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: The difference between the brightness temperatures at 183.31±7.00 and 183.31±1.00 GHz, and a threshold for the brightness temperature at 183.31±1.00 GHz. The robustness of this cloud filter 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 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. The consistent result is that both cloud wet bias (0.8% RH) and cloud filtering dry bias (–2.4% RH) are modest for microwave data, where the numbers given are for the stochastic cloud dataset. This indicates that for microwave data cloud-filtered UTH and unfiltered UTH can be taken as error bounds for errors due to clouds. This is not possible for the more traditional infrared data, since the radiative effect of clouds is much stronger there. 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.

An upper tropospheric humidity data set from operational satellite microwave data

“…Clouds can affect measurements if they are high enough to be seen by Channel 18 and if they contain a sufficient amount of ice. The surface can affect measurements if the atmosphere is so dry that Channel 18 sees the surface, which happens at total column water vapor below 3 kg/m 2 [ Buehler et al , 2007]. This usually only occurs at high latitudes or in high mountain regions.…”

“…The problem of clear‐sky bias in UTH data sets can be overcome, or at least significantly reduced, by using microwave measurements [ Buehler et al , 2007]. Continuous UTH measurements from microwave instruments on board satellites have been available since 1994.…”

Evaluating Instrumental Inhomogeneities in Global Radiosonde Upper Tropospheric Humidity Data Using Microwave Satellite Data

“…Data not fulfilling either condition are considered cloud contaminated. T thr for different viewing angles is given in [36].…”

“…Microwave radiances are less sensitive to clouds than infrared radiances [33], but sufficiently optically thick ice clouds can affect microwave data [34]. A cloud filter explained in [35] and [36] was used to exclude cloud-affected pixels from the satellite data. The cloud filter works as follows.…”

“…Therefore, we limited our simulations to those profiles where the total precipitable water vapor was above 5 kg/m 2 . In this case, the simulated radiance is not affected by the surface emissivity anymore even in very cold conditions [36].…”