The representation of the West African monsoon vertical cloud structure in the Met Office Unified Model: an evaluation with CloudSat

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...

Full description

Saved in:
Bibliographic Details
Published in:Quarterly journal of the Royal Meteorological Society 2015-10, Vol.141 (693), p.3312-3324
Main Authors: Stein, Thorwald H. M., Parker, Douglas J., Hogan, Robin J., Birch, Cathryn E., Holloway, Christopher E., Lister, Grenville M. S., Marsham, John H., Woolnough, Steven J.
Format: Article
Language:English
Subjects:
African monsoon
Atmospheric precipitations
Cascade
Climate models
Cloud cover
Cloud structure
Cloud types
Clouds
CloudSat
Convection
Distribution
Freezing
Freezing level
Length
Meteorology
Microphysics
model evaluation
Monsoon circulation
Monsoon clouds
Monsoon rainfall
Monsoons
Precipitation
Radar
Radar reflectivity
Rain
Rainfall
Rainfall distribution
Reflectance
Simulation
Simulators
Tropical climate
Tropical convection
Vertical distribution
West African monsoon
Wind
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary: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.
ISSN:0035-9009
1477-870X
DOI:10.1002/qj.2614