study. The recycling model is applied in studying the hydrologic cycle in the Amazon basin. It is estimated that about 25% of all the rain that falls in the Amazon basin is contributed by evaporation within the basin. This estimate is based on analysis of a data set supplied by the European Centre for Medium-range Weather Forecasts. The same analysis is repeated using a different data set from the Geophysical Fluid Dynamics Laboratory. Based on this data set, the recycling ratio is estimated to be 35%. The seasonal variability of the recycling ratio is small compared with the yearly average. The new estimates of the recycling ratio are compared with results of previous studies, and the differences are explained. Sensitivity of regional climate to deforestation
in the Amazon basin Abstract: Deforestation results in several adverse effects on the natural environment. The focus of this paper is on the effects of deforestation on land-surface processes and regional climate of the Amazon basin. In general, the effects of deforestation on climate are likely to depend on the scale of the deforested area. In this study, we are interested in the effects due to deforestation of areas with a scale of about 250km. Hence, a meso-scale climate model is used in performing numerical experiments on the sensitivity of regional climate to deforestation of areas with that size. It is found that deforestation results in less net surface radiation, less evaporation, less rainfall, and warmer surface temperature. The magnitude of the change in temperature is of the order of 0 5°C, the magnitudes of the changes in the other variables are of the order of 10%. In order to verify some of the results of the numerical experiments, the model simulations of net surface radiation are compared to recent observations of net radiation over cleared and undisturbed forest in the Amazon. The results of the model and the observations agree in the following conclusion: the difference in net surface radiation between cleared and undisturbed forest is, almost equally partitioned between net solar radiation and net long-wave radiation. This finding contributes to our understanding of the basic physics in the deforestation problem. Relationship between surface conditions and
subsequent rainfall in convective storms Abstract: This paper describes the relationship between surface conditions (temperature and humidity) and subsequent rainfall. The focus is on convective storms that are forced and maintained locally due to conditional instability in the vertical distribution of atmospheric temperature. These storms are described using two probabilistic measures: (1) the probability of occurrence of storms given surface conditions and (2) the average storm rainfall. The surface conditions are described by a single variable: surface wet-bulb temperature. The proposed theoretical relationships are tested using an hourly data set on rainfall and wet-bulb temperature from the Amazon region. These observations confirm that both measures increase linearly with wet-bulb temperature. However, for the occurrence of any storm the wet-bulb temperature has to exceed a threshold of about 22°C. The sensitivity of the frequency of storms to changes in the climatology of surface wet-bulb temperature is larger than the corresponding sensitivity of the average storm rainfall. These general concepts are applied in discussing the potential impact of changes in land cover on rainfall patterns using two specific examples: deforestation in the Amazon region and development of irrigation projects in the Columbia River basin. Stochastic modeling of the thermally induced atmospheric
flow at mesoscale Abstract: This paper presents a three-dimensional stochastic linear model of the atmospheric flow induced by the variability of heat flux over land surface. The primitive equations relating perturbation terms of wind field, geopotential and buoyancy are formulated as a system of stochastic partial differential equations and solved analytically. The solution is based on spectral representations of the homogeneous random fields. The flow intensity is found to be proportional to the standard deviation of the heat flux into the atmosphere. The intensity of the vertical motion becomes more sensitive to the differential heating with a larger length scale as altitude goes higher. Stability and synoptic wind inhibit the development of the flow. The proposed theory improves the understanding of the role that heterogeneous land surface plays in atmospheric circulations at the mesoscale. A stochastic linear theory of mesoscale
circulation induced by the thermal heterogeneity of the land surface Abstract: This paper presents a three-dimensional stochastic linear model of the mesoscale circulation induced by the variability of turbulent sensible heat flux over land surface. The primitive equations relating wind field, geopotential, and potential temperature are formulated as a system of stochastic partial differential equations and solved analytically. The solution is based on spectral representations of homogeneous random fields. The flow intensity is found to be proportional to the standard deviation of the turbulent sensible heat flux into the atmosphere. Large (small) scales of spatial variability in the surface heating preferably impact circulations at high (low) altitudes. The mesoscale fluxes associated with the atmospheric flow are related to explicit functions of atmospheric stability, variance of turbulent heat flux, and synoptic wind. The authors find that the vertical momentum flux is significant in the presence of synoptic wind and that the flow perpendicular to the direction of the synoptic wind is responsible for this momentum flux. The proposed linear theory identifies the synoptic conditions under which the land-surface heterogeneity may play a role in atmospheric circulations at the mesoscale. Numerical Simulation of
