RETRIEVAL OF AGEOSTROPHIC WIND FROM A RADIOSOUNDING NETWORK AND A SINGLE ST RADAR

Previous studies have shown that ageostrophic circulations play an imp ortant role in frontogenesis phenomena. They result from the maintenan ce of the balanced state of the atmosphere which tends to be broken do wn by the large-scale forcing of the atmosphere. Their existence expla ins how a frontal discontinuity can be produced in a few days. From an experimental and forecasting point of view it would be interesting to obtain these circulations in real time. Unfortunately, the ageostroph ic wind is not a measurable quantity and its retrieval from experiment al data is difficult, particularly at the mesoscale, since instrumenta l noise leads to a need for significant data correction. In this conte xt, this paper provides answers to some of the main problems related t o the retrieval of ageostrophic winds in frontal systems. It focuses o n the feasibility of retrieval while retaining, at small enough scale, details of physical interest. The framework of this study is the futu re implementation of regular meteorological wind-profiler networks wit h a sampling time of one hour. This paper proposes to overcome numerou s practical problems using an approach based on the variational Analys e du Vent AGeostrophique (AVAG) analysis. Tests of such an approach ar e done on real data extracted from the Mesoscale Frontal Dynamic Proje ct/FRONTS 87 data base for a typical two-dimensional frontal case (the IOP 7 case). They confirm that the AVAG analysis is able to provide a non-noisy and consistent ageostrophic circulation. Comparisons with c lassical methods demonstrate the robustness of AVAG and its capability to obtain physically meaningful ageostrophic wind fields. This study also estimates the best and most suitable experimental sampling for pr actical applications. Sensitivity tests on the quality of the retrieve d wind field with respect to (i) the sampling time used, (ii) the avai lability of additional thermodynamic data and (iii) the geometrical pa ttern of the network are presented. These tests provide evidence of th e importance of accounting for thermodynamical- and dynamical-field al ong-front variations.