Abstract
Weather and climate model simulations of the West African Monsoon (WAM) have generally poor representation of the rainfall distribution and monsoon circulation because key processes, such as clouds and convection, are poorly characterized. The vertical distribution of cloud and precipitation during the WAM are evaluated in Met Office Unified Model simulations against CloudSat observations. Simulations were run at 40 and 12 km horizontal grid length using a convection parametrization scheme and at 12, 4, and 1.5 km grid length with the convection scheme effectively switched off, to study the impact of model resolution and convection parametrization scheme on the organisation of tropical convection. Radar reflectivity is forward‐modelled from the model cloud fields using the CloudSat simulator to present a like‐with‐like comparison with the CloudSat radar observations. The representation of cloud and precipitation at 12 km horizontal grid length improves dramatically when the convection parametrization is switched off, primarily because of a reduction in daytime (moist) convection. Further improvement is obtained when reducing model grid length to 4 or 1.5 km, especially in the representation of thin anvil and mid‐level cloud, but three issues remain in all model configurations. Firstly, all simulations underestimate the fraction of anvils with cloud‐top height above 12 km, which can be attributed to too low ice water contents in the model compared to satellite retrievals. Secondly, the model consistently detrains mid‐level cloud too close to the freezing level, compared to higher altitudes in CloudSat observations. Finally, there is too much low‐level cloud cover in all simulations and this bias was not improved when adjusting the rainfall parameters in the microphysics scheme. To improve model simulations of the WAM, more detailed and insitu observations of the dynamics and microphysics targeting these non‐precipitating cloud types are required.
Highlights
Global climate models (GCMs) show large uncertainties in the radiative impact of clouds (Jakob, 2002) as cloud processes are often poorly represented in parametrization schemes (Randall et al, 2003; Stevens and Bony, 2013)
Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society
A comprehensive overview by Roehrig et al (2013) of the state of the West African Monsoon (WAM) in simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5) highlighted three cloud features that GCMs struggle with: the vertical extent of deep convection, the amount and occurrence of mid-level cloud over the Sahara, and the depth and occurrence of stratus over the Gulf of Guinea. We focus on these cloud features to analyse the vertical structure of clouds and precipitation over West Africa in the Met Office Unified Model (MetUM) using a set of simulations which were run as part of the Cascade project (Pearson et al, 2010)
Summary
Global climate models (GCMs) show large uncertainties in the radiative impact of clouds (Jakob, 2002) as cloud processes are often poorly represented in parametrization schemes (Randall et al, 2003; Stevens and Bony, 2013). Pearson et al (2010, 2014) evaluated the Cascade simulations at 12, 4, and 1.5 km horizontal grid length against observations from GERB (Geostationary Earth Radiation Budget), analysing the diurnal cycle of the size distribution of clusters of outgoing long-wave radiation (OLR) Their results showed that, when using a convection parametrization scheme, the cloud and precipitation occurrences peaked in the early afternoon at all cluster-size scales, whereas GERB observations indicated a gradual shift from smaller OLR clusters in the early afternoon which lasted until the evening, towards larger clusters peaking in the evening and lasting until the early morning. The aims of this article are to evaluate the representation of WAM vertical cloud structure in the suite of Cascade simulations against CloudSat observations and thereby to infer the differing roles of model resolution and convection parametrization in the model errors. The vertical distribution of reflectivity and ice water content is discussed in section 5, including an additional sensitivity study which focuses on precipitating low-level cloud, followed by conclusions and outlook for further research on clouds in the WAM
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