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Ian L. Wittmeyer and Thomas H. Vonder Haar

Abstract

A climatological examination of the global water vapor field based on a multiyear period of successful satellite-based observations is presented. Results from the multiyear global ISCCP TOVS water vapor dataset as operationally produced by NESDIS and ISCCP are shown. The methods employed for the retrieval of precipitable water content (PWC) utilize infrared measurements collected by the TOVS instrument package flown aboard the NOAA series of operational polar-orbiting satellites. Strengths of this dataset include the nearly global daily coverage, availability for a multiyear period, operational internal quality checks, and its description of important features in the mean state of the atmosphere. Weaknesses of this PWC dataset include that the infrared sensors are unable to collect data in cloudy regions, the retrievals are strongly biased toward a land-based radiosonde first-guess dataset, and the description of high spatial and temporal variability is inadequate. Primary consequences of these factors are seen in the underestimation of ITCZ water vapor maxima, and underestimation of midlatitude water vapor mean and standard deviation values where transient atmospheric phenomena contribute significantly toward time means. A comparison of TOVS analyses to SSM/I data over ocean for the month of July 1988 shows fair agreement in the magnitude and distribution of the monthly mean values, but the TOVS fields exhibit much less temporal and spatial variability on a daily basis in comparison to the SSM/I analyses. The emphasis of this paper is on the presentation and documentation of an early satellite-based water vapor climatology, and description of factors that prevent a more accurate representation of the global water vapor field.

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Graeme L. Stephens, Darren L. Jackson, and Ian Wittmeyer

Abstract

This paper describes a physically based method for the retrieval of upper-tropospheric humidity (UTH) and upper-tropospheric column water vapor (UTCWV) based an the use of radiance data collected by the TIROS Operational Vertical Sounder (TOVS), principally channels 4 (14.2 μm), 6 (13.7 μm), and 12 (6.7 μm) of High-Resolution Infrared Radiation Sounder. This paper demonstrates how TOVS radiance data, particularly that of the upper-tropospheric water vapor channel 12, can be modeled usefully using a single band Malkmus model with parameters tuned to a particular sensor on a particular satellite. A significant uncertainty arises from the treatment of continuum absorption, even in regions where line absorption is dominant. This uncertainty can introduce a bias as large as 2 K, which in turn leads to an uncertainty of approximately 15%–20% in the retrieved UTH and UTCWV. The research described in this paper points to the critical need for high-accuracy measurements of upper-tropospheric water vapor to test retrievals such as the one described herein. The results suggest that the relative humidity of the upper troposphere, especially over the domain of the Hadley circulation taken to be between 30°N and 30°S, undergoes a significant seasonal change. This is contrary to the usual assumption of fixed relative humidity adopted in simple climate feedback studies. Large seasonal changes in the region from 30°N to 30°S are possibly associated with the seasonal swings in the Hadley circulation. Similar seasonal changes in the 350-hPa overburden indicate that these swings in relative humidity occur as a result of significant seasonal shifts in the upper-tropospheric water vapor content. In the region equatorward of 30° latitude, the Southern Hemisphere winter is shown to be significantly drier than the Northern Hemisphere winter. This enhanced drying is consistent with the existence of more extensive regions of subsidence producing larger regions of dry upper-tropospheric air in the SH during winter than in the corresponding NH during winter, especially in the subtropical Eastern Hemisphere. Analyses of the data show the clear effects of moistening in the NH subtropics through the monsoonal circulations over Asia and North America and little effect of monsoon circulation in the Southern Hemisphere.

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