Abstract
Abstract. We compare atmospheric total precipitable water (TPW) derived from the SSM/I (Special Sensor Microwave Imager) and SSMIS (Special Sensor Microwave Imager/Sounder) radiometers and WindSat to collocated TPW estimates derived from COSMIC (Constellation System for Meteorology, Ionosphere, and Climate) radio occultation (RO) under clear and cloudy conditions over the oceans from June 2006 to December 2013. Results show that the mean microwave (MW) radiometer – COSMIC TPW differences range from 0.06 to 0.18 mm for clear skies, from 0.79 to 0.96 mm for cloudy skies, from 0.46 to 0.49 mm for cloudy but non-precipitating conditions, and from 1.64 to 1.88 mm for precipitating conditions. Because RO measurements are not significantly affected by clouds and precipitation, the biases mainly result from MW retrieval uncertainties under cloudy and precipitating conditions. All COSMIC and MW radiometers detect a positive TPW trend over these 8 years. The trend using all COSMIC observations collocated with MW pixels for this data set is 1.79 mm decade−1, with a 95 % confidence interval of (0.96, 2.63), which is in close agreement with the trend estimated by the collocated MW observations (1.78 mm decade−1 with a 95 % confidence interval of 0.94, 2.62). The sample of MW and RO pairs used in this study is highly biased toward middle latitudes (40–60∘ N and 40–65∘ S), and thus these trends are not representative of global average trends. However, they are representative of the latitudes of extratropical storm tracks and the trend values are approximately 4 to 6 times the global average trends, which are approximately 0.3 mm decade−1. In addition, the close agreement of these two trends from independent observations, which represent an increase in TPW in our data set of about 6.9 %, are a strong indication of the positive water vapor–temperature feedback on a warming planet in regions where precipitation from extratropical storms is already large.
Highlights
Clouds are important regulators for Earth’s radiation and hydrological balances
While water vapor retrievals from visible and infrared satellite sensors are limited to clear skies over both land areas and oceans, passive microwave (MW) imagers on satellites can provide all sky water vapor products, but only over oceans. These water vapor products are mainly verified by comparing to reanalyses, radiosonde measurements, or other satellite data (i.e., Soden, and Lanzante, 1996; Sohn and Smith, 2003; Noël et al, 2004; Palm et al, 2010; Sohn and Bennartz, 2008; Wick et al, 2008, hereafter Wick2008; Milz et al, 2009; Prasad and Singh, 2009; Pougatchev et al, 2009; Knuteson et al, 2010; Larar et al, 2010; Wang et al, 2010; Ho et al, 2010a, b)
The mean total precipitable water (TPW) differences are equal to 0.06 mm with a σ of 1.65 mm for F15, 0.07 mm with a σ of 1.47 mm for F17, and 0.18 mm with a σ of 1.35 mm for WindSat
Summary
Clouds are important regulators for Earth’s radiation and hydrological balances. Water vapor is a primary variable that affects cloud radiative effects and hydrological feedbacks. The measured radiances at 19.35, 22.235, and 37.0 GHz from SSMIS and 18.7, 23.8, and 37.0 GHz from WindSat are used to derive TPW, total cloud water (TCW), wind speed, and rainfall rates over oceans (Wentz and Spencer, 1998) These four variables are retrieved by varying their values until the brightness temperatures calculated using a forward model match satellite-observed brightness temperatures. The various radiometers from the different satellites have been precisely intercalibrated at the radiance level by analyzing the measurements made by pairs of satellites operating at the same time This was done for the explicit purpose of producing versions of the data sets that can be used to study decadal-scale changes in TPW, wind, clouds, and precipitation; special attention was focused on interannual variability in instrument calibration. The all sky daily RSS ocean products for F15, F16, F17, and WindSat are downloaded from http://www.remss.com/missions/ssmi
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