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Tropical clouds

Results from MT-Africa campaign (August 2010)

«Under construction»

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 ( Vt ) deduced from Vd 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 Vd 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, NO* (intercept parameter) and Dm (equivalent weighted diameter) and the Mass-size prefactor. Once the couple NO*-Dm retrieved, we can easily derive IWC, Nt and so on.

The cloud properties retrievals are limited to the area were both Vt 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, Nt , Dm and NO* (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. Dm agrees with the in-situ but with discrepancies in the region before 16.88 UTC. NO* and Nt , 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, NO* and Dm depend on the Mass size relationship, Nt is a direct measurement and does not require any assumption.