Development and application of an algorithm for the estimation of mixing height with the use of a SODAR-RASS remote sensing system

Abstract

This work introduces a newly developed algorithm for the estimation of mixing height with the use of a SODAR-RASS system (sound detection and ranging; radio acoustic sounding system) and in-situ instrumentation. The algorithm utilizes the temperature, acoustic back-scattered signal and the Richardson number profiles, estimated by the remote sensing system, and incorporates information on the basic meteorological parameters related to the surface flow given by in-situ measurements. It was applied to a two-year period (2007–2008) data set of a SODAR-RASS installed in the Messogia Plain, a coastal region of the eastern Mediterranean Sea. The profiles of the parameters used by the algorithm showed good agreement among the different criteria used for various atmospheric conditions. The mixing height estimates are analysed with respect to the cold and warm seasons of the two-year period, and the frequency distributions of the estimated 30-min mixing height values from the entire time series show clear peaks, associated with certain synoptic or meso-scale phenomena observed over the area. The mean diurnal cycle of the mixing height during the warm period exhibited higher daytime values compared with the corresponding values during the cold period. The dominating synoptic and meso-scale phenomena over the area were analysed, and selected plots of mixing height estimates together with time–height distributions of SODAR-RASS parameters are given. Mixing height values during the evolution of low-level jets show weak diurnal variability, following the lift or the descent of the core of the jet, and in most cases mixing height estimates nearly coincided with the height of the jet's core, while night-time values were kept high due to the increased mixing. For the case of frontal passages, the mixing height follows the intense wind shift line, with values close to the temperature inversion height that separates the two air masses. Under sea–land breeze conditions, mixing height obtains low daytime values due to the elevated inversion that is formed at the upper part of the lower current of the sea breeze cell. The algorithm exhibited unsatisfactory performance during the transition periods (early morning and night-time hours – neutral conditions) of the diurnal cycle.