Open AccessObserved relationship between surface specific humidity, integrated water vapor, and longwave downward radiation at different altitudes
Author(s) -
Ruckstuhl Christian,
Philipona Rolf,
Morland June,
Ohmura Atsumu
Publication year - 2007
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2006jd007850
Subject(s) - environmental science , longwave , water vapor , humidity , atmospheric sciences , modtran , atmosphere (unit) , meteorology , relative humidity , altitude (triangle) , outgoing longwave radiation , radiation , radiative transfer , geography , physics , geometry , mathematics , quantum mechanics , convection
Atmospheric water vapor and surface humidity strongly influence the radiation budget at the Earth's surface. Water vapor not only absorbs solar radiation in the atmosphere, but as the most important greenhouse gas it also largely absorbs terrestrial longwave radiation and emits part of it back to the surface. Using surface observations, like longwave downward radiation ( LDR ), surface specific humidity ( q ) and GPS derived integrated water vapor ( IWV ), we investigated the relation between q and IWV and show how water vapor influences LDR . Radiation data from the Alpine Surface Radiation Budget (ASRB) network, surface humidity from MeteoSwiss and GPS IWV from the STARTWAVE database are used in this analysis. Measurements were taken at four different sites in Switzerland at elevations between 388 and 3584 m above sea level and for the period 2001 to 2005. On monthly means the analysis shows a strong linear relation between IWV and q for all‐sky as well as for cloud‐free situations. The slope of the IWV ‐ q linear regression line decreases with increasing altitude of the station. This is explained by the faster decrease of IWV than of q with height. Both q and IWV are strongly related with LDR measured at the Earth's surface. LDR can be parameterized with a power function, depending only on humidity. The estimation of LDR with IWV has an uncertainty of less than 5% on monthly means. At lower altitudes with higher humidity, the sensitivity of LDR to changes in q and IWV is smaller because of saturation of longwave absorption in the atmospheric window.