Regional climate modelling of the 2006 West African monsoon: sensitivity to convection and planetary boundary layer parameterisation using WRF

Abstract

International audienceRegional climate model (RCM) is a valuable scientific tool to address the mechanisms of regional atmospheric systems such as the West African monsoon (WAM). This study aims to improve our understanding of the impact of some physical schemes of RCM on the WAM representation. The weather research and forecasting model has been used by performing six simulations of the 2006 summer WAM season. These simulations use all combinations of three convective parameterization schemes (CPSs) and two planetary boundary layer schemes (PBLSs). By comparing the simulations to a large set of observations and analysis products, we have evaluated the ability of these RCM parameterizations to reproduce different aspects of the regional atmospheric circulation of the WAM. This study focuses in particular on the WAM onset and the rainfall variability simulated over this domain. According to the different parameterizations tested, the PBLSs seem to have the strongest effect on temperature, humidity vertical distribution and rainfall amount. On the other hand, dynamics and precipitation variability are strongly influenced by CPSs. In particular, the Mellor–Yamada–Janjic PBLS attributes more realistic values of humidity and temperature. Combined with the Kain–Fritsch CPS, the WAM onset is well represented. The different schemes combination tested also reveal the role of different regional climate features on WAM dynamics, namely the low level circulation, the land–atmosphere interactions and the meridional temperature gradient between the Guinean coast and the Sahel

West African monsoon intraseasonal activity and its daily precipitation indices in regional climate models: diagnostics and challenges

“…This is consistent with the findings of Flaounas et al (2011), who showed drastic changes depending on the physical parameterizations.…”

“…Crétat et al (2015) conducted a more detailed analysis on the relationship between AEWs and daily rainfall, using various regional simulations, and revealed the fairly good performance of RCM in terms of spatial and temporal phases. Although a set of arguments tends to confirm the real added value of RCMs, Flaounas et al (2011) have shown a decisive impact of physical parameterizations on the monsoon onset and precipitation intraseasonal variability, implying that substantial work is still needed to calibrate or to adapt RCMs for Africa.…”

A review on regional dynamical downscaling in intraseasonal to seasonal simulation/prediction and major factors that affect downscaling ability

“…Using MM5, Solman and Pessacg (2012) found that based on model performance in sea level pressure, surface temperature, and precipitation simulation from October through November, no single combination was found to perform the best over the entire domain and during their entire integration period for their La Plata Basin study. Flaounas et al (2011) found that the PBLs seem to have the strongest effect on the vertical distributions of temperature and humidity, and rainfall amount, and convective schemes strongly influence precipitation variability in their WRF/ARW simulations for the year 2006 West African Monsoon simulation.…”

“…Over South Africa, Ratna et al (2013) indicated that in WRF/ARW, the BMJ scheme seemed to reproduce the intensity of rainfall anomalies and also exhibited the highest correlation with observed interannual summer rainfall variability there compared with other schemes. Due to the development of the new general mesoscale forecast models, such as WRF, which consist of broad selections of different physical parameterizations, studies have combined different cumulus schemes, radiation schemes, and planetary boundary layer (PBL) schemes in an attempt to define the optimal combination of physical parameterizations for certain regional climate studies (e.g., Xu and Small, 2002;Leung et al, 2003b;Lynn et al, 2010;Flaounas et al, 2011;Solman and Pessacg, 2012;Yuan et al, 2012). Using MM5, Solman and Pessacg (2012) found that based on model performance in sea level pressure, surface temperature, and precipitation simulation from October through November, no single combination was found to perform the best over the entire domain and during their entire integration period for their La Plata Basin study.…”

Validating the dynamic downscaling ability of WRF for East Asian summer climate

“…For example, studies on the impact of soil moisture on regional climate (Kim and Hong 2007;Sato and Xue 2013;Liu et al 2014) hardly consider the different model performance between physical schemes combinations. Although the new general meso-scale forecast models (such as Weather Research and Forecasting (WRF) model) were developed to consist of broad selections of physical parameterizations, sensitivity studies were mainly conducted for different model settings and physical processes, respectively (Flaounas et al 2011;Waliser et al 2011;Yang et al 2012;Ratna et al 2014).…”