Observation of Water Vapor Greenhouse Absorption over the Gulf of Mexico Using Aircraft and Satellite Data

Through its interaction with radiation, water vapor provides an important link between the ocean and atmosphere. One way this occurs is through the greenhouse effect; observations of water vapor greenhouse absorption in the Gulf of Mexico during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment during July 2002 are reported here. The quantity dG(a)/dT(s) is the change in the amount of upwelling infrared flux absorbed by water vapor as the sea surface temperature increases, and therefore parameterizes the strength of the evaporative feedback between the ocean and atmosphere. Using hemispherical infrared broadband (IRBR) and narrow field of view (NFOV) radiometers aboard a NASA ER-2 aircraft, dG(a)/dT(s) was measured during flights on 9 and 26 July marked by large-scale convective and quiescent conditions, respectively. Using the NFOV over the wavelength range 4-40 mum, dG(a)/dT(s) = 13.4 +/- 1.0 W m(-2) K-1 on 9 July, while on 26 July dG(a)/dT(s) = 9.7 +/- 0.3 W m(-2) K-1. The NFOV measurement of dG(a)/dT(s) in the 8-12-mum wavelength range yielded values of similar to2.5 W m(-2) K-1 for both days, indicating that most of the change in greenhouse absorption with increasing ocean temperature occurs in the rotational and vibrational spectral regions of water vapor. The IRBR measurements yielded higher values of dG(a)/dT(s) on both days, but were likely affected by cold clouds in the hemispherical radiometer field of view. These results support the link between greenhouse efficiency, mid- to upper-tropospheric water vapor concentration, and convection.