David E. Weissman

Effects of Rain Rate and Wind Magnitude on SeaWinds Scatterometer Wind Speed Errors

Abstract Rain within the footprint of the SeaWinds scatterometer on the QuikSCAT satellite causes more significant errors than existed with its predecessor, the NASA scatterometer (NSCAT) on Advanced Earth Observing Satellite-I (ADEOS-I). Empirical relations are developed that show how the rain-induced errors in the scatterometer wind magnitude depend on both the rain rate and on the wind magnitude. These relations are developed with collocated National Data Buoy Center (NDBC) buoy measurements (to provide accurate sea surface winds) and simultaneous Next Generation Weather Radar (NEXRAD) observations of rain reflectivity. An analysis, based on electromagnetic scattering theory, interprets the dependence of the scatterometer wind errors on volumetric rain rate over a range of wind and rain conditions. These results demonstrate that the satellite scatterometer responds to rain in a manner similar to that of meteorological radars, with a Z–R relationship. These observations and results indicate that the com...

The relationship between the microwave radar cross section and both wind speed and stress: Model function studies using Frontal Air-Sea Interaction Experiment data

The Frontal Air-Sea Interaction Experiment (FASINEX) provided a unique data set with coincident airborne scatterometer measurements of the ocean surface radar cross section (RCS) (at Ku band) and near-surface wind and wind stress. These data have been analyzed to study new model functions which relate wind speed and surface friction velocity (square root of the kinematic wind stress) to the radar cross section and to better understand the processes in the boundary layer that have a strong influence on the radar backscatter. Studies of data from FASINEX indicate that the RCS has a different relation to the friction velocity than to the wind speed. The difference between the RCS models using these two variables depends on the polarization and the incidence angle. The radar data have been acquired from the Jet Propulsion Laboratory airborne scatterometer. These data span 10 different flight days. Stress measurements were inferred from shipboard instruments and from aircraft flying at low altitudes, closely following the scatterometer. Wide ranges of radar incidence angles and environmental conditions needed to fully develop algorithms are available from this experiment.

Challenges to Satellite Sensors of Ocean Winds: Addressing Precipitation Effects

AbstractMeasurements of global ocean surface winds made by orbiting satellite radars have provided valuable information to the oceanographic and meteorological communities since the launch of the Seasat in 1978, by the National Aeronautics and Space Administration (NASA). When Quick Scatterometer (QuikSCAT) was launched in 1999, it ushered in a new era of dual-polarized, pencil-beam, higher-resolution scatterometers for measuring the global ocean surface winds from space. A constant limitation on the full utilization of scatterometer-derived winds is the presence of isolated rain events, which affect about 7% of the observations. The vector wind sensors, the Ku-band scatterometers [NASA’s SeaWinds on the QuikSCAT and Midori-II platforms and Indian Space Research Organisation’s (ISRO’s) Ocean Satellite (Oceansat)-2], and the current C-band scatterometer [Advanced Wind Scatterometer (ASCAT), on the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT)’s Meteorological Operation ...

Measurements of the Effect of Rain-Induced Sea Surface Roughness on the QuikSCAT Scatterometer Radar Cross Section

Radar measurements of the sea surface, with satellite scatterometers that operate at Ku-band, are affected by the presence of rain through modification of the sea surface roughness by rain impacts. This is in addition to wind driven roughness, atmospheric scattering, and attenuation that affect the measured normalized radar cross section (NRCS). This paper presents a case study of the increase of the total radar cross section, averaged across surface illuminated areas (individual footprints) of the SeaWinds scatterometer (on QuikSCAT) caused by rain striking the sea surface. This effort combines satellite-based Ku-band data with high-resolution 3-D volumetric rain measurements, from simultaneous collocated Next Generation Weather Radar data. The results to be presented were acquired during a significant rain event in the Gulf of Mexico, to the east of Corpus Christi, and just south of Houston, TX, in May 2005. The results of this paper show dependence on wind speed, rainrate, and polarization. They agree with numerous surface-based studies (single point measurements), using ocean platforms and wind-wave tanks, whose data were collected under similar conditions. For example, at rainrates less than 10 mm/hr, the relative change in surface roughness is seen to decrease as the wind magnitude increases from 5 to 7 m/s. Another consistent observation is that the vertical polarization NRCS shows less sensitivity to rainrate than does horizontal.

Ocean model validation of the NASA scatterometer winds

The suitability of basin-scale, satellite-based scatterometer winds for forcing of numerical ocean models is examined using a reduced gravity, primitive equation model of the tropical Pacific Ocean. Three surface forcing fields are validated in a comparison of upper layer thickness (ULT) from the ocean model with observed sea level data. The forcing fields are the Florida State University observed winds, winds derived from the NASA Scatterometer (NSCAT), and stresses derived directly from NSCAT. The sea level data sets are the World Ocean Circulation Experiment “fast” sea level data set from island measurements and sea level anomalies from TOPEX/POSEIDON. Results of this comparison demonstrate that while the three model results are qualitatively similar, the results are quantitatively better when forcing with the NSCAT derived stresses. This is particularly true in the eastern tropical Pacific and in convergent zones where forcing with the NSCAT stresses can lead to large differences in ULT (>40 m) compared with results from the other two wind products.

