Forced Precipitation Response in a Single Column Model with Parameterized Dynamics
Rain falling in sheets and torrents is a common image of the tropics, and the broad regions of warmest tropical sea surface temperature (SST) are, in an averaged sense, the rainiest places on earth. Heavy rain is both a threat and an essential resource for the populous countries of the tropics, and the deep convective clouds that produce it serve as the "boiler" of the heat engine that drives the global atmospheric circulation. The study of tropical precipitation, including its dependence on SST, sensitivity to greenhouse warming, depth of convective clouds, and other factors is thus a primary research area in climate dynamics.
A characteristic feature of the tropics is the lack of strong temperature contrasts, particularly at levels above the turbulent motions generated near the earth's surface. The uniformity of atmospheric temperature has motivated theories of tropical precipitation based on the weak temperature gradient (WTG) approximation, in which the net effect of large-scale atmospheric dynamics is to impose a vertical temperature profile in the atmospheric columns where convection and precipitation are occurring. Under this assumption convection and precipitation can be understood as a consequence of physics and thermodynamics occurring locally within a single atmospheric column, without taking the large-scale three-dimensional circulation into account. Thus, single column models (SCMs) using the WTG approximation have become important tools for understanding tropical convection and precipitation.
Work performed here develops and uses SCMs built on variants of the WTG approximation to address the response of tropical precipitation to external forcing. One goal is to examine theories of the response of precipitation to greenhouse warming framed in terms of gross moist stability (GMS), a stability measure based on the exchange of thermodynamic energy between the column and its surroundings. Reductions in GMS due to the moistening of the atmosphere with increasing temperature tend to increase precipitation but GMS can also increase if warming causes convective clouds to become taller. A further complication is that GMS increases if warmer conditions cause the height of the strongest updrafts within clouds to increase, even if the clouds themselves do not get taller. This effect can be captured in SCMs using the WTG approximation but there are large discrepancies in results from SCMs with different representations of column physics. Another form of external forcing examined here is heating in the stratosphere above the column, and the PIs attempt to reconcile differing precipitation responses to stratospheric heating found in earlier studies.
The work has broader impacts due to the substantial societal impacts of changes in tropical precipitation, as work performed here has direct relevance to the development of models used to predict the weather and climate of the tropics. The PIs also serve the broader climate science community by making their SCM versions available as part of the Community Earth System Model (CESM). SCMs are commonly used as part of a hierarchy of models, in which full-complexity climate models are used to simulate phenomena of interest and simpler models are used to isolate particular physical mechanisms and test hypotheses regarding their roles in the full-complexity simulations. The SCMs developed in this project use column physics representations taken from CESM and are fully compatible with CESM software, thus they can be easily incorporated into the model hierarchy developed for CESM. In addition, the work provides support and training to a graduate student, thereby providing for the future workforce in this research area.