Douglas Vandemark

Hurricane Directional Wave Spectrum Spatial Variation in the Open Ocean

Abstract The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricanes inner core over open water. The NASA airborne scanning radar altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane research aircraft at 1.5-km height acquired the open-ocean data on 24 August 1998 when Bonnie, a large hurricane with 1-min sustained surface winds of nearly 50 m s−1, was about 400 km east of Abaco Island, Bahamas. The NOAA aircraft spent more than five hours within 180 km of the eye and made five eye penetrations. Grayscale coded images of Hurricane Bonnie wave topography include individual waves as high as 19 m peak to trough. The dominant waves generally propagated at significant angles to the downwind direction. At some positions, three different wave fields of comparable energy crossed each other. Partitioning the SRA directional wave spectra enabled determination of the characteristics of the various components of the hurricane wave field and mapping of their sp...

A Global View of Swell and Wind Sea Climate in the Ocean by Satellite Altimeter and Scatterometer

Abstract Numerous case reports and regional studies on swell and wind sea events have been documented during the past century. The global picture of these common oceanic phenomena, however, is still incomplete in many aspects. This paper presents a feasibility study of using collocated wind speed and significant wave height measurements from simultaneous satellite scatterometer and altimeter sources to observe the spatial and seasonal pattern of dominant swell and wind wave zones in the worlds oceans. Two energy-related normalized indices are proposed, on the basis of which global statistics of swell/wind sea probabilities and intensities are obtained. It is found that three well-defined tongue-shaped zones of swell dominance, termed “swell pools,” are located in the eastern tropical areas of the Pacific, the Atlantic, and the Indian Oceans, respectively. Regions of intensive wave growth are observed in the northwest Pacific, the northwest Atlantic, the Southern Ocean, and the Mediterranean Sea. Seasonal...

A Two-Parameter Wind Speed Algorithm for Ku-Band Altimeters

Globally distributed crossovers of altimeter and scatterometer observations clearly demonstrate that ocean altimeter backscatter correlates with both the near-surface wind speed and the sea state. Satellite data from TOPEX/Poseidon and NSCAT are used to develop an empirical altimeter wind speed model that attenuates the sea-state signature and improves upon the present operational altimeter wind model. The inversion is defined using a multilayer perceptron neural network with altimeter-derived backscatter and significant wave height as inputs. Comparisons between this new model and past single input routines indicates that the rms wind error is reduced by 10%‐15% in tandem with the lowering of wind error residuals dependent on the sea state. Both model intercomparison and validation of the new routine are detailed, including the use of large independent data compilations that include the SeaWinds and ERS scatterometers, ECMWF wind fields, and buoy measurements. The model provides consistent improvement against these varied sources with a wind-independent bias below 0.3 m s21. The continuous form of the defined function, along with the global data used in its derivation, suggest an algorithm suitable for operational application to Ku-band altimeters. Further model improvement through wave height inclusion is limited due to an inherent multivaluedness between any single realization of the altimeter measurement pair [s o, HS] and observed near-surface winds. This ambiguity indicates that HS is a limited proxy for variable gravity wave properties that impact upon altimeter backscatter.

Ocean Wave Slope Observations Using Radar Backscatter and Laser Altimeters

Abstract Combination of laser and radar aboard an aircraft is used to directly measure long gravity wave surface tilting simultaneously with nadir-viewing microwave backscatter from the sea surface. The presented dataset is extensive, encompassing varied wind conditions over coastal and open-ocean wave regimes. Laser-derived slope statistics and Ka-band (36 GHz) radar backscatter are detailed separately to document their respective variations versus near-surface wind speed. The slope statistics, measured for λ > 1–2 m, show good agreement with Cox and Munks oil-slickened sea measurements. A notable exception is elevated distribution peakedness and an observed wind dependence in this likely proxy for nonlinear wave–wave interactions. Aircraft Ka-band radar data nearly mimic Ku-band satellite altimeter observations in their mean wind dependence. The present calibrated radar data, along with relevant observational and theoretical studies, suggest a large (−5 dB) bias in previous Ka-band results. Next, wave-...

A new bistatic model for electromagnetic scattering from perfectly conducting random surfaces

Abstract In this paper, we extend the Kirchhoff approach, which is widely used for near-nadir backscattering calculations, to include the proper polarization sensitivity for general bistatic scattering from gently sloping, perfectly conducting surfaces. Previously, Holliday has shown how the inclusion of terms from the second iteration of the surface-current integral equation is required to obtain agreement with the small perturbation method for backscattering conditions. Here we employ a similar approach by retaining all terms in this iterative expansion through first order in the surface slope to derive expressions for the standard Kirchhoff field as well as for a supplementary field that contains the polarization sensitivity. A polarization vector notation is introduced to simplify the inclusion of tilting effects from larger-scale features on the scattering surface. In connection with this latter development, we provide a clarification of the earlier work by Valenzuela on this topic together with an e...

