Convective inhibition explains regional differences in tropical precipitation
Submitter
Fast, Jerome D — Pacific Northwest National Laboratory
Area of research
Cloud-Aerosol-Precipitation Interactions
Journal Reference
Science
Understanding what controls rainfall in tropical regions is important for accurate climate modeling. A team examined radar-derived precipitation data from three tropical field campaigns and the corresponding large-scale environmental variables from global reanalysis data to quantify the roles of various environmental factors in the transition from a suppressed to an active rainfall regime. They identified the variability of environmental factors that explain the difference between the rainfall statistics in the studied regions. They developed a simple machine learning (ML) model that predicts the probability of a transition from a suppressed to an active precipitation regime as a function of five large-scale environmental variables. The work demonstrates a potential application of the ML model as a trigger function, a set of conditions used to determine whether the convection should be activated, for climate model convection parameterizations.
Impact
A comparative and quantitative insight into the environmental factors controlling precipitation characteristics and differences among tropical regions is essential for better understanding rainfall statistics and their representations. This will ultimately lead to improved representations in global and regional models. This work demonstrates how marginal statistical analysis can be used in tandem with ML models to obtain a quantitative and qualitative understanding of the relationship between rain clouds and the large-scale environment. The ML model captures several qualitative relationships obtained from a statistical analysis of frequencies. This work can easily be generalized to account for additional environmental variables and to include other regions.
Summary
A quantitative insight into the environmental factors that control rainfall characteristics and regional variation is needed to better understand rainfall statistics and extremes. Such information is also important for accurately representing rainfall in global and regional models. Using statistical analysis and a ML model, researchers examined the roles of various environmental variables in the transition from suppressed to active precipitation regimes. They documented the origins of differences in between environmental conditions and radar observed precipitation regime statistics across three tropical regions— the Amazon, northern Australia, and the equatorial Indian Ocean. For widespread rain to start in the three tropical areas, an abrupt increase in the growth rate of precipitable water (PW, the total water in the atmospheric column) ~ 60 mm and convective inhibition (CIN, the amount of buoyant energy required to enable convection) <100J kg-1 is needed. Differences in precipitation among the three areas can be primarily explained by regional differences in CIN statistics and, to a lesser extent, PW. High CIN and low PW are comparatively common over the Amazon, while high CIN is less frequent over the equatorial Indian Ocean. Darwin, Australia has the most frequent active regimes with high PW and moderate CIN.
Figure 1. The specific conditions required to transition to a precipitation event are similar, but slightly different for three different tropical regions. A shallow-to-deep convection transition event over the equatorial Indian Ocean during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign, winter 2011-2012. (Photo by Samson Hagos|Pacific Northwest National Laboratory.)