Tropical clouds

RASTA measurements: Top figure illustrates the nadir and zenith radar reflectivity at 95GHz corresponding to a tropical anvil (cirrus and stratiform precipitations regions) over West Africa (Niger). Middle panel represents nadir and zenith Doppler velocity (note that the Doppler velocity includes both air motion and cloud/rain velocity). The lower panel is the vertical terminal fall velocity ( V_t ) deduced from V_d using the Dop-Z-H technique (Protat and Williams 2011).

Vertical Doppler Velocity is obtained using the WIRE method (Papazzoni 2010). During this campaign RASTA was flying in a 5 beams configuration (3 beams downwards, 2 upwards), which explains why V_d cannot be retrieved at the top of the cloud.

We are using an evolution of the RadOn method (Delanoë et al. 2007) combining RASTA reflectivity and terminal fall velocity, which allows one to retrieve two moments of the particle size distribution, N_O* (intercept parameter) and D_m (equivalent weighted diameter) and the Mass-size prefactor. Once the couple N_O*-D_m retrieved, we can easily derive IWC, N_t and so on.

The cloud properties retrievals are limited to the area were both V_t and Z are available. Total number concentration decreases as we approach the base of the cloud and shows very high concentration at the top of the cloud. re values lie between a few microns and 90 microns. IWC reaches very large values for large reflectivity values, i.e. up to 10 gm-3. A such value of IWC is not surprising in continental African anvils. The most striking result is the fact that the particle mass increases near the melting layer without necessary involving very large IWC values (at least not before 16.94 UTC). Large IWC can also be associated to large particles falling slowly (less than 0.5 ms^-1) due to their density.

The Falcon 20 aircraft was also carrying in-situ measurements. Ice cloud particles were collected by 2DS, PIP and CIP probes. Lamp provided us with particle size distribution (PSD) derived by combining these probes. The PSD is used to compute in-situ IWC, N_t , D_m and N_O* (Mass size is obtained by minimising the difference between valid reflectivity measurement at closest distance to the plane and the one derived from the PSD).

Note that RadOn also retrieves the Mass-Size relationship in an independent way.

It is obvious that RadOn IWC at the closest distance to the plane (above and below) is in a very good agreement with the in-situ values. D_m agrees with the in-situ but with discrepancies in the region before 16.88 UTC. N_O* and N_t , at least for the values below the aircraft, are also in good agreement with in-situ data. It is important to note that, although IWC, N_O* and D_m depend on the Mass size relationship, N_t is a direct measurement and does not require any assumption.