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Title: Sub-cloud moist entropy curvature as a predictor for changes in the seasonal cycle of tropical precipitation

Journal Article · · Climate Dynamics
ORCiD logo [1];  [1]; ORCiD logo [1]
  1. BATTELLE (PACIFIC NW LAB)
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Convective Quasi-Equilibrium (CQE) may be a useful framework for understanding the precipitation minus evaporation (P-E) response to CO2-induced warming. To explore this proposition, a suite of aquaplanet simulations with a slab ocean from TRACMIP (the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project) is analyzed. A linear relationship between P-E and the curvature of sub-cloud moist entropy, a marker for the spatial distribution of sub-cloud moist energy and onset of a tropical direct overturning circulation under CQE conditions, is shown to exist across many of the TRACMIP simulations. Furthermore, this linear relationship is a skillful predictor of changes in P-E in response to CO2-induced warming. The curvature metric also shows improvement in predicting P-E changes compared to the simpler method of relating P-E directly to the sub-cloud moist entropy field or a simple ‘wet-get-wetter’ type null hypothesis, especially on seasonal and shorter timescales. Using fixed relative humidity in the curvature metric and sub-cloud moist entropy degrades their ability to predict P-E changes, implying that both temperature and relative humidity changes in the boundary layer are important for characterizing future precipitation changes. To understand why the curvature metric is a skillful predictor of hydrological changes, a moist static energy (MSE) budget analysis is performed, which shows that MSE divergence on sub-daily timescales is well parameterized as a downgradient diffusive process. Additionally, this transient MSE divergence has a similar structure to the curvature term. Assuming that the transient MSE divergence is related to convective activity, these findings suggest that the theoretical underpinning of the linear relationship between the curvature term and P-E resides in cloud processes that both remove spatial gradients of sub-cloud moist entropy and generate precipitation at sub-seasonal and shorter timescales.

Research Organization:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Organization:
USDOE
DOE Contract Number:
AC05-76RL01830
OSTI ID:
1633055
Report Number(s):
PNNL-SA-132417
Journal Information:
Climate Dynamics, Vol. 53, Issue 5-6
Country of Publication:
United States
Language:
English

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Related Subjects

Convective Quasi-Equilibrium
CQE
CO2-induced warming
aquaplanet simulations
TRACMIP
precipitation minus evaporation (P-E)
sub-cloud moist energy
curvature metric
fixed relative humidity
boundary layer
future precipitation changes
hydrological changes
moist static energy
downgradient diffusive process
transient MSE divergence