F. Tridon

Signal Postprocessing and Reflectivity Calibration of the Atmospheric Radiation Measurement Program 915-MHz Wind Profilers

AbstractThe Department of Energy Atmospheric Radiation Measurement (ARM) Program has recently initiated a new research avenue toward a better characterization of the transition from cloud to precipitation. Dual-wavelength techniques applied to millimeter-wavelength radars and a Rayleigh reference have a great potential for rain-rate retrievals directly from dual-wavelength ratio measurements. In this context, the recent reconfiguration of the ARM 915-MHz wind profilers in a vertically pointing mode makes these instruments the ideal candidate for providing the Rayleigh reflectivity/Doppler velocity reference. Prior to any scientific study, the wind profiler data must be carefully quality checked. This work describes the signal postprocessing steps that are essential for the delivery of high-quality reflectivity and mean Doppler velocity products—that is, the estimation of the noise floor from clear-air echoes, the absolute calibration with a collocated disdrometer, the dealiasing of Doppler velocities, and...

Disentangling Mie and attenuation effects in rain using a Ka‐W dual‐wavelength Doppler spectral ratio technique

[1]xa0A novel technique that enables to disentangle Mie and attenuation effects in coincident, beam-matched Ka- and W-band radar observations is presented here. The ratio of the measured radar Doppler spectra at the two frequencies is estimated, and the Doppler velocity regime that corresponds to Rayleigh scatterers is determined. The range variation of the Rayleigh regime “plateau” is directly linked to the differential attenuation between the two wavelengths and does represent the attenuation component of the dual-wavelength ratio. The retrieval technique is applied to a light stratiform rain event and provides plausible results. The proposed Doppler spectral ratio methodology has potential for applications in precipitating snow, liquid and ice clouds and can be extended to other wavelength pairs.

First observations of triple-frequency radar Doppler spectra in snowfall: Interpretation and applications

The potential of multifrequency Doppler spectra to constrain precipitation microphysics has so far only been exploited for dual-frequency spectra in rain. In this study, we extend the dual-frequency concept to triple-frequency Doppler radar spectra obtained during a snowfall event which included rimed and unrimed snow aggregates. A large selection of spectra obtained from low-turbulence regions within the cloud reveals distinctly different signatures of the derived dual spectral ratios. Due to the third frequency, a characteristic curve can be derived which is almost independent of the underlying particle size distribution and velocity-size relation. This approach provides new opportunities for validating existing and future snow scattering models and reveals how the information content of triple-frequency radar data sets can be further exploited for snowfall studies.

Dual‐frequency radar Doppler spectral retrieval of rain drop size distributions and entangled dynamics variables

A novel technique based on Ka-W band dual-wavelength Doppler spectra has been developed for the simultaneous retrieval of binned rain drop size distributions (DSD) and air state parameters like vertical wind and air broadening caused by turbulence and wind shear. The rationale underpinning the method consists in exploiting the peculiar features observed in Doppler spectra caused by the wavelength dependence of scattering and absorption properties. A notional study based on a large data set of DSDs measured by a two-dimensional video disdrometer demonstrates that the retrieval performs best for small/moderate air broadening spectral width and when mean volume diameters exceed at least 1u2009mm. The retrieval is also limited to ranges below cloud base and where the signal-to-noise ratio of both radars exceed 10u2009dB, which rules out regions affected by strong attenuation. Broadly speaking, it is applicable to rain rates comprised between roughly 1 and 30u2009mmu2009h−1. Preliminary retrieval for observations at the Atmospheric Radiation Measurement Southern Great Plains site shows very good agreement with independent reflectivity measurements from a 0.915u2009GHz wind profiler. The proposed methodology shows great potential in linking microphysics to dynamics in rainfall studies.

Using a multiwavelength suite of microwave instruments to investigate the microphysical structure of deep convective cores

Abstract Due to the large natural variability of its microphysical properties, the characterization of solid precipitation is a longstanding problem. Since in situ observations are unavailable in severe convective systems, innovative remote sensing retrievals are needed to extend our understanding of such systems. This study presents a novel technique able to retrieve the density, mass, and effective diameter of graupel and hail in severe convection through the combination of airborne microwave remote sensing instruments. The retrieval is applied to measure solid precipitation properties within two convective cells observed on 23–24 May 2014 over North Carolina during the IPHEx campaign by the NASA ER‐2 instrument suite. Between 30 and 40 degrees of freedom of signal are associated with the measurements, which is insufficient to provide full microphysics profiling. The measurements have the largest impact on the retrieval of ice particle sizes, followed by ice water contents. Ice densities are mainly driven by a priori assumptions, though low relative errors in ice densities suggest that in extensive regions of the convective system, only particles with densities larger than 0.4 g/cm3 are compatible with the observations. This is in agreement with reports of large hail on the ground and with hydrometeor classification derived from ground‐based polarimetric radars observations. This work confirms that multiple scattering generated by large ice hydrometeors in deep convection is relevant for airborne radar systems already at Ku band. A fortiori, multiple scattering will play a pivotal role in such conditions also for Ku band spaceborne radars (e.g., the GPM Dual Precipitation Radar).

Multiple-scattering-induced "ghost echoes" in GPM DPR observations of a tornadic supercell

AbstractEvidence of multiple-scattering-induced pulse stretching for the signal of both frequencies of the Dual-Frequency Precipitation Radar (DPR) on the Global Precipitation Measurement (GPM) mission Core Observatory satellite is presented on the basis of collocated ground-based WSR-88D S-band observations of an extreme case: a tornadic supercell. The ground-based observations clearly show a tilted convective core with a so-called bounded weak-echo region—that is, locations where precipitation is absent or extremely light at the ground while large amounts of liquid or frozen precipitation are present aloft. The satellite observations in this region show reflectivity profiles that extend all the way to the surface despite the absence of near-surface precipitation: these are here referred to as “ghost echoes.” Furthermore, the Ku- and Ka-band profiles exhibit similar slopes, which is a typical sign that the observed power is almost entirely due to multiple scattering. A novel microphysical retrieval that ...

Validation of the GPM Version-5 Surface Rainfall Products over Great Britain and Ireland

AbstractInstantaneous surface rain rate estimates from the Global Precipitation Measurement (GPM) mission’s Dual-Frequency Precipitation Radar (DPR) and combined DPR and multifrequency microwave im...

Evaporation in action sensed by multiwavelength Doppler radars

This work documents a rain case dominated by evaporation which occurred at the Atmospheric Radiation Measurement site in Oklahoma on the 15th Sept 2011. A recently developed algorithm, applied to radar Doppler spectra measured at Ka and W band, provides the vertical evolution of binned drop size distributions (DSD) and of the vertical wind. Such retrieved quantities are used in connection with relative humidity (RH) profiles to derive evaporation rates and atmospheric cooling rates. In addition, in regions of stationarity and of light rain — when other microphysical processes are negligible, the presented case study suggests the possibility of retrieving RH profiles from the vertical evolution of the drop size distributions. The key is to characterize the gradient of the rain mass flux between successive levels. Such signal is particularly weak and can be enhanced thanks to a substantial averaging of the retrieved DSD over approximately 5 min and 250 m (eight range gates). The derived profile agrees with the retrieval from coincident Raman lidar observations within a 10% RH difference. These results suggest that other rain microphysical processes could be studied by combining the radar-based DSD retrieval with ancillary RH observations.