W.T. Liu

QuikSCAT wind retrievals for tropical cyclones

The use of QuikSCAT data for wind retrievals of tropical cyclones is described. The evidence of QuikSCAT /spl sigma//sub 0/ dependence on wind direction for >30-m/s wind speeds is presented. The QuikSCAT /spl sigma//sub 0/s show a peak-to-peak wind direction modulation of /spl sim/1 dB at 35-m/s wind speed, and the amplitude of modulation decreases with increasing wind speed. The decreasing directional sensitivity to wind speed agrees well with the trend of QSCAT1 model function at near 20 m/s. A correction of the QSCAT1 model function for above 23-m/s wind speed is proposed. We explored two microwave radiative transfer models to correct the attenuation and scattering effects of rain for wind retrievals. One is derived from the collocated QuikSCAT and Special Sensor Microwave/Imager (SSM/I) dataset, and the other one is a published parametric model developed for rain radars. These two radiative transfer models account for the effects of volume scattering, scattering from rain-roughened surfaces and rain attenuation. The models suggest that the /spl sigma//sub 0/s of wind-roughened sea surfaces for 40-50-m/s winds are comparable to the /spl sigma//sub 0/s of rain contributions for up to about 10-15 mm/h. Both radiative transfer models have been used to retrieve the ocean wind vectors from the collocated QuikSCAT and SSM/I rain rate data for several tropical cyclones. The resulting wind speed estimates of these tropical cyclones show improved agreement with the wind fields derived from the best track analysis and Hollands model for up to about 15-mm/h SSM/I rain rate. A comparative analysis of maximum wind speed estimates suggests that other rain parameters likely have to be considered for further improvements.

QuikSCAT geophysical model function for tropical cyclones and application to Hurricane Floyd

The QuikSCAT radar measurements of several tropical cyclones in 1999 have been studied to develop the geophysical model function (GMF) of Ku-band radar /spl sigma//sub 0/ values (normalized radar cross section) for extreme high wind conditions. To account for the effects of precipitation, the authors analyze the co-located rain rates from the Special Sensor Microwave/Imager (SSM/I) and propose the rain rate as a parameter of the GMF. The analysis indicates the deficiency of the NSCAT2 GMF developed for the NASA scatterometer, which overestimates the ocean /spl sigma//sub 0/ for tropical cyclones and ignores the influence of rain. It is suggested that the QuikSCAT /spl sigma//sub 0/ is sensitive to the wind speed of up to about 40-50 m s/sup -1/. The authors introduce modifications to the NSCAT2 GMF and apply the modified GMF to the QuikSCAT observations of Hurricane Floyd. The QuikSCAT wind estimates for Hurricane Floyd in 1999 was improved with the maximum wind speed reaching above 60 m s/sup -1/. The authors perform an error analysis by comparing the QuikSCAT winds with the analyses fields from the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD). The reasonable agreement between the improved QuikSCAT winds and the HRD analyses supports the applications of scatterometer wind retrievals for hurricanes.

Power density of ocean surface wind from international scatterometer tandem missions

For 6 months between April and October 2003, two identical scatterometers flew in tandem. Their observations demonstrate the need for more than one scatterometer in the polar orbit to include sufficient temporal variability and reduce aliasing of ocean surface wind‐stress measurements required for applications such as estimating electricity generation potential and ocean–atmosphere gas exchange. The energy deficiency over a 12‐h period, evident in the data from one scatterometer, is eliminated with the additional scatterometer. The missions in tandem allow an improved understanding of the diurnal variability from coastal regions to the open ocean. The power density distributions were found to be very different at the different sampling times of the two satellites. Two scatterometers will be launched by India and China in the next few years and will fly in tandem with the scatterometers of the USA and Europe, which are already in operation. The potential improvement in the coverage of ocean wind stress by this constellation is analysed and discussed. The constellation is found to meet the 6‐hourly revisit requirement of operational weather forecasting over most of the ocean.

Flood mapping over the Asian continent during the 1999 summer monsoon season

Backscatter data from the SeaWinds scatterometer on the QuikSCAT satellite are used to delineate floods over the Asian, continent. SeaWinds acquires Ku-band (13.4 GHz) data at the vertical polarization over a very large swath of 1800 km, and at the horizontal polarization over a 1400-km swath. The authors present the flood areas together with topography on land and wind field on ocean. Results show extensive floods in China, India, Bangladesh, and other Asian countries. Timely flood mapping can provide crucial information for flood relief efforts.

