Structure of marine atmospheric boundary layer under contrasting conditions of tropospheric dynamics, convection and cloudiness
Abstract
Atmospheric boundary layer (ABL) processes play a pivotal role in the exchange of energy and minor constituents between the surface and free-troposphere. Temporal variations of the marine ABL (MABL) are mainly controlled by large-scale circulation and surface conditions. Diurnal variation of mixing height over the oceanic regions is significantly smaller than that over the continental regions. This study investigates the MABL characteristics under contrasting conditions of large-scale circulation, sea surface temperature (SST), convection and cloudiness based on the altitude profiles of atmospheric thermo-dynamic variables observed using balloon-borne GPS Radiosondes launched during three ship based campaigns in the tropical Indian Ocean, the Arabian Sea and the Bay of Bengal. These observations were carried out during July-November period and the ship tracks transacted through the ITCZ, regions of deep convection and the descending limb of Hadley cell. The prevailing cloud conditions corresponding to the time of radiosonde observations were obtained using collocated data from geostationary satellite Kalpana-1-VHRR, which were used to classify the data into highly cloudy and clear sky categories. The mixing height at the descending limb of the Hadley cell typically extends up to an altitude of ∼1200 m, which is capped by a region of thin cloud that is sandwiched between the top of MABL and the strong trade wind inversion at ∼2 km altitude. The moist static energy in the surface layer at the deep convective regions is significantly larger than that at the descending limb of Hadley cell. Notwithstanding this, the mixing height in the deep convective cases is rather shallow, though significantly humid. Remarkably, the MABL over these deep convective regions are generally capped by a relatively cloud-free altitude band marked by a distinct inversion in virtual potential temperature, which is followed by a deep convective cloud layer in the altitude band of ∼2-13 km. In general, the mixing height under highly cloudy conditions is lower than that during the clear-sky periods. Histograms of the mixing height peak at ∼400 m during highly cloudy conditions and at ∼700 m during clear-sky conditions.
- Publication:
-
40th COSPAR Scientific Assembly
- Pub Date:
- 2014
- Bibcode:
- 2014cosp...40E2870S