Dry Bias in Vaisala RS90 Radiosonde Humidity Profiles over Antarctica

Abstract

Middle to upper tropospheric humidity plays a large role in determining terrestrial outgoing longwave radiation. Much work has gone into improving the accuracy of humidity measurements made by radiosondes. Some radiosonde humidity sensors experience a dry bias caused by solar heating. During the austral summers of 2002/03 and 2003/04 at Dome C, Antarctica, Vaisala RS90 radiosondes were launched in clear skies at solar zenith angles (SZAs) near 83°and 62°. As part of this field experiment, the Polar Atmospheric Emitted Radiance Interferometer (PAERI) measured downwelling spectral infrared radiance. The radiosonde humidity profiles are used in the simulation of the downwelling radiances. The radiosonde dry bias is then determined by scaling the humidity profile with a height-independent factor to obtain the best agreement between the measured and simulated radiances in microwindows between strong water vapor lines from 530 to 560 cm Ϫ1 and near line centers from 1100 to 1300 cm Ϫ1. The dry biases, as relative errors in relative humidity, are 8% Ϯ 5% (microwindows; 1) and 9% Ϯ 3% (line centers) for SZAs near 83°; they are 20% Ϯ 6% and 24% Ϯ 5% for SZAs near 62°. Assuming solar heating is minimal at SZAs near 83°, the authors remove errors that are unrelated to solar heating and find the solar-radiation dry bias of 9 RS90 radiosondes at SZAs near 62°to be 12% Ϯ 6% (microwindows) and 15% Ϯ 5% (line centers). Systematic errors in the correction are estimated to be 3% and 2% for microwindows and line centers, respectively. These corrections apply to atmospheric pressures between 650 and 200 mb.

Ground-based lidar measurements from Ny-Ålesund during ASTAR 2007

“…From the comparison of MPL data with radiosondes, we found some cases with relative humidity down to 60% sufficient for cloud existence especially in altitudes above 4 km. Hence a dry bias of the RS-92 in extreme environments must be considered, similar to the findings of Rowe et al (2008).…”

Comparison of Vaisala radiosondes RS41 and RS92 at the ARM Southern Great Plains site

“…Solar heating of the radiosonde sensors has been known to have an impact on radiosonde measurements (e.g., Vömel et al, 2007;Rowe et al, 2008;Milosevich et al, 2009;Immler et al, 2010;Wang et al, 2013;Dirksen et al, 2014). In order to investigate solar heating impacts on the differences between RS92 and RS41 radiosondes, we have computed the differences separately for daytime and nighttime soundings (as indicated in Table 5).…”

“…These measurements provide critical input to weather forecasting and climate models, quantification of atmospheric thermodynamic stability, input to remote-sensing retrievals, and important constraints for atmospheric process studies. The long history of radiosonde observations includes many changes in instrumentation, practices, processing, and other issues (e.g., Elliot and Gaffen, 1991;Gaffen, 1993;Elliot et al, 1998;Wang et al, 2003;Haimberger, 2007;Vömel et al, 2007;Haimberger et al, 2008;Rowe et al, 2008;Sherwood et al, 2008;McCarthy et al, 2009;Milosevich et al, 2004Milosevich et al, , 2009Seidel et al, 2009;Dai et al,2010;Immler et al, 2010;Thorne et al, 2011;Moradi et al, 2013;Wang et al, 2013;Dirksen et al, 2014;Yu et al, 2015;Bodeker et al, 2016).…”

Thin ice clouds in the Arctic: cloud optical depth and particle size retrieved from ground-based thermal infrared radiometry

“…To avoid the issue of interpolating radiosondes over extensively long periods of time, the cases were selected to be as close as possible to radiosonde launch times. Radiosonde humidity sensors are known to be subject to dry bias, especially in dry conditions and could yield relative humidity underestimates of 10 % (Rowe et al, 2008). The six channels are, however, far less sensitive to WVC than to COD (see Fig.…”