Atmospheric Water Content over the Tropical Pacific Derived from the Nimbus-6 Scanning Microwave Spectrometer

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  • 1 National Environmental Satellite Service, NOAA, Washington, DC 20233
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Abstract

The scanning microwave spectrometer (SCAMS) aboard Nimbus-6 contains a 22.23 GHz water vapor channel and 31.65 GHz window channel for deriving integrated water vapor (precipitable water) and cloud liquid water through a column over the oceans. The SCAMS derived parameters are based on a nonlinear relationship between the two channel measurements, and is shown to be more accurate than a linear model, particularly for the large values of precipitable water found in the tropics. Comparisons are made between the SCAMS derived precipitable water and 163 radiosonde measurements collected from 19 island stations over the tropical Pacific for the period between 18 August and 4 September 1975. The SCAMS values of precipitable water are shown to display the same variability as the radiosonde data, with a 0.5 cm rms difference.

Fields of both precipitable water and liquid water derived by the SCAMS were averaged for the 2-week period for the ocean area between 130°E–100°W and 35°N–35°S. The Intertropical Convergence Zone was delineated near 10°N and contained greater than 5 cm of precipitable water and greater than 0.4 mm of liquid water. The Southern Convergence Zone is similarly well defined. Also exhibited were the dry zones in both hemispheres containing less than 3.5 cm of moisture and less than 0.2 mm of liquid water. These microwave derived parameters are of sufficient detail to be useful in studying both synoptic and climatic variations in the atmospheric water cycle.

Abstract

The scanning microwave spectrometer (SCAMS) aboard Nimbus-6 contains a 22.23 GHz water vapor channel and 31.65 GHz window channel for deriving integrated water vapor (precipitable water) and cloud liquid water through a column over the oceans. The SCAMS derived parameters are based on a nonlinear relationship between the two channel measurements, and is shown to be more accurate than a linear model, particularly for the large values of precipitable water found in the tropics. Comparisons are made between the SCAMS derived precipitable water and 163 radiosonde measurements collected from 19 island stations over the tropical Pacific for the period between 18 August and 4 September 1975. The SCAMS values of precipitable water are shown to display the same variability as the radiosonde data, with a 0.5 cm rms difference.

Fields of both precipitable water and liquid water derived by the SCAMS were averaged for the 2-week period for the ocean area between 130°E–100°W and 35°N–35°S. The Intertropical Convergence Zone was delineated near 10°N and contained greater than 5 cm of precipitable water and greater than 0.4 mm of liquid water. The Southern Convergence Zone is similarly well defined. Also exhibited were the dry zones in both hemispheres containing less than 3.5 cm of moisture and less than 0.2 mm of liquid water. These microwave derived parameters are of sufficient detail to be useful in studying both synoptic and climatic variations in the atmospheric water cycle.

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