A seamless assessment of the role of convection in the water cycle of the West African Monsoon

Abstract

A suite of 40 day UK Met Office Unified Model simulations over West Africa during summer 2006 are analyzed to investigate the causes of biases in the position of the rainbelt and to understand the role of convection in the regional water budget. The simulations include climate, global operational, and limited area runs (grid spacings from 1.5 to 40 km), including two 12 km runs, one with parameterized and one with explicit convection. The most significant errors in the water cycle terms occur in the simulations with parameterized convection, associated with the diurnal cycle and the location of the convection. Errors in the diurnal cycle increase the northward advection of moisture out of the Sahel toward the Sahara but decrease the advection of moisture into the Sahel from further south, which limits the availability of moisture for Sahelian rainfall. These biases occur within the first 24 h, showing that they originate from the representation of fast physical processes, specifically, the convection scheme. Once these rainfall regimes have been established, the terms of the water budgets act to reinforce the biases, effectively locking the rainbelt's latitude. One of the simulations with parameterized convection does, however, produce a better latitudinal distribution of rainfall because on the first day it is better able to trigger convection in the Sahel. Accurate representation of the diurnal cycle of convection and the ability to trigger convection in a high convective inhibition environment is key to capturing the water cycle of the region and will improve the representation of the West African Monsoon.

Connecting spatial and temporal scales of tropical precipitation in observations and the MetUM-GA6

“…Such biases can have implications for weather and climate modelling on a wide range of temporal and spatial scales, from inhibiting moisture transport through intra-seasonal propagation of convection (e.g. Bush et al, 2015;Kim et al, 2016) to contributing to uncertainty in projections of future tropical rainfall (e.g. Kent et al, 2015).…”

“…Such deficiencies can have a significant impact on the regional-scale circulation and water cycle (e.g. Birch et al, 2014). Studies such as Kendon et al (2014) illustrated that representing rainfall characteristics on short and small scales may be paramount in order to eliminate these biases and thereby provide confidence in projections of the spatial and temporal characteristics of heavy rainfall in a future climate.…”

A process-based evaluation of dust-emitting winds in the CMIP5 simulation of HadGEM2-ES

“…H2ES-A shows a reduction of dust emission in the western Sahel compared to H2ES-C for June to September (not shown). Earlier studies suggest an association with larger bare soil fractions due to a vegetation die-back in H2ES-I caused by a deficit of monsoon rainfall Martin and Levine 2012;Birch et al 2014). Interestingly, dust emission from the Sahel is even larger in winter and spring with around 100 gm −2 per season in H2ES-I.…”

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“…triggered by the sea-breeze convergence along the Guinea coast (Fink et al, 2010). The moist convection embedded within the monsoon flow has been shown to be intrinsic to the monsoon, and the poor representation of convection in models leads to biases in the WAM (Birch et al, 2014;GarciaCarreras et al, 2013;Marsham et al, 2013).…”