Nonlinear Mesoscale Circulation Induced by Thermal Heterogeneity
of Land Surface Mesoscale circulations forced by a random distribution of surface sensible heat flux have been investigated using a three-dimensional numerical model. The complex land surface is modeled as a homogeneous random field characterized by a spectral distribution. Standard deviation and length scale of the sensible heat flux at the surface have been identified as the important parameters that describe the thermal variability of land surface. The form of the covariance of the random surface forcing is not critical in driving the mesoscale circulation. The thermally induced mesoscale circulation is significant, and extends up to about five kilometers when the atmosphere is neutral. It becomes weak and is suppressed when the atmosphere is stable. The mesoscale momentum flux is much stronger than the corresponding turbulent momentum flux in the neutral atmosphere, while the two are comparable in the stable atmosphere. The mesoscale heat flux has a different vertical profile than turbulent heat flux, and may provide a major heat transport mechanism beyond the planetary boundary layer. The impact of synoptic wind on the mesoscale circulations is relatively weak. Nonlinear advection terms are responsible for momentum flux in the absence of synoptic wind. The role of clouds in the surface energy balance over the Amazon ForestElfatih A.B. Eltahir and E. James Humphries Jr. Abstract: Deforestation in the Amazon region will initially impact the energy balance at the land surface through changes in land cover and surface hydrology. However, continuation of this human activity will eventually lead to atmospheric feedback, including changes in cloudiness which may play an important role in the final equilibrium of solar and terrestrial radiation at the surface. In this study, the different components of surface radiation over an undisturbed forest in the Amazon region are computed using data from the Amazon region micrometerological experiment (ARME). Several measures of cloudiness are defined: two estimated from the terrestrial radiation measurements, and one from the solar radiation measurements. The sensitivity of the surface fluxes of solar and terrestrial radiation to natural variability in cloudiness is investigated to infer the potential role of the cloudiness feedback in the surface energy balance. The results of this analysis indicate that a 1% decrease in cloudiness would increase net solar radiation by ca. 1.6 W/m2. However, the overall magnitude of this feedback, due to total deforestation of the Amazon forest, is likely to be of the same order as the magnitude of the decrease in net solar radiation due to the observed increase in surface albedo following deforestation. Hence, the total change in net solar radiation is likely to have a negligible magnitude. In contrast to this conclusion, we find that terrestrial radiation is likely to be more strongly affected; reduced cloudiness still decrease net terrestrial radiation; a 1% decrease in cloudiness induces a reduction in net terrestrial radiation of ca. 0.7 W/m2; this process augments the similar effects of the predicted warming and drying in the boundary layer. Due to the cloudiness feedback, the most significant effect of large-scale deforestation on the surface energy balance is likely to be in the modification of the terrestrial radiation field rather than the classical albedo effect on solar radiation fields. The net effect of clouds is to reduce net radiation; a 1% increase in cloudiness induces a reduction in net radiation of ca. I W/m2. The implications of this negative feedback on large-scale land-atmosphere interactions over rainforests are discussed. c 1998 Royal Meteorological Society. A See-Saw Oscillation Between the
Amazon and Congo Basins Abstract: The climate of Earth is shaped to a significant degree by the occurrence of intense storms over three regions: the Amazon and Congo basins and the Pacific Ocean. However, little is known about natural oscillations in the amounts of rainfall over the Amazon and Congo basins. Here, we present new satellite observations on tropical rainfall distribution and historical river flow observations to document a natural see-saw oscillation across the Atlantic Ocean: floods over the Amazon basin tend to coincide with droughts over the Congo basin and vice versa. This phenomenon is most significant during the southern hemisphere summer, and was observed most clearly during the decades of 1945-1955, 1960s, and 1970s. The mechanism responsible for this see-saw phenomenon is based on the Gill model of tropical circulations since rising motions associated with floods over either of the two basins is likely to force subsidence and droughts over the other basin. Soil Moisture-Rainfall FeedbacksA feedback mechanism in annual rainfall, Central SudanElfatih