Validation of Tropospheric Water Vapor as Measured by the 183-GHz HAMSTRAD Radiometer Over the Pyrenees Mountains, France

The H₂O Antarctica Microwave Stratospheric and Tropospheric Radiometers (HAMSTRAD) 183-GHz radiometer has been developed to measure vertical profiles of tropospheric water vapor above Dome C (Concordia station), Antarctica ( 75?06´S, 123?21´E, 3233 m asml), which is an extremely cold and dry environment, over decades. Prior to its installation at Dome C in January 2009, the instrument was deployed at the Pic du Midi (PdM) station ( 42?56´N, 0?08´E, 2877 m asml) in the Pyrenees Mountains, France, over the period covering February-June 2008. Vertical profiles of absolute humidity and integrated water vapor (IWV) as measured by HAMSTRAD were compared with measurements from radiosondes launched in three different sites: Lannemezan (43?07´N, 0?23´E, 610 m asml), France (~30 km northeast from PdM), Bordeaux-Me?rignac Airport (44?49´N, 0?42´W, 50 m asml), France ( ~ 220 km northwest from PdM), and Zaragoza (41?39´N, 0?53´W, 263 m asml), Spain ( ~170 km southwest from PdM). The validation process also used the vertical profiles of tropospheric H₂O as measured by the nadir-viewing infrared atmospheric sounding interferometer (IASI) instrument aboard the MetOp-A space platform. The temporal evolution of the HAMSTRAD H₂O measurements above the PdM station is very consistent with IASI, sonde, and in situ measurements, tracking the same atmosphere from a dry period in February to a wet period in June. HAMSTRAD showed unrealistic values in periods of well-established snow tempest. While the sensitivity of the HAMSTRAD measurements tends to be degraded 6 km above the altitude of the instrument, namely, above 8877 m asml, the HAMSTRAD measurements seem reasonable at the uppermost retrieval level (namely, 10 km, 12 877 m asml). In May, the wet periods are systematically showing a good agreement between sonde and HAMSTRAD IWV fields and H₂O below 6777 m asml but a dry bias of IASI by more than 4-kg m^-2 IWV, where- - as outside of these periods, the three data sets behave consistently. Since the best results (mean, standard deviation, bias, and correlation) are obtained when the HAMSTRAD radiometer operates in the very dry conditions of February, namely, in dryness conditions comparable to Dome C summertime values, we are very confident in the optimal use of the instrument when deployed in Antarctica.