J. Zec

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.

NSCAT normalized radar backscattering coefficient biases using homogenous land targets

The NASA scatterometer (NSCAT) is a spaceborne radar sensor designed to measure the normalized radar backscattering coefficient σ0 of the Earths surface. Over the ocean, backscatter measurements are used to infer surface wind vectors. Wind retrieval is based on a statistical relationship between short-ocean wave roughness (that causes the backscatter) and the surface wind speed and direction. For NSCAT geometry, multiple antennas are used to provide backscatter measurements at several azimuth directions to resolve wind direction ambiguities. To achieve the desired wind vector accuracy, these antenna beams must be calibrated within a few tenths of a decibel. A simple relative-calibration method is applied to the NASA scatterometer backscatter from homogenous, isotropic, large-area targets. These targets exhibit both azimuth and time invariant radar response. A simple polynomial model for incidence angle dependence of σ0 is used, and the mean radar response from all antenna beams is taken as the reference. Corrections (σ0 biases) are calculated as differences (in log space) between measurements from particular beam and the reference. This simple model is applied to data from the Amazon rain forest and the Siberian plain. These areas are tested for temporal stability within the calibration period (several weeks). High-resolution masks are applied to extract suitable calibration data sets. Calculated corrections for each antenna beam are added to NSCAT σ0 measurements as a function of incidence angle. The magnitudes of corrections show the necessity of on-orbit calibration.

An improved NASA Scatterometer geophysical model function for tropical cyclones

The NASA Scatterometer (NSCAT) has been used to infer ocean surface wind vectors in tropical cyclones, but these measurements have generally under-estimated the high winds. This research provides an improved relationship, between the radar backscatter and the ocean surface wind vector, known as the geophysical model function (GMF). Satellite and airborne scatterometer backscatter measurements, obtained during tropical cyclones, are used to develop a neural network based GMF. NSCAT wind retrievals in a Pacific typhoon are presented that yield higher surface wind speeds for this GMF than produced using the current NSCAT GMF (NSCATI).

A feasibility study for a wide-swath, airborne, hurricane imaging microwave radiometer for operational hurricane measurements

Presents a conceptual design of an airborne Hurricane Imaging (microwave) Radiometer (HIRad) instrument for use in operational hurricane surveillance. The basis of the HIRad design is the Stepped Frequency Microwave Radiometer (SFMR) that has successfully measured surface wind speed and rain rate in hurricanes from the NOAA Hurricane Research Divisions P-3 aircraft. Unlike the SFMR that views only at nadir, the HIRad provides wide-swath measurements between /spl plusmn/45 degrees in incidence angle with a spot-beam spatial resolution of approximately 1-3 km. The system operates at four equally spaced frequency channels that cover a range between 4 GHz and 7 GHz.

Validation of QuikScat radiometric estimates of rain rate

The SeaWinds scatterometer on the QuikScat satellite simultaneously measures the active (scattering) and passive (emission) microwave characteristics of the Earths surface. This paper describes the use of the QuikScat Radiometer (QRad) brightness temperature measurements to infer rain rate over the oceans. A discussion of the rain rate algorithm is presented, and comparisons are made with near-simultaneous integrated rain rate measurements from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These results demonstrate the potential for using QRad rain index to infer moderate to high rain rates over the ocean.

Tropical Cyclone Geophysical Model Function for ocean surface wind retrievals from NASA Scatterometer measurements at high wind speeds

Describes a hybrid model function, denoted as Tropical Cyclone Geophysical Model Function (TCGMF) that was derived from the NSCAT1b model function and airborne KUSCAT scatterometer measurements during tropical cyclones. At higher wind speeds, TCGMF assigns lower backscatter values to a given wind vector; therefore compared to other model functions, its use results in higher winds. TCGMF was used for wind retrieval from the NASA Scatterometer measurements during several high-wind events (tropical cyclones). When compared to numerical models, results show that wind speeds retrieved using the TCGMF agree better than do those from other scatterometer model functions for model winds >20 m/s.

SeaWinds beam and slice balance using data over Amazonian rainforest

The primary application of a scatterometer is estimation of wind vectors over the sea surface, but a variety of other geophysical parameters can be retrieved as well. To eliminate ambiguity in retrieved wind direction, multiple looks are required at the observed area. These looks are provided by multiple scatterometer antenna beams. To achieve the desired accuracy of wind vectors and other retrieved geophysical products, the antenna beams must be calibrated to within /spl plusmn/0.2 dB. Pre-launch calibration alone is insufficient for such a level of beam balance. Postlaunch calibration/validation activities are therefore required for scatterometer missions. This paper describes beam balance procedure applied following the recent launch of the SeaWinds scatterometer on the QuikScat spacecraft. This work was similar to the NSCAT post-launch verification. A brief description is given of the SeaWinds instrument. A calibration data set is introduced. An azimuth beam balance and a comparison with preceding NSCAT instrument is presented. High-resolution slice-balance results are also shown.

Validation of QuikSCAT Radiometer rain rates using the TRMM microwave radiometer

The primary mission of the SeaWinds scatterometer on the QuikSCAT satellite is to infer surface wind vector from ocean backscatter measurements. Occasionally the backscatter measurements are contaminated by the presence of rain; therefore a reliable method of identifying rain is needed. Fortunately, the SeaWinds scatterometer simultaneously obtains active (scattering) and passive (emission) measurements of the ocean; thus, the QuikSCAT Radiometer (QRad) measured brightness temperatures can be used to infer rain rate within the scatterometer antenna field-of-view. This paper describes a new QuikSCAT Level-2B science product of rain rate over oceans. The principal use of this product for quality control purposes to provide a quantitative rain flag associated with QuikSCAT wind vector cells. The QRad rain rate algorithm is described and the characteristics of the rain rates product are presented. This product has been validated by near-simultaneous comparisons with rain rate measurements from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). An example of QuikSCAT retrieved winds in the presence of rain is presented with collocated QRad rain measurements. Results demonstrate that the QRad rain rate product provides a reliable, quantitative wind vector quality flag.

Relative calibration of scatterometer antennas using land targets

In this paper a simple method for relative antenna calibration is applied to NSCAT (NASA Scatterometer), and the model used is described. Calibration data sets are extracted using high-resolution masks. Results are presented for the Amazon rain forest (traditionally used) and the Siberian plain. Beam correction tables are derived for balancing normalized radar backscatter coefficient measurements from different antennas.

Effects of spacecraft attitude on the NASA Scatterometer antenna calibration

Radar scatterometers are expected to become a main source of marine surface winds. Wind vector retrieval is based on the relation (geophysical model function) between the radar cross-section and wind induced surface roughness. Multiple antennas, pointed at different azimuth angles, are required to remove wind direction ambiguity inherent in a single cross-section wind vector observation. Scatterometer antennas must be well calibrated to ensure desired accuracy of the retrieved wind. Pre-launch calibration alone proved insufficient in the past so post-launch calibration and validation is planned for scatterometer missions. During cal/val activities for the NASA Scatterometer (NSCAT), a consistent difference was noted between calibration corrections calculated based on ascending vs. descending passes. This difference cannot be attributed to geophysical parameters and must be instrument related. In this paper, an attempt is made to attribute ascending/descending discrepancy to imperfect spacecraft attitude. Calibration is performed at multiple spacecraft attitude sets (roll, pitch, and yaw). The set producing the lowest difference between ascending and descending based corrections is the suggested attitude. After brief introduction to the NSCAT in the next section, calibration method using homogenous land targets is outlined.