Vincent J. Cardone

NSCAT high-resolution surface wind measurements in Typhoon Violet

NASA scatterometer (NSCAT) measurements of the western Pacific Supertyphoon Violet are presented for revolutions 478 and 485 that occurred in September 1996. A tropical cyclone planetary boundary layer numerical model, which uses conventional meteorological and geostationary cloud data, is used to estimate the winds at 10-m elevation in the cyclone. These model winds are then compared with the winds inferred from the NSCAT backscatter data by means of a novel approach that allows a wind speed to be recovered from each individual backscatter cell. This spatial adaptive (wind vector) retrieval algorithm employs several unique steps. The backscatter values are first regrouped in terms of closest neighbors in sets of four. The maximum likelihood estimates of speed and direction are then used to obtain speeds and directions for each group. Since the cyclonic flow around the tropical cyclone is known, NSCAT wind direction alias selection is easily accomplished. The selected wind directions are then used to convert each individual backscatter value to a wind speed. The results are compared to the winds obtained from the tropical cyclone boundary layer model. The NSCAT project baseline geophysical model function, NSCAT 1, was found to yield wind speeds that were systematically too low, even after editing for suspected rain areas of the cyclone. A new geophysical model function was developed using conventional NSCAT data and airborne Ku band scatterometer measurements in an Atlantic hurricane. This new model uses the neural network method and yields substantially better agreement with the winds obtained from the boundary layer model according to the statistical tests that were used.

Laser and Microwave Observations of Sea-Surface Condition for Fetch-Limited 17- to 25-m/s Winds

The variability of sea-surface conditions has been observed from a low-flying aircraft by a laser-wave profiling system and a scanning horizontally polarized 19.35-GHz passive microwave radiometer for fetch-limited wind speeds of 17 to 25 m/s in the North Sea. Wave profiles obtained with the laser system have been analyzed and show that wave growth occurs simultaneously at all frequencies and that an equilibrium value for the higher frequency components is eventually reached, but not before substantially higher (overshoot) values are obtained. Simultaneous observations of the microwave brightness temperature at vertical incidence show an increase with wind speed (or roughness of the sea surface) of 1°K/m/s. This increase, not in accord with theory, is shown to be a function of the percentage of foam coverage of the ocean surface.

Chapter 4 Oceanic Surface Winds

Publisher Summary This chapter presents a brief review of radar backscatter that provides a foundation to the satellite approach. The vicissitudes of the marine surface wind field is discussed with explanation of how it is affected by varying degrees of surface roughness and thermodynamic influences. Mechanism for measurement of surface wind speed using microwave systems is explained. Concepts related to the marine surface boundary layer are elaborated. Results of a global data assimilation experiment using 2 days (July 16, OOZ-July 18, OOZ, 1978) of real global scatterometer winds from Seasat-A are presented. These experiments were designed to give a preliminary assessment of the impact of SASS winds on the operational numerical weather analysis and forecasting system used at the National Meteorological Center (NMC). The alias-removal scheme used to obtain the most nearly correct direction from the SASS data is elaborated. Some history is provided regarding the development of algorithms, and an evaluation of the effect on a numerical atmospheric circulation model of inclusion of satellite data in the standard data base is discussed.

Seasat microwave wind and rain observations in severe tropical and midlatitude marine storms

Publisher Summary This chapter presents initial results of the studies concerning Seasat measurements in and around tropical and severe midlatitude cyclones over the open ocean, and provides an assessment of their accuracy and usefulness. Sensors flown on Seasat provided complementary measurements of surface wind speed and direction, rainfall rate, significant wave height and wave length, and sea surface temperature. These measurements were made with the Seasat-A Satellite Scatterometer (SASS), the Scanning Multichannel Microwave Radiometer (SMMR), the Seasat altimeter, and the Seasat Synthetic Aperture Radar (SAR). The general tropical-storm structure is also discussed in the chapter. The progress, to date, with microwave remote sensing of tropical storms are reviewed. The nature of severe marine storms is analyzed, and microwave measurements in severe marine storms are reviewed. Seasat observations of rain rate and microwave attenuation in tropical cyclones are presented. The accuracy is assessed of derived wind, precipitation, and sea surface temperature fields in and around tropical cyclones. Initial efforts to demonstrate the usefulness of Seasat data for operational applications, such as storm warning and forecasting are also reviewed.

NASA Scatterometer high resolution winds for Hurricane Lili

Aircraft flights that measured the normalized radar backscattering coefficient in hurricanes for rain free areas show that the backscatter has properties that differ from those in larger scale extratropical cyclones. The backscatter values are lower for the higher winds and increase more slowly at upwind and downwind with increasing wind speed, and the values near crosswind increase in such a way that the upwind crosswind difference decreases with increasing wind speed. A Tropical Cyclone Geophysical Model Function, named the TCGMF, has been developed explicitly for tropical cyclones. It differs from NSCAT-1a and NSCAT-1b for winds above about 15 meters per second. This model function is used to obtain a high resolution wind field for Hurricane Lili whose eye has been located for Rev 900 on Julian day 292 in 1996 to be at 23.4 degrees north and 283.1 degrees east. The analysis involves the large gradients in wind speed and pressure and the effects on the measured backscatter resulting from the spiral bands of thick clouds and rain. Conventional data sources such as ship reports, data from a NOAA research aircraft that flew into the storm at the time of the pass and from other spacecraft are used to develop an independent wind field analysis called TC96 for the winds in the boundary layer for comparison to the winds obtained from NSCAT. For winds above 25 m/s, the values of the averages, of TCGMF-TC96 are 0.1 m/s for Rev. 900 and -1.3 m/s for Rev. 907. For NSCAT-1b, the corresponding values are -5.32 m/s and -5.95 m/s.