The 12/13 January 1988 Narrow Cold-Frontal Rainband Observed during MFDP/FRONTS 87. Part II: Microphysics

Abstract

The microphysics of a narrow cold-frontal rainband (NCFR) observed during the MFDP/FRONTS87 experiment is investigated by using a microphysical retrieval model. The equations of evolution of the water substance and of the temperature are solved using a wind field prescribed from dual-Doppler radar observations. Different runs of the model were performed to investigate the role of various microphysical processes. All of them use a two-dimensional version of the model and give a solution for the steady state corresponding to the input wind field. The validity of this approach was checked a posteriori by comparing the results obtained from vertical cross sections at two different locations and two different times. In each case, the consistency of the results was controlled through comparisons with in situ measurements (aircraft, ground stations, and radiosondes) and radar reflectivity observations. The main result obtained from this study was that the precipitation associated with the NCFR was mostly composed of graupel particles, essentially formed by riming. Rain was produced by accretion of cloud water in the condensation zone and by melting of graupel. The choice of the type of ice-precipitating particles introduced in the model appeared very important. Only rimed particles (graupel) could reproduce observed precipitation. The precipitation efficiency was rather high (73%). The zone of light precipitation in which the NCFR was embedded seemed to play no-seeder role in the growth of precipitation in the NCFR, probably due to the overturning airflow located in the prefrontal zone. Another important result concerns the role of the microphysical processes on the thermodynamics. The temperature drop observed at low levels just behind the frontal discontinuity could be explained at the time of the observations by two cooling effects of equal importance: the melting of graupel and the evaporation of precipitation.