Danièle Hauser

A New Optical Instrument for Simultaneous Measurement of Raindrop Diameter and Fall Speed Distributions

Abstract This paper presents a recently developed optical spectro-pluviometer. The principle of the device is based upon the optical occultation of an infrared light beam by falling drops. This allows the retrieval of raindrop-size and velocity distributions. The characteristics of the instrument are described in detail. Possible errors resulting either from instrumental limitations or from environment conditions of measurement are reviewed. Preliminary results regarding a 20 min rainshower observed near Paris in September 1980 are presented and discussed. The deduced total precipitation amount is in good agreement with other independent measurements. The results reveal small but significant differences between the deduced fall velocities and Gunn and Kinzers observations. The relationship between rainfall bulk parameters (rainfall rate, water content, reflectivity factor and mean fall velocity) are obtained with small relative dispersion.

A study of the slope probability density function of the ocean waves from radar observations

[1] Radar observations of the sea surface at C-Band and small incidence angles are used to investigate some properties of the surface slope probability density function (pdf). The method is based on the analysis of the variation of the radar cross-section with incidence angle, assuming a backscattering process following the Geometrical Optics theory. First, we assess the limit of this model in our experimental configuration by using simulations of radar cross-sections with a more accurate backscattering model, namely the Physical Optics model. We show that roughness properties with scales larger than 12 cm can be analyzed in our configuration (C-Band, incidence 7 to 16). The radar data are then analyzed in terms of filtered mean square slope under the assumption of a Gaussian slope pdf. Dependence of the radar-derived mean square slopes (mss) with wind speed is analyzed, thanks to wind estimates obtained by using coincident observations of the same radar at larger incidence (around 32). Furthermore an analysis of the anisotropy of the mean square slope is proposed. The results are discussed in comparison with those of Cox and Munk (1954a, 1954b), and with the mean square slopes derived from two surface models (Elfouhaily et al., 1997 and Kudryavtsev et al., 2003). We find that the radar-derived values are in good agreement with Cox and Munk results, taking into account the filtering effect on radar-derived values. We also show that the surface model of Elfouhaily et al. yields good agreement for the omni directional mss, but a too large anisotropy of the mss. The model of Kudryavtsev provides a reasonable anisotropy of the mss, but overestimates the mss values in all directions. Finally, we propose an analysis of the radar data under a non-Gaussian assumption for the slope pdf, by applying the compound model suggested by Chapron et al. (2000) to our observations. To our knowledge, it is the first time that peakedness values are explicitly derived from radar observations, and documented as a function of azimuth and wind speed. We show that the peakedness (or kurtosis) of the slope pdf is not zero but weak (peakedness factor reaching about 0.20), and slightly increases with wind speed.

Radar scattering of the ocean surface and sea-roughness properties: A combined analysis from dual-polarizations airborne radar observations and models in C band

[1] An analysis of radar observations in C band combined with models is proposed to study some of the ocean surface properties and their relation with the sea surface backscatter. The electromagnetic part of the models is of different kinds: composite Bragg model with or without including effect of wave breaking zones on the normalized radar cross-section (NRCS), geometrical optics approximation and small-slope approximation model. The surface description is based on the wave spectrum proposed by Kudryavtsev et al. (2003), but tests with the spectrum of Elfouhaily et al. (1997) are also discussed to assess our conclusions. The originality is to use not only the NRCS in HH and VV polarizations, but also their difference in linear units. First, we show that the upwind-to-downwind anisotropy of the radar signal cannot be explained entirely by the modulation of Bragg waves by longer surface waves, but that an additional nonpolarized contribution must be invoked to explain it, consistently with scattering from zones of enhanced roughness associated with breaking waves. Then, combining a composite model and observations in the two polarizations, we assess the contribution of the nonpolarized backscatter on the total NRCS. Finally, the proposed full model, which takes into account the nonpolarized contribution over breaking zones, gives good agreement with the observed polarization ratio and with the NRCS in each polarization.

A Tropical Squall Line Observed during the COPT 81 Experiment in West Africa. Part II: Water Budget

Abstract The relative contributions of the different processes involved in the water budget of the 22 June 1981 tropical squall line are investigated. The kinematic and thermodynamic fields derived from Doppler radar data are used to calculate the sources and sinks of condensate in the convective and trailing stratiform regions of the system. Both regions play an important role in providing the surface rain. Fifty-five to 65% of the total rain precipitated at the surface is convective, while the remaining 35–45% is supplied by the trailing stratiform cloud. This partition corresponds to a precipitation efficiency of 47–57% for the convective region, and 45-57% for the stratiform region. Though these efficiencies are of the same order of magnitude, the sink of water substance in each region (before reaching the surface) is attributable to different processes. In the convective region, low of water substance is mainly due to the transfer of condensate into the trailing anvil cloud. This transfer represents ...

