Comparison of AMSR-E Retrievals of Total Water Vapor over the Ocean with Ship based Measurements

ABSTRACT

Atmospheric water vapor is an important part of the Earth's hydrological cycle and plays a crucial role in many aspects of the climate system. The main source of the atmospheric moisture is the ocean. Until recently the information we had about the distribution of atmospheric water vapor over the oceans was based on a relatively sparse distribution of radiosonde profiles. In order to cover the global distribution of water vapor in the atmosphere measurements from spaceborne instruments are necessary.

The Advanced Microwave Scanning Radiometer (AMSR-E) was launched on May 4, 2002, aboard NASA's Aqua spacecraft. The National Space Development Agency of Japan (NASDA) provided AMSR-E to NASA as an indispensable part of Aqua's global hydrology mission. AMSR-E provides measurements of the total atmospheric water vapor column over the oceans. Additionally, a number of other important geophysical parameters are measured that include sea surface temperature (SST), wind speed, atmospheric water vapor, cloud water, and rain rate. A key feature of AMSR-E is its capability to see through clouds, thereby providing an uninterrupted view of global water vapor path, SST and surface wind fields.

Here the measurements of the total water vapor (TWV) path from AMSR-E are compared with measurement from ship based instruments. The shipboard instruments include Vaisala radiosondes (RS), microwave radiometer (MWR), and the Marine-Atmosphere Emitted Radiance Interferometer (M-AERI). The satellite and ship measurements were also compared with the European Center of Medium-Range Weather Forecasts (ECMWF) analysis data. The measurements were taken during two deployments, a 2 months deployment in the Caribbean on the Explorer of the Seas and during 1 month navigation around the North America on the USCGC Healy.

RS90-A radiosondes were generally launched from the ships twice a day during the deployments while the vessels were underway, coincident with Aqua overpasses. The microwave radiometer used in this study is the Radiometrics Corporation WVR-1100, which operated continuously, measuring atmospheric emission at 23.8 GHz and 31.4 GHz. The TWV and the precipitable liquid water (PLW) are retrieved using coefficients that were derived by a bilinear regression between the atmospheric brightness temperature at the two frequencies and the TWV and PLW obtained from radiosonde soundings. The coefficients are site-dependent and can vary with seasons. For deployment on ships a generic set of calibration coefficients is being used. The validity of the coefficients for the Caribbean basin was tested in Miami by comparing PWV measured by the MWR and the GPS system at the Miami airport. A set of general coefficients was used for the Healy deployment.

The M-AERI is a sea-going instrument that measures spectra on atmospheric infrared radiation in range of 4 to 18 µm with ~10 min temporal resolution. These spectra can be used to retrieve profiles of temperature and humidity in the atmosphere, and can thus be employed for continuous monitoring of the distribution of temperature and humidity in the marine atmosphere. M-AERI measurements can be used to validate both modeling results and satellite measurements.

M-AERI retrieved profiles of atmospheric humidity were used to compute total water vapor path in the marine atmosphere. We will present the comparison of AMSR-E measurements of TWV from space with the ship based measurements by M-AERI, MWR and the radiosondes.