Aircraft Observations of Air-Mass Modification Upstream of the Sea of Japan during Cold-Air Outbreaks

Abstract

Russian research aircraft observed the atmospheric boundary layer in 2001 during weak (29 Jan.), intense (2 Feb.) and very intense (3 Feb.) cold air outbreaks. These observations occurred during a field experiment of Winter Meso-scale convective systems Observations, over the Sea of Japan in 2001 (WMO-0l). Conditional sampling techniques were used to elucidate air-mass modification processes upstream of the Sea of Japan during cold-air outbreaks. Buoyancy fluxes in the lower boundary layer near the sea surface differed significantly among the three cases. TKE budgets suggest that the differences were characterized mainly by differences in buoyancy production. However, common properties in heat transfer did exist, namely intensity (3.0) and fractional coverage (28%) of rising thermals (w⁺T_v⁺). Scale analysis showed that heating by convective-scale motions dominated in the lower boundary layer. Heating by meso-γ scale motions became more important in the upper boundary layer. Near the cloud top, on the other hand, contributions by cold downdrafts (w⁻T_v⁻) to the total buoyancy flux (w´T_v&actute;) dominated. Thecloud-top w´T_v´ was about half of that near the sea surface, suggesting that cooling processes near the cloud top are also important in TKE production. Comparisons of fractional spatial coverage of w⁺T_v⁺ with thermal motions over other warm oceans show good agreement with the AMTEX and GALE cases. However, flux intensities in the present case, and in the GALE case, are stronger than in the AMTEX case. Impacts on ocean convection by the upstream, large surface heat fluxes are discussed.