RESSAC: a new airborne FM/CW radar ocean wave spectrometer

The RESSAC radar system, used from aircraft platforms for measuring directional spectra of ocean waves in the gravity domain (wavelengths from 30 to 400 m), is presented. The instrument consists of a C-band (5.35-GHz) FM/CW radar system. The transmitting and receiving antennas look toward the surface at a low incidence angle ( approximately=14 degrees for the center of the antenna beam) and rotate around a vertical axis. When the data are processed, the known antenna gain pattern is removed from the recorded signal, making it possible to estimate the sea surface slope variance, which in turn is used to determine the tilt modulation transfer function, without the need of any external wind measurement. Fully normalized spectra obtained from RESSAC are compared to other data sets. >

A New Method for Deducing Hydrometeor-Size Distributions and Vertical Air Motions from Doppler Radar Measurements at Vertical Incidence

Abstract Using the hypothesis of an exponential hydrometeor-size distribution characterized by the two classical intercept (N0) and slope (λ) parameters, a method is presented for simultaneously deducing N0, λ and the vertical air velocity w from a Doppler spectrum measured at vertical incidence. It is based on a least-squares fitting of a theoretical spectrum depending on N0, λ and w, which are the adjusted parameters, to the measured one. The principle, sensitivity and limitations of the method are discussed in detail. It is expected that this method applies mainly to the case of stratiform rain.

SWIMSAT: A Real-Aperture Radar to Measure Directional Spectra of Ocean Waves from Space—Main Characteristics and Performance Simulation

Abstract The project SWIMSAT aims to measure the directional spectra of waves from space using a real-aperture radar with a low-incidence, conical-scanning beam. This system’s design is based on airborne versions developed in France and the United States. In this paper, the authors present the satellite measurement principle and instruments. For this study, the authors developed a simulation method to analyze the sensitivity of wave spectra, taking into account radar observation conditions (spatial resolution, signal-to-noise ratio, integration time, etc.) and inversion processing parameters (noise level and range displacements during temporal integration). The simulation method and results are presented in this paper. The study enabled validation and refinement of the satellite concept. The simulations show that in the chosen configuration, SWIMSAT is capable of measuring wave spectral properties in wind–sea conditions (at dominant wavelengths over approximately 70 m) and swell conditions (at significant...

CAROLS: A New Airborne L-Band Radiometer for Ocean Surface and Land Observations

The “Cooperative Airborne Radiometer for Ocean and Land Studies” (CAROLS) L-Band radiometer was designed and built as a copy of the EMIRAD II radiometer constructed by the Technical University of Denmark team. It is a fully polarimetric and direct sampling correlation radiometer. It is installed on board a dedicated French ATR42 research aircraft, in conjunction with other airborne instruments (C-Band scatterometer—STORM, the GOLD-RTR GPS system, the infrared CIMEL radiometer and a visible wavelength camera). Following initial laboratory qualifications, three airborne campaigns involving 21 flights were carried out over South West France, the Valencia site and the Bay of Biscay (Atlantic Ocean) in 2007, 2008 and 2009, in coordination with in situ field campaigns. In order to validate the CAROLS data, various aircraft flight patterns and maneuvers were implemented, including straight horizontal flights, circular flights, wing and nose wags over the ocean. Analysis of the first two campaigns in 2007 and 2008 leads us to improve the CAROLS radiometer regarding isolation between channels and filter bandwidth. After implementation of these improvements, results show that the instrument is conforming to specification and is a useful tool for Soil Moisture and Ocean Salinity (SMOS) satellite validation as well as for specific studies on surface soil moisture or ocean salinity.

Comparison of Two Methods for the Retrieval of Thermodynamic and Microphysical Variables from Doppler Radar Measurements: Application to the Case of a Tropical Squall Line

Abstract Microphysical and thermodynamic retrieval studies using a specified wind field can provide a means for analysing the different processes occurring within an observed precipitating system. Up to now, the retrieval of microphysical variable fields or thermodynamic fields have been performed separately, though the interest of associating both types of retrieval has been already noted by several authors. The research reported here presents a new retrieval method allowing consistent and simultaneous derivation of the microphysical and thermodynamic variable fields using the whole set of governing equations (momentum, thermodynamic, and microphysical equations) with the wind field specified from Doppler radar observations. The microphysical retrieval makes use of the continuity equation for the total water substance and for the precipitating substance. Two types of precipitating particles are considered (rain and graupel), and a parameterization derived from that proposed by Kessler is chosen. In pract...

A Tropical Squall Line Observed during the COPT 81 Experiment in West Africa. Part III: Heat and Moisture Budgets

Abstract The role of moist convective processes in the heat and moisture budgets of the 22 June 1981 tropical squall line is investigated. Detailed kinematic structure from Doppler radar observations, and thermodynamic and microphysical fields diagnosed from a steady-state model respectively presented in Parts I and II of this paper, are used to estimate the total apparent heat source and moisture sink. The relative contributions of the system components (the convective-scale component associated with the leading convective region, and the mesoscale component of the trailing stratiform region) and of the physical processes (latent heat release and eddy transports) are examined. Consistent results are examined which are obtained in qualitative agreement with those of previous studies. Condensation and evaporation, through the release of latent heat, are the dominant terms in the total apparent heating profile although the melting process is also important. The total apparent moisture sink is also dominated...