APPLICATION OF SODAR TO STUDY THE URBAN CLIMATE: A REVIEW Margarita A. Kallistratova * Obukhov Institute of Atmospheric Physics, Moscow, Russia Abstract Merits and demerits of application of remote acoustic sensing to study heat islands in the urban boundary layer (UBL) are discussed. An overview of climatological results obtained in some cities with help of acoustic sounders (sodars) is presented. Outcome given includes data on UBL stratification, mixing height, wind field, turbulence characteristics. The use of the sodar data in air pollution study is exemplified. Peculiarities of the UBL parameters as compared with surrounding countryside, which were investigated by sodar networks, are outlined. Key words: sodar, urban climate, urban boundary layer 1. INTRODUCTION The deep understanding of genesis of heat islands in the urban boundary layer (UBL), and determination of its basic features and its effects on airflow and air quality was achieved with the help of traditional in situ measurements taken from meteorological towers, radiosonde, tethered balloons, and aircraft [1]. Such researches proceed till now [2]. At the same time, the development of ground based remote sensing of the atmosphere has expanded opportunities of UBL studies. Acoustic sounding by sodar is one of the simpler and less expensive means to measure wind velocities profiles and intensity of turbulence up to several hundred meters agl, and to estimate a type of stratification, mixing height and depth of inversion layers as well [3]. Being able to operate continuously under any weather conditions (excluding heavy precipitation and storms) sodars can supply the study of UBL dynamics, and provide a real-time monitoring of wind field affecting the transfer and dispersion of air pollution. Besides, using a set of several sodars one can investigate a spatial inhomogeneity of the urban airshed. The use of sodars in cities began in the 1970s. Among the first works on acoustic sounding of the UBL we shall mention papers of Russel and Uthe [4], Mellling and List [5], Walczewski [6], Singal [7], and Asimakopoulos [8]. The main shortcoming of sodar, which is essential for a heat island investigation, is its incapacity for measurement of air temperature. This restriction is compensated in many respects by its ability to visualize the meso-scale structure of turbulence, and to measure profile of temperature structure parameter, whose both patterns are closely allied to the temperature profile. It was due to sodar that the intricate structure of thermal stratification (which is wide of its idealized form) was sometimes revealed over cities, even those located in a flat terrain. For example, such complex structures are shown in the middle and right parts of Fig. 1, where three fragments of facsimile record of sodar echo-signal obtained over Berlin are given [9]. Fig. 1. Structure of UBL by sodar and radiosonde. Berlin, May 1985 [9]. The thin elevated layer (see right part of Fig. 1) observed under cloudless skies during the whole day hindered to the removal of pollution. Such a long-living structure unlikely can be revealed without sodar, and scarcely can be modeled with help of the similarity theory. Over cities located in a complex terrain the multilayer structures are observed regularly. In Fig. 2 an example of facsimile record, and appropriate wind speed and direction profiles obtained by Doppler sodar over Alma-Ata (Kazakhstan, at the Tien Shan foothills) is given [10]. Wind velocities often are weak there, and wind field at height 100-200 m does not correlate with that in the surface layer. Such conditions lead to accumulation of air pollution in this city. Despite a qualitative character of such information, sodar echograms till now give valuable data for diagnostics of ability of urban airshed to self-purification. For example, analysis of the echogram (along with wind velocity profiles) helped to Neff and King [11] investigate the * Corresponding author address: Margarita A. Kallistratova, Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevskii 3, 119017, Moscow, Russia; e-mail: margo@ifaran.ru