A. B. Eltahir Abstract: Annual rainfall in many parts of the world is an independent process. Yet annual rainfall series in some regions of Africa show characteristics incompatible with such an hypothesis. The annual rainfall process in Central Sudan is weakly dependent. The hypothesis that the dependence is due to a "feedback mechanism" is investigated using a mathematical model based on the water balance of the neighboring region, Bahr Elghazal. Precipitation Recycling Abstract. The water cycle regulates and reflects natural variability in climate at the regional and global scales. Large-scale human activities that involve changes in land cover, such as tropical deforestation, are likely to modify climate through changes in the water cycle. In order to understand, and hopefully be able to predict, the extent of these potential global and regional changes, we need first to understand how the water cycle works. In the past, most of the research in hydrology focused on the land branch of the water cycle, with little attention given to the atmospheric branch. The study of precipitation recycling, which is defined as the contribution of local evaporation to local precipitation, aims at understanding hydrologic processes in the atmospheric branch of the water cycle. Simply stated, any study on precipitation recycling is about how the atmospheric branch of the water cycle works, namely, what happens to water vapor molecules after they evaporate from the surface, and where will they precipitate? Estimation of precipitation recycling over any large basin, such as the Mississippi or the Amazon, is a necessary step before developing a quantitative description of the regional water cycle. This paper reviews the research on the concept of precipitation recycling and emphasizes the basic role of this process in defining the different components of the atmospheric branch in any regional water cycle. To illustrate the assumptions and limitations involved in estimation of precipitation recycling, we present and discuss a general formula for estimation of precipitation recycling. The recent estimates of annual precipitation recycling ratio from different regions are reviewed and compared. Finally, the dependence of precipitation recycling over any region on the spatial scale is discussed and illustrated by the example of the Amazon basin. An analysis of the soil moisture-rainfall feedback, based on direct observations from IllinoisKirsten L. Findell and Elfatih A. B. Eltahir Abstract. Many global and regional climate modeling studies have demonstrated the importance of the initial soil water condition in their simulations of regional rainfall distribution. However, none of these modeling studies has been tested against directly observed data. This study tests the hypothesis that soil saturation is positively correlated with subsequent precipitation by analyzing a 14-year soil moisture data set from the state of Illinois. The linear correlation between an initial soil saturation condition and subsequent rainfall is significant during the summer months, reaching a peak of r2 > 0 4 in mid-June. This result is consistent with the hypothesis that knowledge of late spring/ early summer soil moisture conditions can aid in the prediction of drought or flood years, but it does not necessarily prove that feedback from anomalous soil moisture reservoirs is the cause of anomalous summer conditions. Further analyses indicate that from early June to mid-August, persistence in rainfall cannot fully account for the observed correlations, suggesting the likelihood of a physical feedback mechanism linking early summer soil saturation with subsequent precipitation. However, spatial and temporal data limitations restrict the potential for drawing strong new conclusions from the Illinois Study. A soil moisture-rainfall feedback mechanism 1. Theory and
observations Abstract. This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large-scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper [Zheng and Eltahir, this issue] using a simple numerical model. A soil moisture-rainfall feedback mechanism 2. Numerical
experiments Abstract. Here we develop a numerical model to investigate the hypothesis proposed by a companion paper [Eltahir, this issue], which describes a soil moisture-rainfall feedback mechanism. The model is designed to describe the seasonal evolution of the West African monsoon rainfall and is used to perform numerical experiments that elucidate the mechanisms of the response of rainfall to soil moisture anomalies. A significant rainfall anomaly is simulated by the model in response to a hypothetical soil moisture anomaly that has been imposed during early summer. However, the magnitude of this anomaly almost vanishes when the net radiation at the surface is not allowed to respond to the soil moisture anomaly. Hence the results of the numerical experiments support the proposed hypothesis and highlight the crucial importance of the radiative and dynamical feedbacks in regulating the rainfall anomalies that result from the soil moisture anomalies. Hydroclimatology of Illinois: A comparison of monthly
