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) 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. This project has involved working with Eli Tziperman, a professor at Harvard, and Harrison Li, a summer PRISE fellow and research assistant in 2015. We found that the process of cold air formation is highly sensitive to the ocean temperature at high latitudes, with warmer initial air masses cooling much less rapidly.

2) Self-aggregation of convection

I've also been working with Allison Wing on numerical modeling 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. We worked together on a paper from 2014-2015 on self-aggregation in a long channel, where self-aggregation takes the form of multiple dry and moist bands. See the Movies! link for more detail, or Wing and Cronin (2015), QJRMS.

3) Diurnal Cycle and Island Rainfall

As a dissertation topic, I studied the influence of land-atmosphere interactions and the diurnal cycle on the climate system. One goal is to understand 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. Another goal is understand why islands are typically rainier than nearby ocean areas in the tropics. See Cronin, Emanuel, and Molnar (2015), QJRMS, and Molnar and Cronin (2015), Paleoceanography for more information, or the Movies! link for some animations!

4) Boundary Layer Sensitivity

The human footprint on the global land surface is large, and rapidly changing, and changes in land surface properties can affect the climate. But real land surfaces are highly complex and heterogeneous, and thus difficult to represent in climate models. 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.

5) Details about Radiative-Convective Equilibrium

Radiative-convective equilibrium (RCE) is 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, and radiative heating of the surface 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.

6) Tropical Cyclones in odd environments

I did a project for Tropical Meteorology (12.811, Spring 2011) on the unusual reintensification of Tropical Cyclone Erin (2007) over land. My findings were somewhat inconclusive, but I hope to revisit the subject of tropical cyclone intensification over land at some point. I'm also interested in the possibility for studying "dry" tropical cyclones, as well as the possibility of hurricane-like storms in the Arctic in much warmer climates.

7) Hadley Circulation and Eddies

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.