Nelly M. Mognard

Satellite observations of a polar low over the Norwegian Sea by Special Sensor Microwave Imager, Geosat, and TIROS-N Operational Vertical Sounder

Many polar lows are generated at the boundary between sea ice and the ocean, in regions of large temperature gradients, where in situ observations are rare or nonexistent. Since satellite observations are frequent in high-latitude regions, they can be used to detect polar lows and track their propagation and evolution. The Special Sensor Microwave/I mager (SSM/I) providing estimates of surface wind speed, integrated cloud liquid water content, water vapor content, and precipitation size ice-scattering signal over the ocean; the Geosat radar altimeter measuring surface wind speed and significant wave height; and the TIROS-N Operational Vertical Sounder (TOVS) allowing the determination of temperature and humidity profiles in the atmosphere have been used in synergy for a specific case which occurred in the Norwegian Sea on January, 23–24 1988. All three instruments show sharp atmospheric gradients associated with the propagation of this low across the ocean, which permit the detection of the polar low at a very early stage and tracking it during its development, propagation, and decay. The wind speed gradients are measured with good qualitative agreement between the altimeter and SSM/I. TOVS retrieved fields prior to the formation of the low confirm the presence of an upper level trough, while during the mature phase baroclinicty can be observed in the 1000–500 hPa geopotential thicknesses. Comparisons between satellite-retrieved products and analyses of the operational Norwegian limited area model (150- and 50- km mesh) highlight the importance of satellites for the monitoring of such mesoscale phenomena.

A new method for the validation of altimeter‐derived sea state parameters with results from wind and wave models

A new approach for the validation of altimeter-derived sea state parameters using data from wind and wave models is proposed. Wind speeds and significant wave heights (SWH) acquired with the Geosat altimeter over the North Atlantic are compared with the wind analysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model and with wave hindcast from the VAG model for the week of October 12–19, 1987. The statistical results with this 1-week data set agree with those of a 7-month comparison study between Geosat and the 43 National Data Buoy Center (NDBC) buoys. An advantage with the models is the large number of comparison points which allows an investigation of high sea state conditions and statistical analyses by binning in various ways. The underestimation of SWH by the Geosat altimeter, compared with the VAG hindcast, is confirmed and underestimation is shown to be proportional to the SWH. In order to use model hindcasts to validate altimeter-derived wind parameters, a method to improve the model wind data set is proposed. This method relies on the SWH comparisons between altimeter and model that are simultaneously available with the wind data. Tests on the 1-week data set show the validity of this new method. Applied to the three algorithms, Brown, smooth Brown, and Chelton-Wentz, these tests show that the statistical differences between the various algorithms are a function of wind speed range and indicate that the wind speeds derived with the Brown algorithm (which already agree best with buoy data) also agree best with the ECMWF 10-m wind fields.

Southern Ocean Waves and Winds Derived from SEASAT Altimeter Measurements

SEASAT altimeter measurements of significant wave height in the Southern Ocean during August and September 1978 have been examined. Significant wave heights of greater than 10 m were observed. Background levels of 2–4 m indicate generally rougher seas than occur in the North Atlantic during a comparable season. The altimeter data set is examined statistically and the spectrum of the worst case situations is determined parametrically based upon a JONSWAP form of the spectrum. Results of a statistical analysis of the altimeter data are presented together with an evaluation of the probable spectral characteristics based upon a parametric derivation of the spectrum.