Air–Sea Interaction during an Extreme Cold Air Outbreak from the Eastern Coast of the United States

Abstract

The area east of the coast of North Carolina chosen for enhanced observation during the Genesis of Atlantic Lows Experiment (GALE) has one of the highest average wintertime energy transfers from ocean to atmosphere on earth. A substantial part of this transfer occurs in the aftermath of winter storms as cold, dry air flows off of the continent over the warm Gulf Stream. We report on an aircraft investigation of boundary layer mean and turbulent structure and evaluate the Lagrangian budgets of temperature and moisture in the subcloud layer following a streamline during an extreme cold air outbreak. The maximum sea-air temperature difference was 23 K. Two aircraft were used: the NCAR Electra, which measured turbulent fluxes and investigated subcloud layer conditions, while the NASA Electra, using a radar, measured the height of cloud tops. A stratocumulus overcast was found from about 60 km offshore to the Gulf Stream core with cloud top rising downstream. East of the Gulf Stream cumulus congestus and snow showers were observed. Cloud base decreased downstream and numerous steam plumes filled the subcloud layer. Temperatures were corrected for the substantial effects of diurnal variation in order to isolate air-sea interaction processes. Cross sections show most warning, and moistening of the subcloud layer occurred before the Gulf Stream core. Windspeeds increased downstream and maxima were observed near cloud top (inversion) and in the subcloud layer. Lagrangian budgets showed most warming, and moistening of the layer between 70 m and about 100 m below mean cloud base was due to turbulent flux divergence. At 70 m a maximum total heat flux of 1174 W m?2 (364 W m?2 sensible, 799 W m?2 latent) was observed over the Gulf Stream core. In the temperature budget, the radiative flux divergence term was relatively small and a residual condensation warming was inferred. Complementary drying was estimated from the residual of the moisture budget. The budgets were combined using a graphical technique on a conserved parameter (? ? q) diagram and were extrapolated into the surface layer with reasonable results. This technique was also applied to the entire subcloud layer with results that implied that east of the Gulf Stream entrainment fluxes at cloud base and evaporation of falling precipitation may act to cool and dry the subcloud layer, reducing the effects of flux convergence (which would warm and moisten). The Lagrangian warming and moistening rates we estimated indicate that cold, dry continental air can be transformed to air which can participate in deep convection (which appears to be an integral part of rapid cyclogenesis) in about 20?30 hours.