Abstract
Passive imagers on polar-orbiting satellites provide long-term, accurate integrated water vapor (IWV) data sets. However, these climatologies are affected by sampling biases. In Germany, a dense Global Navigation Satellite System network provides accurate IWV measurements not limited by weather conditions and with high temporal resolution. Therefore, they serve as a reference to assess the quality and sampling issues of IWV products from multiple satellite instruments that show different orbital and instrument characteristics. A direct pairwise comparison between one year of IWV data from GPS and satellite instruments reveals overall biases (in kg/m
 
 
 
 
 2
 
 
 
 ) of 1.77, 1.36, 1.11, and −0.31 for IASI, MIRS, MODIS, and MODIS-FUB, respectively. Computed monthly means show similar behaviors. No significant impact of averaging time and the low temporal sampling on aggregated satellite IWV data is found, mostly related to the noisy weather conditions in the German domain. In combination with SEVIRI cloud coverage, a change of shape of IWV frequency distributions towards a bi-modal distribution and loss of high IWV values are observed when limiting cases to daytime and clear sky. Overall, sampling affects mean IWV values only marginally, which are rather dominated by the overall retrieval bias, but can lead to significant changes in IWV frequency distributions.
Highlights
The important role of water vapor in the hydrological cycle as well as in climate as the most effective greenhouse gas has been presented in numerous studies on weather and climate [1,2]
This study provides an assessment of various well-established integrated water vapor products from several passive instruments on polar-orbiting satellites using measurements from a one-year time period, from June 2012 to May 2013, for a German domain
The instrument characteristics differ in spectral channels, spatial resolution and overpass times, while the corresponding integrated water vapor (IWV) retrieval algorithms are subject to different types of limitations such as daytime and clear-sky situations
Summary
The important role of water vapor in the hydrological cycle as well as in climate as the most effective greenhouse gas has been presented in numerous studies on weather and climate [1,2]. The major role and fast feedback processes of water vapor in a changing climate have been acknowledged for quite some time, yet large uncertainties exist in its variability and changes along with corresponding estimates of radiative forcing and climate sensitivity [3,4,5]. Many aspects of these studies, e.g., gaining improved understanding and model representation of the interaction of water vapor with other variables such as clouds and radiation, depend on accurate, comprehensive water vapor data sets [6,7,8]. It has the major purpose to quantify the current state of the art in water vapor products being constructed for climate applications, i.a. total column water vapor (TCWV), by the analyses and intercomparison of long-term satellite data records, including data records from in situ and ground-based observations as well as from reanalyses [13]
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