Hurricane Directional Wave Spectrum Spatial Variation at Landfall

Abstract The NASA Scanning Radar Altimeter (SRA) flew aboard one of the NOAA WP-3D hurricane research aircraft to document the sea surface directional wave spectrum in the region between Charleston, South Carolina, and Cape Hatteras, North Carolina, as Hurricane Bonnie was making landfall near Wilmington, North Carolina, on 26 August 1998. Two days earlier, the SRA had documented the hurricane wave field spatial variation in open water when Bonnie was 400 km east of Abaco Island, Bahamas. Bonnie was similar in size during the two flights. The maximum wind speed was lower during the landfall flight (39 m s−1) than it had been during the first flight (46 m s−1). Also, Bonnie was moving faster prior to landfall (9.5 m s−1) than when it was encountered in the open ocean (5 m s−1). The open ocean wave height spatial variation indicated that Hurricane Bonnie would have produced waves of 10 m height on the shore northeast of Wilmington had it not been for the continental shelf. The gradual shoaling distributed t...

Measuring sea surface mean square slope with a 36‐GHz scanning radar altimeter

Values of sea surface mean square slope (mss) computed from the backscattered power falloff with incidence angle measured by an airborne 36-GHz scanning radar altimeter are presented. When the mss values are plotted against the 10-m wind speed (extrapolated from the 46- to 110-m aircraft height), for wind speeds up to 7 m s -1 the values scatter about a curve which is scaled from that obtained by Wu [1990] by a factor of 0.85. Wus result was a reanalysis of the optical measurements of Cox and Munk [1954]. The offset between the microwave and optical curves is reasonable because one would expect the surface to appear smoother to the microwave wavelength than to the optical. This is unlike the mss values inferred from absolute power measurements at nadir by 14-GHz satellite altimeters, which indicate a higher mss under light wind conditions than the optical measurements indicate. For wind speeds below 7 m s -1 the peak of the backscattered power distribution is observed at nadir, but for higher speeds the peak shifts off-nadir in the downwind direction.

Estimation of wind stress using dual‐frequency TOPEX data

The TOPEX/POSEIDON satellite carries the first dual-frequency radar altimeter. Monofrequency (Ku-band) algorithms are presently used to retrieve surface wind speed from the altimeters radar cross-section measurement (σ0Ku). These algorithms work reasonably well, but it is also known that altimeter wind estimates can be contaminated by residual effects, such as sea state, embedded in the σ0Ku measurement. Investigating the potential benefit of using two frequencies for wind retrieval, it is shown that a simple evaluation of TOPEX data yields previously unavailable information, particularly for high and low wind speeds. As the wind speed increases, the dual-frequency data provides a measurement more directly linked to the short-scale surface roughness, which in turn is associated with the local surface wind stress. Using a global TOPEX σ0° data set and TOPEXs significant wave height (Hs) estimate as a surrogate for the sea states degree of development, it is also shown that differences between the two TOPEX σ0 measurements strongly evidence nonlocal sea state signature. A composite scattering theory is used to show how the dual-frequency data can provide an improved friction velocity model, especially for winds above 7 m/s. A wind speed conversion is included using a sea state dependent drag coefficient fed with TOPEX Hs data. Two colocated TOPEX-buoy data sets (from the National Data Buoy Center (NDBC) and the Structure des Echanges Mer-Atmosphre, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) campaign) are employed to test the new wind speed algorithm. A measurable improvement in wind speed estimation is obtained when compared to the monofrequency Witter and Chelton [1991] model.

Effect of Long Waves on Ku-Band Ocean Radar Backscatter at Low Incidence Angles Using TRMM and Altimeter Data

This letter uses a large ocean satellite data set to document relationships between Ku-band radar backscatter (sigmao) of the sea surface, near-surface wind speed (U), and ocean wave height (SWH). The observations come from satellite crossovers of the Tropical Rainfall Mapping Mission (TRMM) Precipitation Radar (PR) and two satellite altimeters, namely: 1) Jason-1 and 2) ENVISAT. At these nodes, we obtain TRMM clear-air normalized radar cross-section data along with coincident altimeter-derived significant wave height. Wind speed estimates come from the European Centre for Medium-Range Weather Forecast. TRMM PR is the first satellite to measure low incidence Ku-band ocean backscatter at a continuum of incidence angles from 0deg to 18deg. This letter utilizes these global ocean data to assess hypotheses developed in past theoretical and field studies.

A satellite altimeter model for ocean slick detection

About 5% of Ku-band altimeter ocean data are degraded by the occurrence of high radar return cross sections (σ0), usually called σ0 blooms. During blooms, which occur during no or low wind conditions, the mean altimeter waveform can significantly depart from the expected shape. In about 60% of the cases the waveforms are distorted to such an extent that either the range tracker loses lock or the off-nadir angle estimate becomes unrealistic. The analysis of high data rate altimeter waveforms during bloom events reveals the presence of V-shaped patterns similar to the ones observed during rain events. These patterns trace small-scale (i.e., smaller than the altimeter footprint) changes in surface backscatter. Such variations of surface roughness are commonly observed in SAR images under low wind conditions. On the basis of the experience gained through the analysis of high- resolution altimeter waveforms in the presence of rain cell, a model is developed to analyze the altimeter response to phenomena whose length scale is smaller than the altimeter footprint. The model is applied to simple patterns (linear slicks and circular patches) as well as to realistic surface σ0 estimated by SAR. It is also used to analyze bloom events in terms of surface slicks. The model results shows that the small-scale σ0 variations explain the behavior of altimeter waveforms in bloom events. The results also show that a good proportion of data during bloom events are still valid for estimating geophysical parameters as the Brown model remains valid. Use of high-resolution altimeter waveforms may also offer an interesting mean to study marine slick occurrence rates and type.