A cloud filtering method for microwave upper tropospheric humidity measurements

Abstract

Abstract. The paper presents a cloud filtering method for upper tropospheric humidity (UTH) measurements at 183.31±1.00 GHz. The method uses two criteria: a viewing angle dependent threshold on the brightness temperature at 183.31±1.00 GHz, and a threshold on the brightness temperature difference between another channel and 183.31±1.00 GHz. Two different alternatives, using 183.31±3.00 GHz or 183.31±7.00 GHz as the other channel, are studied. The robustness of this cloud filtering method is demonstrated by a mid-latitudes winter case study. The paper then studies different biases on UTH climatologies. Clouds are associated with high humidity, therefore the possible dry bias introduced by cloud filtering is discussed and compared to the wet biases introduced by the clouds radiative effect if no filtering is done. This is done by means of a case study, and by means of a stochastic cloud database with representative statistics for midlatitude conditions. Both studied filter alternatives perform nearly equally well, but the alternative using 183.31±3.00 GHz as other channel is preferable, because that channel is less likely to see the Earth's surface than the one at 183.31±7.00 GHz. The consistent result of all case studies and for both filter alternatives is that both cloud wet bias and cloud filtering dry bias are modest for microwave data. The recommended strategy is to use the cloud filtered data as an estimate for the true all-sky UTH value, but retain the unfiltered data to have an estimate of the cloud induced uncertainty. The focus of the paper is on midlatitude data, since atmospheric data to test the filter for that case were readily available. The filter is expected to be applicable also to subtropical and tropical data, but should be further validated with case studies similar to the one presented here for those cases.

A review of sources of systematic errors and uncertainties in observations and simulations at 183 GHz

“…For instance, ATMS offers a large suite of channels that can complement each other to separate clear and cloudy scenes (Bormann et al, 2013), while SAPHIR provides only channels in the 183.31 GHz line, giving fewer possibilities for cloud detection (Chambon et al, 2015). Several filtering techniques using only 183 GHz channels exist, relying on the strong reduction in BT induced by scattering and/or absorption, which can reach up to 20 K in regions of intense convection (Greenwald and Christopher, 2002;Hong et al, 2005;Buehler et al, 2007). However, it is difficult to screen all clouds, so residual signals due to clouds must be expected to be present.…”

Evaluating the Diurnal Cycle of Upper Tropospheric Humidity in Two Different Climate Models Using Satellite Observations

“…The channel 1 measurements from AMSU-B and MHS sensors, which monitor UTH variations, were filtered for any impact of cloud as described in Buehler et al [32]. This cloud filter is based on the difference between channel 3 and channel 1 brightness temperatures and a threshold channel 1 value based on the viewing angle of the satellite, which is provided in Buehler et al [32].…”

“…This cloud filter is based on the difference between channel 3 and channel 1 brightness temperatures and a threshold channel 1 value based on the viewing angle of the satellite, which is provided in Buehler et al [32]. The cloud filtered channel 1 brightness temperatures were converted into UTH by the method described in Buehler and John [33] using the equation,…”

High Performance Software Framework for the Calculation of Satellite-to-Satellite Data Matchups (MMS version 1.2)

“…maximal Viewing Zenith Angle (VZA), maximal VZA difference)  BuehlerCloud: a cloud screening for microwave sensors based on (Buehler et al, 2007)  PixelValue: screening that allows to evaluate mathematical expressions on matchup pixels (e.g. reflec_nadir_0870 > 10 7.5 && reflec_fward_0670 > 4.3).…”