The 12/13 January 1988 Narrow Cold-Frontal Rainband Observed during MFDP/FRONTS 87. Part I: Kinematics and Thermodynamics

Abstract

The kinematic and thermodynamic structure of a narrow cold-frontal rainband (NCFR) observed during the English?French?German MFDP/FRONTS 87 experiment is presented. Radiosonde data indicated a very weak convective instability below 1500-m altitude and a low-level jet of 30 m s?1 from SSW before the arrival of the front; a cooling of about 1°C associated with an airflow of 13 m s?1 from WSW after its passage. A composite of wind and reflectivity fields from 17 dual-Doppler radar analyses shows high reflectivity values, large convergence, and relatively intense vertical motions associated with this NCFR at the 1000-m altitude. A mean vertical cross section perpendicular to the surface front, derived from three successive high-resolution dual-Doppler scans, is used to examine the general characteristics of the air circulation. A structure apparently similar to that of a density current is observed. Above the 2-km altitude, however, air flowing at a speed faster than the surface front appreciably modified the kinematic structure as compared to classical schemes. The budgets of the associated mass and momentum fluxes show that only 20% of their vertical divergences were due to the alongfront variations. As deduced from the retrieved pressure and temperature fields, the frontal updraft was essentially maintained by the vertical pressure gradient force since buoyancy remained very small. Examination of the different frontogenetic terms indicates that diabatic heating, with a necessary contribution of the convergence term, was the most important one for maintaining the surface temperature gradient. These results are consistent with those previously deduced for other NCFRs, except the values are smaller here due to the less intense features. Analysis of the successive wind and reflectivity fields reveals some three-dimensional and time-dependent features. In particular, the frontal updraft underwent some evolution related to the formation and fall of precipitation. The pressure and temperature perturbations retrieved from these three-dimensional fields are qualitatively similar to the two-dimensional ones. Their larger amplitudes are, however, closer to those observed during the passage of the front.