Institute of Atmospheric and Climate Sciences, CNR, Bologna - Italy
Cirrus clouds often form on top of intense convective storms and affect the radiation field at the top of the atmosphere. Different mechanisms are most likely responsible for the formation of such clouds and investigations on their origin is necessary using satellite observations and radiative transfer modeling. In fact, in situ measurements are generally not available for these extreme scenarios. Plumes of ice crystals produce a significant increase of cloud-top reflectivity in channel 3 (3.55-3.93 µm) of the Advanced Very High Resolution Radiometer (AVHRR). Levizzani and Setvák (1996) suggested the plumes being composed of small ice crystals whose size is of the order of the channel wavelength. Cirrus features on top of deep convection were detected by analysing multispectral, high-resolution imagery from the AVHRR on board NOAA11.
Reflectivity measurements over a severe storm over Spain are compared with radiative transfer computations. Reflectances and brightness temperatures are computed through the 1-D, plane-parallel radiative transfer model Streamer (Key and Schweiger, 1998). The hydrometeor vertical distribution is assumed that of a cumulonimbus tower composed of a water droplet layer underlying an ice crystal layer. A cirrus deck development is allowed at the top, consisting of ice crystals of various habits. Comparisons between observed and simulated data aim at determining the microphysical characteristics of the plume.
Key, J., and A. J. Schweiger, 1998:
Tools for atmospheric radiative transfer: Streamer and Fluxnet. Computers & Geosciences, 24, 443-451.
Levizzani, V., and M. Setvák, 1996:
Multispectral, high resolution satellite observations of plumes on top of deep convective storms.
J. Atmos. Sci., 53, 361-369.
Melani, S., E. Cattani, V. Levizzani, M. Cervino, F. Torricella, T. Rother, M. Hess, and K. Schmidt, 2000:
Simulations of ice crystal optical properties and cloud top radiative structure of deep convective
storms in the MSG SEVIRI VIS and IR channels. Proc. 2000 EUMETSAT Meteorological Satellite Data
Users' Conf., EUMETSAT, Bologna, 29 May - 2 June, 291-298.