Microphysical characterisation of West African MCS anvils



Deep convection in the Tropics is the source of large tropospheric extended clouds usually called anvils. These anvils may produce precipitation (stratiform region of deep convective storms), and also cirrus shields persisting from several hours to several days. Anvils impact the radiation budget, they induce a storage term in the whole water budget which is still relatively poorly quantified, and dynamical feedbacks may be induced in the upper troposphere. The AMMA field campaign which was held over West Africa during the 2006 wet season provided a unique opportunity to document the microphysics of these anvils from unprecedented airborne observations.

Airborne in situ measurements of ice crystals and observations from a 95 GHz Doppler radar are used to characterize the microphysical properties of tropical anvils. The dataset is binned into stratiform and cirriform regions. Some data in the stratiform regions were likely obtained close enough to convective cores that the particles may have grown primarily within those cores. The data obtained over the continent and over the ocean are also characterized separately. Particle habit and growth processes are inferred from an examination of the collected particle images, from quantitative comparisons of 95 GHz reflectivities calculated from the in situ microphysical observations with the measured radar reflectivities, and from a statistical analysis of the two-dimensional particle images. The predominant precipitation particles above the 0 °C isotherm in the stratiform anvil region are rimed aggregates. These rimed aggregates seem to get less dense and of smaller diameter when moving rearward of the system towards the cirriform region. The retrieved density laws (assumed to be power laws) lie close to the relationship for rimed particles of Locatelli and Hobbs.

Particle size distributions in tropical anvils are also studied. The exponential shape seems to be a good approximation for these particle size distributions overall. The decrease in concentration with diameter is also found to be faster for cirriform regions than for stratiform regions. Normalising these particle size distributions produces a relatively invariant shape (in agreement with earlier studies), with however an increased variability for the smallest and largest values of the normalised diameter.

The characterisation of the bulk microphysical properties using these in situ microphysical observations shows that the ice water content, the effective radius and the reflectivity-weighted fall velocity generally increase with air temperature, in agreement with earlier studies. These parameters are found to be systematically smaller on average in cirriform regions than in stratiform regions, and this is true at all temperatures. These values are then compared with statistical relationships used in cloud-resolving models and general circulation models, since a realistic representation of microphysics in models is very important to understand not only the processes at work, but the dynamical feedbacks and effects on climate. Large differences are found, the current parametrizations being unable to reproduce the large values of the considered microphysical parameters. Copyright © 2010 Royal Meteorological Society