evaporation estimates based on atmospheric water balance and soil water balance Abstract: Here we describe the regional-scale hydrological cycle of Illinois, including both the land and atmospheric branches, using a dataset on most of the hydrological variables i.e., precipitation, streamflow, soil water content, snow depth, groundwater table level, and atmospheric flux of water vapor. Since direct observations on evaporation are not available, two different approaches-soil water balance and atmospheric water balance were applied to estimate the regional evaporation over Illinois from 1983 to 1994. The availability of a comprehensive hydrological dataset covering the large area of Illinois facilitated a comparison between these two approaches for estimation of evaporation. To our knowledge, this is the first time such a comparison has been made. The climatologies of the monthly evaporation estimates from the two approaches agree reasonably well, and within a 10% error; however, substantial differences exist between the two estimates of evaporation for individual months. The seasonal variability of the evaporation estimates based on soil water balance is largely balanced by the seasonal pattern of subsurface storage, whereas the seasonal variability of evaporation estimates from the atmospheric water balance is almost entirely balanced by the seasonal pattern of lateral fluxes of water vapor. This contrast reflects a fundamental difference in the hydrology of the land and atmospheric branches of the regional water cycle. In light of the fact that independent datasets were used in the two approaches, our results are encouraging: the atmospheric water balance approach has the potential for the accurate estimation of the climatology of regional evaporation, at least for humid regions at a scale similar to that of Illinois (~ 105 Km2 ). However, sensitivity analysis suggests that the accuracy of atmospheric water. On the Asymmetric Response of Aquifer Water Level to
Droughts and Floods in Illinois Abstract: Here we analyze observed characteristics of the natural variability in the regional-scale hydrological cycle of Illinois, including the soil and atmospheric branches. This analysis is based on a consistent data-set that describes several hydrological variables: the flux of atmospheric water vapor, incoming solar radiation, precipitation, soil moisture content, aquifer water level, and river flow. The climatology of the average regional hydrological cycle has been estimated. Variability in incoming solar radiation, not precipitation, is the main forcing of the seasonal variability in evaporation, soil moisture content, aquifer water level, and river flow. While precipitation plays a minor role in shaping the natural variability in the regional hydrological cycle at the seasonal time scale; variability in precipitation is the major factor in shaping the natural variability in the regional hydrological cycle at the inter-annual time scale. The anomalies in the different variables of the regional hydrological cycle have been computed and the persistence patterns of extreme floods and droughts have been compared. The 1988 drought left a signature in the aquifer water level that is significantly more persistent than the corresponding signature for the 1993 summer flood. The discharge from unconfined groundwater aquifers to streams (base-flow) provides an efficient dissipation mechanism for the wet anomalies in aquifer water level (groundwater rating curve), which may explain why droughts leave a significantly more persistent signature on groundwater hydrology, in comparison to the signature of floods. The non-linearity has been attributed to the increasing degree by which the unconfined aquifers get connected to the channels network, as the aquifer water level rises leading to higher drainage density. The potential implications of these results regarding the impact on regional water resources due to any future climate change are discussed. Analysis of the Pathways Relating Soil Moisture and
Subsequent Rainfall in Illinois Abstract: This study is a continuation of an earlier work [Findell and Elahir, 1997] on the soil moisture-rainfall feedback using a data set of biweekly neutron probe measurements of soil moisture at up to 19 stations throughout Illinois. This earlier work showed that soil moisture can play a significant role in maintaining drought or flood conditions during the summer. Results of a linear correlation analysis between initial soil saturation and rainfall in the subsequent three weeks showed that a positive correlation between these two variables was present from early June to mid-August. This correlation was more significant than the serial correlation within precipitation, suggesting the likelihood of a physical mechanism linking soil moisture to subsequent rainfall. Further investigations probed the nature of such a physical pathway linking soil moisture to subsequent rainfall: these analyses are presented in this paper. Near-surface hourly observations of pressure, temperature, wet-bulb temperature, and relative humidity from 13 stations in and close to Illinois were used as indicators of near-surface air conditions. Time series of the spatial average of each of these and other quantities were then calculated by averaging data from the 13 stations at each hour. An analysis of the connections between an average daily soil saturation time series for the whole state of Illinois with these state wide average air conditions did not yield the anticipated positive correlation between soil moisture and moist static energy (MSE). It is not clear if this is due to limitations of the data