Two frequency radar interferometry applied to the measurement of ocean wave height

A technique is developed for measuring the rms wave height averaged over an area of the sea that is much greater than any horizontal scale of the surface waves. The method involves a nadir looking radar which transmits and receives two monochromatic signals simultaneously. Signal processing at the receiver involves the computation of the correlation between the two returning signals as a function of theft variable frequency separation. The cross correlation between the amplitude and phase functions of the individual returning carriers depends on the distribution of discrete scatterers along the direction of propagation. This information can be used to determine the rms surface elevation (about the mean); it does not depend on the temporal or spatial frequency spectrum of the wave height or slope. Under conditions which are typical for a microwave signal being normally incident and reflected by the sea, the two frequency correlation function R(\Delta k) is seen to be equal to the characteristic function of the surface elevation of specular points. Laboratory measurements have been conducted on wind driven waves, and the measured correlation function compares favorably with the theory developed here.

Calibrating the Quikscat/SeaWinds Radar for measuring rainrate over the oceans

This effort continues a study of the effects of rain, over the oceans, on the signal retrieved by the SeaWinds scatterometer. It is determined that the backscatter radar cross section can be used to estimate the volumetric rain rate, averaged horizontally, across the surface resolution cells of the scatterometer. The dual polarization of the radar has a key role in developing this capability. The relative magnitudes of the radar backscatter depends on the volumetric rain rate, the rain column height and surface wind velocity, the viewing angle, as well as the polarization (due to the oblateness of raindrops at the higher rain rates). The approach to calibrating the SeaWinds normalized radar cross section (NRCS) is to collect National Weather Service Next Generation Weather Radar (NEXRAD) radar-derived rain rate measurements (4-km spatial resolution and 6-min rotating cycles) colocated in space (offshore) and time with scatterometer observations. These calibration functions lead to a Z-R relationship, which is then used at mid-ocean locations to estimate the rain rate in 0.25/spl deg/ or larger resolution cells, which are compared with Tropical Rainfall Mapping Mission (TRMM) Microwave Imager (TMI) rain estimates. Experimental results to date are in general agreement with simplified theoretical models of backscatter from rain, for this frequency, 14 GHz. These comparisons show very good agreement on a cell-by-cell basis with the TMI estimates for both wide areas (1000 km) and smaller area rain events.

Satellite scatterometer studies of ocean surface stress and drag coefficients using a direct model

This study evaluates the accuracy of estimates of sea surface friction velocity derived using a new model function that operates directly on the NASA scatterometer normalized radar cross-section (NRCS) measurements. These NRCS data are first collocated with numerous National Data Busy Center buoys (within ±0.5° latitude and longitude) and are then processed using the new Weissman et al. [1994] Frontal Air-Sea Interaction Experiment model to produce 25-km by 25-km spatial averages of the friction velocity for several contiguous areas within this region. One method of validation of these estimates involves comparisons with friction velocity estimates derived from data collected by the buoys using bulk methods. This was conducted at numerous locations; the Atlantic and Pacific coastlines and within the Gulf of Mexico. It is found that there is excellent agreement between these two different measurements. In addition, this approach enabled the derivation of estimates of the surface drag coefficient, with the use of the buoy winds, neutral stability winds at 10-m elevation, U10N, and the scatterometer surface friction velocity u*. This permitted the study of the dependence of this drag coefficient CD on wind speed in these different regions on spatial scales, which had not previously been possible. Comparisons are made to previously published models of CD that only depend on wind speed, with generally good agreement at winds over 6 m−1 s, but with significant regional differences at winds less than 6 m−1 s.

Relationship between hurricane surface winds and L-band radar backscatter from the sea surface

Abstract High-altitude, airborne, L-band synthetic aperture radar (SAR) data were collected in Hurricane Gloria on 28 and 30 September 1976. The backscattered power levels (proportional to the surface scattering coefficient) averaged over a few square kilometers of surface area were found to vary with surface wind speed and the angle of the wind relative to the radar. Comparisons between the backscatter from the eye and eye-wall regions of the hurricane were made with low-level aircraft wind measurements that were nearly coincident in space and time. The SAR has the potential advantage over other radar types because of its higher spatial resolution. It also appears to have the ability to penetrate rainfall, with a reduction in the echo from the surface. One difference when compared with higher frequency microwave radars is a decrease in the sensitivity of the backscatter to changes in wind speed. This dependence of L-band radar backscatter on surface winds suggests that the winds associated with hurricane...

Air/sea momentum transfer and the microwave cross section of the sea

Measurements of atmospheric fluxes of heat, moisture, and momentum were made simultaneously and coincidentally with microwave backscatter measurements from an airship flown over the Pacific Ocean in 1993. The measurement technique was well suited to measure fluxes at very low wind speeds because the airship required an air speed near 10 m s−1 in order to maintain altitude. The measurements show that very low wind speeds are always associated with very low microwave cross sections and very high air/sea drag coefficients. The occurrence of regions of very low wind speed is not usually correlated with either the sea surface temperature or the air/sea temperature difference. Nevertheless, these regions can remain in place for time periods of several hours. The rate of increase of the microwave cross section at very low wind speeds agrees with that predicted by Donelan and Pierson [1987], but the absolute value of the threshold wind speed appears to be lower than their prediction. The high drag coefficient at low wind speeds is due to the fact that the friction velocity is nearly constant for wind speeds below 4–5 m s−1. Thus, at these wind speeds the increase of the microwave cross section follows the behavior of the wind speed rather than the wind stress. At higher wind speeds, however, the behavior is reversed with the cross section following the wind stress at a constant wind speed. We suggest that this behavior can be understood if momentum transfer across the air/sea interface is supported by both viscosity and the entire spectrum of waves on the surface, as many investigators have indicated.