Sodar observations of the atmospheric boundary layer over the ocean during ASTEX-91

Abstract

A complex marine experiment was conducted in autumn 1991 on the research vessel Dmitry Mendelev in association with the Atlantic Stratocumulus Transition Experiment (ASTEX). A three-axis Doppler sodar designed at the Institute of Atmospheric Physics, Moscow, was used in this experiment. Total observation time was about 770 hours from 6 October to 23 November. Besides facsimile records illustrating spatial and temporal structure of the turbulence distribution in the atmospheric boundary layer (ABL), routine quantitative measurements of profiles of wind and echo-signal strength were taken. Some main characteristics of the ABL behavior over the ocean were revealed through an analysis of these data as well as the results of other kinds of measurements. An important peculiarity of the ABL observed between the Canary Islands and the Azores was the presence of diurnal variation of convective turbulence strength having a maximum between 04:00 and 07:00 LT. A similar diurnal variation was observed for low-level cloud cover. Occurrence of various types of thermal stratification and their diurnal variation were obtained. Comparison of elevated stable layers and low-level cumulus showed that the lower boundary of clouds correlates well with the height of the bottom of elevated inversion layers (at heights of 200–600 m). Canary and Cabo Verde observations showed that islands strongly affect the ABL structure. The strong effect of a surface water temperature gradient on the ABL stability was observed when crossing the Canary, Azores, and Labrador currents and the Gulf Stream.