or of the theory. Other factors, such as clouds, could potentially be masking the impacts of soil moisture on the energy of the near-surface air. There was evidence , however, that moisture availability (or lack thereof) at the surface has a very strong impact on the wet-bulb depression of near-surface air, particularly from mid-May to the end of August, showing good correspondence to the period of significant soil moisture-rainfall association. The final set of analyses performed included an investigation of hourly boundary layer and rainfall data. Data from 82 hourly rainfall stations were averaged to compare state wide hourly rainfall to state wide hourly boundary layer conditions. A link between high MSE and high rainfall was noted during some summer months, and a link between low wet-bulb depression and high rainfall was evident for all of the months analyzed (April through September). These analyses then suggest that the significant but weak correlation between soil moisture and rainfall in Illinois summers is at least partially due to soil moisture controls on the wet-bulb depression of near-surface air. El Niņo and the Flow in Tropical RiversEl Nino and the natural variability in the flow of the Nile
River Abstract. Natural variability in the annual flow of the Nile River has been the subject of great interest to the civilizations that have historically occupied the banks of that river. Here we report results from analysis on two extensive data sets describing sea surface temperature of the Pacific Ocean, and the flow of water in the Nile River. The analysis suggests that 25% of the natural variability in the annual flow of the Nile is associated with El Nino oscillations. A procedure is developed for using this observed correlation to improve the predictability of the Nile flood. A simple hypothesis is presented to explain physically the occurrence of the Hurst phenomenon in the Nile flow. ENSO and the natural
variability in the flow of tropical rivers Abstract: This paper examines the relationship between the annual discharges of the Amazon, Congo, Parana, and Nile rivers and the sea surface temperature (SST) anomalies of the eastern and central equatorial Pacific Ocean, an index of El Nino-southern Oscillation (ENSO). Since river systems are comprehensive integrators of rainfall over large areas, accurate characterization of the flow regimes in major rivers will increase our understanding of large-scale global atmospheric dynamics. Results of this study reveal that the annual discharges of two large equatorial tropical rivers, the Amazon and the Congo, are weakly and negatively correlated with the equatorial Pacific SST anomalies with 10% of the variance in annual discharge explained by ENSO. Two smaller subtropical rivers, the Nile and the Parana, show a correlation that is stronger by about a factor of 2. The Nile discharge is negatively correlated with the SST anomaly, whereas the Parana river discharge shows a positive relation. The tendency for reduced rainfall/discharge over large tropical convection zones in the ENSO warm phase is attributed to global scale subsidence associated with major upwelling in the eastern Pacific Ocean. Nilometers, El Niņo, and Climate
Variability Abstract: Nilometers have been used for gauging the level of water in the Nile river for more than five millennia. The written records describing some of these measurements represent the longest written records for any hydrological phenomenon. They describe interannual fluctuations in the Nile river flow which are closely associated with El Niņo phenomenon. Here, we use information about long-term variability in El Niņo occurrences that has been extracted from the Nilometers records to test this significance of the recent trend in the frequency of El Niņo years. We show that the observed frequency of El Niņo years during the last two decades is rather high compared to the long-term statistics that are computed from about a thousand years of Nilometers data; however similar levels of activity have been observed during the first millennium. Use of ENSO Information for Medium- and Long-range
Forecasting of the Nile Floods Abstract: The Nile river flow is negatively correlated with the Sea Surface Temperature (SST) in the tropical eastern Pacific Ocean (TEP), an index of El Nino - Southern Oscillation (ENSO). In this paper, we combine several sources of information, including ENSO, rainfall over Ethiopia and the recent history of river flow in the Nile, in order to obtain accurate forecasts of the Nile flood at Aswan. Bayesian theorem is used in developing the discriminant forecasting algorithm. We use conditional categoric probability to describe the flood forecasts, and define a synoptic index to measure the forecasts skill. Our results show that ENSO information is the only valuable predictor for the long-range forecasts (lead time longer than the hydrological response time scale, which is 2-3 months in this study). However, the incorporation of the rainfall and river flow information in addition to the ENSO information significantly improves the quality of the medium-range forecasts (lead time shorter than the hydrological response time scale). Topography and the Distribution of Soil MoistureStochastic analysis of the relationship between topography
