Larry F. Bliven

Radar signature of the sea surface perturbed by rain

The attenuation and reflectivity of drops falling through the atmosphere affects the radar signature of the sea surface. Another, much less documented, effect is the perturbation of the surface roughness due to the impacting drops. It is the subject of the present paper. As for the surface description, the present authors refer to the rain-only experiments performed by Bliven et al. (1997) at the NASA/WallopslRain-Sea Interaction Facility. These experiments consisted of wire-probe measurements of a water surface perturbed by rain, which was generated by a simulator with 2.8 mm drops placed at 13 m height; this height enabled the drops to fall practically at terminal velocity. The observed frequency spectrum S/sub f/ has been modelled by a log-Gaussian shape and then transformed to an isotropic surface spectrum with the help of the dispersion relationship of surface waves w(K). The present authors decided to introduce two minor improvements in this model. First, for very small rain rates, the surface roughness tends toward the superposition of independent ring-waves. Hence, they decided to linearize the peak value of the spectrum S/sub f,peak/ between O and 5 mm hr/sup -1/. Secondly, they introduced an exponential law to represent the high frequency tail (f>12 Hz) of the rain spectrum. As for the wind-only spectrum the used model is the fully-developed limit of the model proposed by Lemaire et al. In a first approach, they assume that the rain spectrum simply adds to the wind-only spectrum. The related nonlinearities are described.

A Ku-band laboratory experiment on the electromagnetic bias

A uniformly valid algorithm relating the normalized (or nondimensional) sea-surface electromagnetic (EM) bias to physical variables has not yet been established. Laboratory experiments are conducted to guide model development. Simultaneous and collocated measurements of surface topography and altimeter backscattered power are made in a wind-wave facility for a wide range of wind and mechanically generated wave conditions. A small microwave footprint on the water surface is produced by a focused-beam 13.5-GHz radar system that has a high signal-to-noise ratio. Specular facets are easily identifiable, and the data show that troughs are on average better reflectors than crests. The laboratory coefficients are considerably greater than those of in situ algorithms, and when the normalized EM bias is displayed as a function of wave height skewness or wave age, laboratory and field data converge into consistent trends. A two-parameter model is proposed using a nondimensional wave height, which is computed for local winds, and a significant slope, which is computed for nonlocally generated waves. Analysis of the laboratory data shows that the normalized EM bias for mixed conditions is well modeled as a product of these two parameters. >

Microphysical Properties of Snow and Their Link to Ze–S Relations during BAECC 2014

AbstractThis study uses snow events from the Biogenic Aerosols–Effects on Clouds and Climate (BAECC) 2014 campaign to investigate the connection between properties of snow and radar observations. The general hydrodynamic theory is applied to video-disdrometer measurements to retrieve masses of falling ice particles. Errors associated with the observation geometry and the measured particle size distribution (PSD) are addressed by devising a simple correction procedure. The value of the correction factor is determined by comparison of the retrieved precipitation accumulation with weighing-gauge measurements. Derived mass–dimensional relations are represented in the power-law form m = . It is shown that the retrieved prefactor am and exponent bm react to changes in prevailing microphysical processes. From the derived microphysical properties, event-specific relations between the equivalent reflectivity factor Ze and snowfall precipitation rate S (Ze = ) are determined. For the studied events, the prefactor o...

Rain rate measurement with an airborne scanning radar altimeter

The NASA Scanning Radar Altimeter (SRA) sweeps a radar beam of 1/spl deg/ half-power width (two-way) across the aircraft ground track within /spl plusmn/22/spl deg/ of nadir, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 points spaced across the swath at 0.7/spl deg/ incidence angle intervals. Flights were made into Hurricane Humberto on 23 and 24 September 2001, aboard a WP-3D hurricane research aircraft of the NOAA Aircraft Operations Center. The ranges produce raster lines of sea surface topography at a 10 Hz rate. The SRA was primarily designed to produce sea surface directional wave spectra, but the backscattered power measurements can be used to determine path integrated rain rate below the aircraft.

Retrieval of Snow Properties for Ku- and Ka-Band Dual-Frequency Radar

The focus of this study is on the estimation of snow microphysical properties and the associated bulk parameters such as snow water content and water equivalent snowfall rate for Ku- and Ka-band dual-frequency radar. This is done by exploring a suitable scattering model and the proper particle size distribution (PSD) assumption that accurately represent, in the electromagnetic domain, the micro/macro-physical properties of snow. The scattering databases computed from simulated aggregates for small to moderate particle sizes are combined with a simple scattering model for large particle sizes to characterize snow scattering properties over the full range of particle sizes. With use of the single scattering results, the snow retrieval look-up tables can be formed in a way that directly links the Ku- and Ka-band radar reflectivities to snow water content and equivalent snowfall rate without use of the derived PSD parameters. A sensitivity study of the retrieval results to the PSD and scattering models is performed to better understand the dual-wavelength retrieval uncertainties. To aid in the development of the Ku- and Ka-band dual-wavelength radar technique and to further evaluate its performance, self-consistency tests are conducted using measurements of the snow PSD and fall velocity acquired from the Snow Video Imager/Particle Image Probe (SVI/PIP) during the winter of 2014 in the NASA Wallops flight facility site in Wallops Island, Virginia.

Comparison between electromagnetic scattering by a rain induced sea surface roughness and field data

Scattering cross-sections for a rough sea surface in presence of wind and rain are simulated at oblique incidences and for various environmental conditions. They are compared with Ku-band radar signatures available from the literature.