The modulation of the seasonal cross‐shelf sea level variation by the cold pool in the Middle Atlantic Bight

This study explores the influence of the cold pool in the Middle Atlantic Bight (MAB) to cross-shelf sea surface slope by fitting an annual harmonic to temperature and salinity profiles from 1993 to 2012, and compares to the 20-year-averaged altimetry sea level anomaly (SLA). The consistency within bottom temperature, thermal steric height, total steric height and altimetry observation validates that the cold pool induces depressed sea level in the middle shelf overlapping with the dominant surface seasonal cycles. Temporally, the cold pool pattern is most apparent in July and August as a result of magnitude competition between the thermal and haline steric height. In addition, Ensemble Empirical Mode Decomposition (EEMD) is employed to reconstruct the altimetry SLA and reveals the middle-shelf depression pattern from single years SLA data. The locations of the SLA depression from 1993 to 2012 agree with the cold pool locations identified from in-situ measurements, suggesting a promising application of altimetry SLA in the cold pool study. Conclusively, this study reveals the modulation of the cross-shelf sea level variation by the cold pool, and contributes to the understanding of the sea level response to water masses on the continental shelf. This article is protected by copyright. All rights reserved.

Oceanic influence on global hydrologic cycle observed from space

The divergence of moisture transport integrated over the depth of the atmosphere over global oceans show similar geographic distribution as the surface fresh water flux (evaporationprecipitation). The temporal variations of the two terms also agree, from intraseasonal to interannual time scales, at selected locations. These two forcing terms were found to lead ocean surface salinity changes by 90° as expected. The interannual anomalies of the hydrologic parameters in the high latitude regions of North American and Eurasia are found the be opposite in phase, and their differences are found to have significant correlation with the moisture transport in the North Atlantic, suggesting N. Atlantic moisture transport is a bridge to the opposing hydrologic phases of the two continents.

Sensitivity of spacebased microwave radiometer observations to ocean surface evaporation

The methodology of spacebased estimation of evaporation (E) and latent heat flux from the ocean is described. Preliminary results of a simulation study with atmospheric radiation transfer model demonstrate that the observed radiances at the frequencies of the Tropical Rain Measuring Mission (TRMM) Microwave Imager (TMI) are sensitive to the change of E. An algorithm to retrieve E directly from the radiances observed by TMI is being constructed.

High Wind and Power Density Over Global Oceans

Spacebased scatterometer measures ocean surface roughness, which is in equilibrium with surface stress (momentum flux). Under general conditions, the variation of stress is reflected in the variation of winds. Eight years of QuikSCAT data are used to give a good representation of the probability distribution and power density of wind speed over global oceans and to provide useful applications. For hurricane-scale winds (> 35 m/s), present scatterometer measurements are not sensitive to increase in winds. Although strong efforts have been made to adjust the model functions for retrieving winds under moderate wind to the strong wind conditions and to improve the sensor design to retrieve strong winds, such effort is likely to be limited by the natural process of turbulent transport. Surface stress does not increase with wind in hurricane-scale winds due to flow separation.

Estimation of sea surface current derived from TRMM sea surface temperature using feature tracking techniques

We estimated Sea Surface Velocity (SSV) derived from daily Sea Surface Temperature (SST) measured during Tropical Rainfall Measure Mission (TRMM) in the South China Sea (SCS) from January 1999 to September 2004. In order to estimate SSV from consecutive SSTs, we used two feature tracking techniques: Maximum Cross Correlation (MCC) and Shape Matching Method (SMM). Because the advantages and disadvantages of two methods compensate each other, we incorporated the two SSV vector fields to obtain more SSV features including translation, rotation, and scaling. Using the daily SSV flow fields produced from MCC and SMM methods, we identified the following features in the SCS: (1) SSV patterns driven by summer and winter monsoon, (2) meridional and zonal velocity variation at the Luzon Strait, and (3) the formation of a jet-like meandering off the coast of Vietnam due to coastal upwelling. These features were compared to SSV fields from Princeton Ocean Model (POM), which displayed similar feature.

Potential scientific applications of seawinds and its follow-on

An example of combined use of spacebased scatterometer, microwave radiometer, and ocean color sensor in the study of extratropical ocean-atmosphere coupling is demonstrated. A new concept that combines both active and passive microwave measurements into a single sensor sharing a large antenna to provide high resolution measurements that will close the hydrologic balances is proposed.