and the spatial distribution of soil moisture Abstract. This paper deals with the issue of the spatial horizontal variability of soil moisture in the root zone of a shallow soil at the large scale. The problem of water flow in the unsaturated zone is formulated so that topography appears explicitly as a forcing for the movement and redistribution of soil moisture. This formulation emphasizes the role of the lateral redistribution of water that is induced by topography. A stochastic theory is developed to relate the statistical distribution of soil moisture to that of elevation. This approach will ultimately facilitate the use of the readily available data sets describing topography for the purpose of defining the large-scale distribution of soil moisture. The steady state horizontal distribution of soil moisture under homogeneous bare soil conditions is regulated by three distinct factors: topography, climate, and soil properties. First, topography, forces a distribution of soil moisture that tends to mimic the elevation field at large scales. The other two factors are the vertical divergence of water in response to the climate forcing (evaporation) and the capillary resistance to water movement. The climate forcing tends to smooth the spatial distribution of soil moisture. However, the capillary forces exerted by the soil matrix tend to resist displacement of water and hence exert adverse effects against the topography and climate forcings. The variance of the soil moisture distribution increases with the variance of the elevation field and decreases with the correlation scale of the elevation field and the magnitude of the climate forcing. The impact of capillary forces on the vertical fluxes of water is more significant than their impact on the topographically induced horizontal fluxes, owing to the larger hydraulic gradient in the vertical direction resulting from the disparity in scale between the vertical and horizontal directions. Role of topography in facilitating coexistence
of trees and grasses within savannas The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that "variations in elevation at relatively short horizontal scales of similar to1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas.'' To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator ( IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen ( SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9degreesN the model simulates trees even when the GWT is assumed to be infinitely deep; at 13degreesN the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11degreesN, trees are simulated when the GWT is at similar to2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas. Hydrology and HydroclimatologyAggregation-disaggregation
properties of a stochastic rainfall model Abstract: A statistical approach based on the modified Bartlett-Lewis rectangular pulses model is presented to disaggregate rainfall statistics from daily data. Six model parameters are estimated from 24- and 48-hour accumulated rainfall data. Based on these estimated parameters, in addition to reproducing 24- and 48-hour statistics, the model is shown to infer 1-, 2-, 6-, and 12-hour historical statistics satisfactorily. An upper limit for disaggregation scale (about 2 days) for this model has been identified. This characteristic behavior of the model is related to the power law dependence of the power spectrum for timescales smaller than 2 days. A detailed comparison between observed and modeled statistics of rainfall data is presented for two rain gages, one from central Italy and the other from the midwestern United States. On the sensitivity of drainage density to climate change Abstract: Drainage density reflects the signature of climate on the topography and dictates the boundary conditions for surface hydrology. Hence defining the relationship between drainage density and climate is important in assessing the sensitivity of water resources and hydrology to climate change. Here we analyze the equilibrium relationship between drainage density and climate and estimate the relative sensitivity of drainage density to climate change. We conclude that the sign of the resulting change in drainage density depends not only on the direction of the change in climate but also on the prevailing climatic regime. Geomorphic
controls on regional base flow Abstract: The subsurface hydrological response plays an important role in the hydrology of humid regions. In particular, the physical relationship between base flow dynamics and the fluctuations in spatially averaged water table depth, as described by the groundwater rating curve, determine to a significant extent the nature of statistical persistence of hydrological anomalies in the unconfined aquifers level and river flow. In this paper, we propose that the scale and shape of the groundwater rating curve reflect some of the geomorphological characteristics of the region such as relief, drainage density, and the hypsometric distribution of the elevation field. These connections between geomorphology and hydrology of river basins are investigated using a simple model of unconfined groundwater flow applied to synthetic basins as well as observed basins from Illinois. Prediction
of regional water balance components based on climate, soil,
and vegetation parameters, with application to the Illinois
