A cloud filtering method for microwave upper tropospheric humidity measurements

[摘要] The paper presents a cloud filtering method for upper tropospherichumidity (UTH) measurements at 183.31±1.00 GHz. The method usestwo criteria: a viewing angle dependent threshold on the brightnesstemperature at 183.31±1.00 GHz, and a threshold on thebrightness temperature difference between another channel and183.31±1.00 GHz. Two different alternatives, using183.31±3.00 GHz or 183.31±7.00 GHz as the other channel,are studied. The robustness of this cloud filtering method isdemonstrated by a mid-latitudes winter case study.

The paper then studies different biases on UTH climatologies. Cloudsare associated with high humidity, therefore the possible dry biasintroduced by cloud filtering is discussed and compared to the wetbiases introduced by the clouds radiative effect if no filtering isdone.This is done by means of a case study, and by means of astochastic cloud database with representative statistics formidlatitude conditions.

Both studied filter alternatives perform nearly equally well, butthe alternative using 183.31±3.00 GHz as other channel ispreferable, because that channel is less likely to see the Earth'ssurface than the one at 183.31±7.00 GHz.

The consistent result of all case studies and for both filteralternatives is that both cloud wet bias and cloudfiltering dry bias are modest for microwave data. The recommendedstrategy is to use the cloud filtered data as an estimate for thetrue all-sky UTH value, but retain the unfiltered data to have anestimate of the cloud induced uncertainty.

The focus of the paper is on midlatitude data, since atmosphericdata to test the filter for that case were readily available. Thefilter is expected to be applicable also to subtropical and tropicaldata, but should be further validated with case studies similar tothe one presented here for those cases.

[学科分类] 大气科学