Research Overview

The Cronin group on convection, radiation, and clouds combines theory and idealized numerical modeling experiments to understand the stability of Earth's climate, and explore possible climates of other worlds.

A (non-exhaustive) list of my research interests is below.

1) Self-aggregation of convection

How and why might clouds cluster together in the tropics, and what impact does this have on climate? I've worked on this subject with Allison Wing, a professor at Florida State Univeristy, as well as Tom Beucler, a fourth-year PhD student at MIT. We have used computer models to simulate and analyze the phenomenon of "self-aggregation" of convection, where large cloud clusters can develop spontaneously and generate large-scale moist and dry regions. Allison worked on the general problem of self-aggregation for her thesis at MIT, and developed some nice methodology to identify what physical mechanisms aid or hinder the process. Allison and I have together on two papers in the past few years on self-aggregation in a long channel, where self-aggregation takes the form of multiple dry and moist bands, and about how analyzing these simulations might tell us something about climate feedbacks. See the Movies! link for more detail, as well as Wing and Cronin (2015), QJRMS, and Cronin and Wing (2017), JAMES. Tom and I have worked together to try to develop a bottom-up understanding of how the responses of radiation and convection to moisture anomalies contribute to self-aggregation -- see Beucler and Cronin (2016), JAMES.

2) Cold air formation in warmer climates

How does very cold air form, and how is that process sensitive to a changing climate? The process of Arctic air formation is that by which high-latitude maritime air is transformed, by radiative cooling, into high-latitude continental air, which is much colder at the surface and often has temperature inversions. I've worked on this problem with Eli Tziperman, a professor at Harvard, Harrison Li, a summer PRISE fellow and research assistant in 2015, and Zeyuan Hu, a Peking University undergraduate who was at Harvard in the summer of 2017. We have found, using several different model configurations, that very cold air masses form less quickly in warm climates. For more details, see Cronin and Tziperman (2015), PNAS, and Cronin, Li, and Tziperman (2017), JAS.

3) Tropical Cyclones in unusual environments

Do hurricanes require an ocean surface, or even moisture at all, to form and exist? The answer, based on the present climate, might seem like an emphatic "of course!" -- but other climates and vortices on other planets might disagree! A paper from several years ago also found formation of a hurricane-like storm in a completely dry atmospheric model, fueled by a big jump in the temperature between the surface and the air just above it (much like the beach on a hot summer day). I'm working with Dan Chavas, a professor at Purdue, on hurricanes that form in this dry limit in numerical models, as well as a range of conditions between a normal moist ocean surface and a completely dry surface. I'm also interested the possibility of hurricane-like storms in the Arctic in much warmer climates.

4) Glacial Inception

How do ice ages begin? How a small glacier, or set of small glaciers, expand to cover much of a continent (which we think happened around 115 thousand years ago) is called glacial inception, and is a fundamental but still largely open problem in climate science. Harvard PhD student Leah Birch (now at WHRC) was co-advised by Eli Tzpiperman and me, and worked on this problem using a regional climate model, with and without asynchronous coupling to an ice sheet model. See Birch, Cronin, and Tziperman (2017), J. Climate for more information.

5) Diurnal Cycle and Island Rainfall

Why does it rain more over even flat islands than nearby ocean areas in the tropics, and what consequences might this have for the climate system? As a dissertation topic, I looked at whether the diurnal cycle of land-atmosphere interaction and convection might be able to explain this island rainfall enhancement. I worked together with advisor Kerry Emanuel and thesis committee member Peter Molnar to explore whether geologic changes in the island cover and elevation of Indonesia over the past few million years could have moved the climate away from the "permanent El Nino" state that was thought to exist about 5 million years ago. See Cronin, Emanuel, and Molnar (2015), QJRMS, and Molnar and Cronin (2015), Paleoceanography for more information, or the Movies! link for some animations!

6) Boundary Layer Sensitivity

How do human-caused changes in land surface properties affect the climate? The complexity of the land surface formulations used in climate models often makes it difficult to understand what is going on when we attempt to simulate something like the impacts of land cover change on climate. I have developed a (relatively) simple theory which can be used to qualitatively understand how changes in surface wetness, roughness, and reflectivity of the land surface lead to changes in near-surface climate. See Cronin (2013), JAMES for more detail.

This work ultimately grew out of my generals project (Spring 2011), where I worked on understanding the climate response to "physiological forcing" -- the tendency of plants to close their stomata (leaf pores, through which CO2 enters leaves and water vapor exits them) as the atmospheric CO2 concentration is raised.

7) Details about Radiative-Convective Equilibrium

What are some of the nuances of our simplest credible model of planetary climate? Radiative-convective equilibrium (RCE) is this simplest credible climate model, and represents a hypothetical state of the atmosphere-surface system where radiative cooling of the atmosphere to space is balanced by convective heating, mainly by rising warm and moist air in clouds. Radiative heating of the surface by the sun is balanced primarily by evaporative cooling. Study of RCE has been extremely important for our understanding of sensitivity and stability of climate. However, there are some details of RCE that are not well understood by the field in general.

One element of RCE that seems not to be widely understood is that there is a long approach time scale to equilibrium, and that this long time scale persists even without the thermal inertia of the ocean. See Cronin and Emanuel (2013), JAMES for more detail.

Another issue is that idealized models (such as of RCE) often use a solar energy input that is averaged over day and night. However, it is not completely obvious how to do this averaging appropriately, since both the sun's angle and intrinsic brightness can be tuned. I suggest that the best way to do this averaging is by insolation-weighting; see Cronin (2014), JAS for details.

8) Hadley Circulation and Eddies

How do midlatitude weather systems alter the overturning cells of the Tropical Atmosphere? Early in my time as a grad student at MIT, I did a few term papers exploring theories of the Hadley circulation. For a Harvard Journal Club in 2013, I led discussion on a paper about a simple model for the Hadley circulation. See the Code For Numerical Models page for more information.