River Basin Abstract: This paper presents a framework for studying regional water balance in which the physical processes are first described at the local instantaneous scale and then integrated to the annual, basin-wide scale. The integration treats the relative soil saturation (i.e., the soil moisture divided by the porosity) and precipitation intensities as stochastic variables in space and time. A statistical equilibrium characterizes the annual water balance, resulting in a specific relation that predicts the space-time average of soil saturation in terms of soil, climate, and vegetation parameters. Specific relationships are proposed to relate the space-time average soil saturation to runoff, groundwater recharge, and evapotranspiration. This framework is applied to the Illinois River Basin. The shape of the spatial and temporal distributions of soil saturation are determined from observations. The other parameters are determined from the physical characteristics of the basin and calibration procedures. The resulting model is able to reproduce an observed relation between the space-time average soil saturation and precipitation. It is also able to reproduce observed relations between space-time average soil saturation and space-time average evapotranspiration, surface runoff, and groundwater runoff. Representation
of Water Table Dynamics in a Land-Surface Scheme: 1. Model Development Abstract: Most of the current land surface parameterization schemes lack any representation of regional groundwater aquifers. Such a simplified representation of subsurface hydrological processes would result in significant errors in the predicted land-surface states and fluxes especially for the shallow water table areas in humid regions. This study attempts to address this deficiency. To incorporate the water table dynamics into a land surface scheme, a lumped unconfined aquifer model is developed to represent the regional unconfined aquifer as a nonlinear reservoir, in which the aquifer simultaneously receives the recharge from the overlying soils, and discharges runoff into streams. The aquifer model is linked to the soil model in the land surface scheme LSX through the soil drainage flux. The total thickness of the unsaturated zone varies in response to the water table fluctuations, thereby interactively couples the aquifer model with the soil model. The coupled model (called LSXGW) has been tested in Illinois for an 11-year period from 1984-1994. The results show reasonable agreements with the observations. However, there are still secondary biases in the LSXGW simulation partially resulting from not accounting for the spatial variability of water table depth. The issue of sub-grid variability of water table depth will be addressed in a companion paper [Yeh and Eltahir, this issue]. Representation
of Water Table Dynamics in a Land-Surface Scheme: 2. Sub-grid
Variability Abstract: A lumped unconfined aquifer model has been developed and interactively coupled to a land surface scheme [Yeh and Eltahir, this issue]. Here, we address the issue of the representation of sub-grid variability of water table depths (WTD). A statistical-dynamical (SD) approach is used to account for the effects of the unresolved sub-grid variability of WTD in the grid-scale groundwater runoff. The dynamic probability distribution function (PDF) of WTD is specified as a two-parameter Gamma distribution based on observations. The grid-scale groundwater rating-curve (i.e., aquifer storage-discharge relationship) is derived statistically by integrating a point groundwater runoff model with respect to the PDF of WTD. Next, a mosaic approach is utilized to account for the effects of sub-grid variability of WTD in the grid-scale groundwater recharge. A grid-cell is categorized into different sub-grids based on the PDF of WTD. The grid-scale hydrologic fluxes are computed by averaging all the sub-grid fluxes weighted by their fractions. This new methodology combines the strengths of the SD approach and the mosaic approach. The results of model testing in Illinois from 1984-1994 indicate that the simulated hydrologic variables (soil saturation and WTD) and fluxes (evaporation, runoff, and groundwater recharge) agree well with the observations. Due to the paucity of the large-scale observations on WTD, the development of a practical parameter estimation procedure is indispensable before the global implementation of the developed LSXGW in climate models. Forest
on the edge: Seasonal cloud forest in Oman creates its own ecological
niche Abstract: Cloud forests usually grow in the moist tropics where water is not a limiting factor to plant growth. Here, for the first time, we describe the hydrology of a water limited seasonal cloud forest in the Dhofar mountains of Oman. This ecosystem is under significant stress from camels feeding on tree canopies. The Dhofar forests are the remnants of a moist vegetation belt, which once spread across the Arabian Peninsula. According to our investigation the process of cloud immersion during the summer season creates within this desert a niche for moist woodland vegetation.Woodland vegetation survives in this ecosystem, sustained through enhanced capture of cloud water by their canopies (horizontal precipitation). Degraded land lacks this additional water source, which inhibits re-establishment of trees. Our modeling results suggest that cattle feeding may lead to irreversible destruction of one of the most diverse ecosystems in Arabia.
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