Air-Sea Interaction during cold-air outbreaks over the Bering and Okhotsk Seas using Advanced Microwave Sounding Unit-A and others active and passive microwave measurements

Improvement knowledge on the exchange of heat, moisture and momentum between ocean and atmosphere is a priority in the study of climate change. Currently there is increasing interest in high latitude regions, where climate change appears most clearly. Estimates of the ocean-atmosphere fluxes in the Arctic and northern seas are however extremely limited. Intensive sea weather systems (polar lows, cold air outbreaks, etc.) and accompaniing them gale winds, extremely low temperatures and the presence of sea ice make the heat fluxes estimations complicated. Differences in mean sensible heat fluxes in the high latitude regions provided by J-OFURO v2, OAFlux, HOAPS-3 and others products are significant, reaching sometimes 50-100% of flux values. The aerodynamic method is widely used to determine surface turbulent heat fluxes. The method is based on bulk formulae which were derived from the Monin-Obukhov similarity approach. These formulae relate turbulent fluxes to mean values of sea surface temperature t, surface wind speed W, and near surface air temperature ta and humidity qa. More precise estimates of fluxes significantly improve results of simulations of climate variability and development of marine weather systems. This work investigates the potential of the Advanced Microwave Sounding Unit-A (AMSU-A), operated on NOAA satellites for determination of the turbulent heat fluxes over the Bering and Okhotsk Seas. Special focus is given to the relationship of brightness temperatures Tb at frequencies f = 52.8 and 53.6 GHz in the oxygen band channels with near surface temperature and humidity during cold air outbreaks regularly observed over the high latitude regions. AMSU-A is a 15-channel microwave sounder that scans across the track in the range of viewing angles ± 48 °. Computations of Tb(52.8) and Tb(53.6) were carried out with the use of Radiative Transfer Model (RTM) and radiosonde data collected at St. Paul Island (57.18°N, 170.3°W), Shemya Island (52.71°N, 174.1°W) and Cold Bay (55.20°N, 162.71°E) stations in the Bering Sea. The sensitivity of the brightness temperatures to variations of the surface wind speed W, integrated water vapor content V, cloud liquid water content Q, the vertical profiles of temperature and humidity in the lower troposphere (up to 3 km) was estimated. Tb(52.8) values are the most sensitive to temperature stratification of the lower troposphere. Due to turbulent mixing in the atmospheric boundary layer there is a close relationship of Tb(52.8) with near surface temperature and humidity used in the bulk formulae. The RTM computations show that the influence of Q on Tb(52.8) is significant: the appearance of clouds with Q = 0.1 kg/m2, that is typical for cold air outbreaks leads to the Tb(52.8) increase on more than 1 K. Correlation between Tb(52.8) and ta increases after correction of Tb values. Five cases of the most extensive cold air outbreaks over the Bering Sea were selected from the laboratory archive. Collocated data set of satellite and in situ measurements was formed. Data set included the NOAA – 15, 16, 18, and MetOp-A AMSU-A brightness temperatures at 52.8 and 53.6 GHz; Aqua AMSR-E- and F–13-17 SSM/I-derived values of surface wind speed, integrated water vapor content, and cloud liquid water content and the meteorological characteristics from moored buoys located in the Bering Sea and Gulf of Alaska. Regional retrieving algorithm of near surface air temperature was developed using data set and results of RTM simulations. Algorithm application is demonstrated for two cases of cold-air outbreaks over Bering and Okhotsk Seas.