Evaluation of Evaporation Climatology for the Congo Basin Wet Seasons in 11 Global Climate Models

David M Crowhurst,Simon J Dadson,Richard Washington

doi:10.1029/2019jd030619

David M Crowhurst, Simon J Dadson + Show 1 more

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https://doi.org/10.1029/2019jd030619

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Abstract

AbstractAcross the Congo, there is a wide spread in rainfall in the two wet seasons in Coupled Model Intercomparison Project 5 global climate models (GCMs). As the Congo is believed to be a moisture recycling hot spot, the evaporation of excess water from the land surface in some models could be amplifying the model spread in rainfall. This study performs an exploratory process‐based evaluation of Congo Basin evaporation in 11 Coupled Model Intercomparison Project 5 GCMs that took part in the Atmospheric Model Intercomparison Project. Our aims are to improve scientific understanding about Congo evaporation, and to determine whether there are opportunities to improve how models produce Congo evaporation. Climatologically, we find that models with “realistic” rainfall simulate higher rainfall in November, the peak of the second wet season, than March, the peak of the first. However, models with “realistic” evaporation simulate lower evaporation in November than March, because these models suppress the transpiration component of the evaporation in November relative to March. In both wet seasons, subgrid rainfall schemes make these models simulate a credible ratio of transpiration to canopy evaporation, and cause them to generate evaporation in a more realistic manner. We therefore trust how these models produce evaporation in the wet seasons, and argue that lower transpiration is likely to explain why evaporation is lower in November than March in reality. We also suggest that using subgrid rainfall schemes in all GCMs could improve how models produce Congo evaporation during the wet seasons. This might reduce the model spread in Congo rainfall.

Highlights

The Congo Basin is one of three major global hot spots of convective rainfall (Webster, 1983)
We suggest that using subgrid rainfall schemes in all global climate models (GCMs) could improve how models produce Congo evaporation during the wet seasons
As subgrid rainfall schemes enable models to simulate evaporation with increased realism during the wet seasons, by making plausible adjustments to the ratio of transpiration to canopy evaporation, we suggest that using subgrid rainfall schemes in the land surface components of all GCMs could improve how models produce evaporation from the Congo Basin during the wet seasons

Summary

Introduction

The Congo Basin is one of three major global hot spots of convective rainfall (Webster, 1983). There is currently a wide spread in global climate model (GCM) simulations of Congo Basin rainfall amount and distribution during the two wet seasons, which have peak rainfall in March and November (Haensler et al, 2013). As the models differ vastly in their simulation of present‐ day wet season rainfall, credibility in model projections of future rainfall is undermined. This problem is hampering the development of adaptation strategies for the Congo Basin, which are required to lessen vulnerability to changes in rainfall (Bele et al, 2015). It is essential to establish whether some models produce realistic and credible simulations of Congo Basin rainfall in the wet seasons, as these models are more likely to have trustworthy projections. If certain model schemes are responsible for improving model rainfall, these schemes could be used more widely to reduce the model spread

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