Retrieval of ageostrophic wind from a radiosounding network and a single ST radar

Abstract

AbstractPrevious studies have shown that ageostrophic circulations play an important role in frontogenesis phenomena. They result from the maintenance of the balanced state of the atmosphere which tends to be broken down by the large‐scale forcing of the atmosphere. Their existence explains 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 ageostrophic wind is not a measurable quantity and its retrieval from experimental data is difficult, particularly at the mesoscale, since instrumental noise leads to a need for significant data correction.In this context, this paper provides answers to some of the main problems related to the retrieval of ageostrophic winds in frontal systems. It focuses on the feasibility of retrieval while retaining, at small enough scale, details of physical interest. The framework of this study is the future implementation of regular meteorological wind‐profiler networks with a sampling time of one hour.This paper proposes to overcome numerous practical problems using an approach based on the variational Analyse du Vent AGéostrophique (AVAG) analysis. Tests of such an approach are done on real data extracted from the Mesoscale Frontal Dynamic Project/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 classical 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 practical applications. Sensitivity tests on the quality of the retrieved wind field with respect to (i) the sampling time used, (ii) the availability of additional thermodynamic data and (iii) the geometrical pattern of the network are presented. These tests provide evidence of the importance of accounting for thermodynamical‐ and dynamical‐field along‐front variations.