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Nicolas Viltard, Corinne Burlaud, and Christian D. Kummerow

Abstract

This study focuses on improving the retrieval of rain from measured microwave brightness temperatures and the capability of the retrieved field to represent the mesoscale structure of a small intense hurricane. For this study, a database is constructed from collocated Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and the TRMM Microwave Imager (TMI) data resulting in about 50 000 brightness temperature vectors associated with their corresponding rain-rate profiles. The database is then divided in two: a retrieval database of about 35 000 rain profiles and a test database of about 25 000 rain profiles. Although in principle this approach is used to build a database over both land and ocean, the results presented here are only given for ocean surfaces, for which the conditions for the retrieval are optimal. An algorithm is built using the retrieval database. This algorithm is then used on the test database, and results show that the error can be constrained to reasonable levels for most of the observed rain ranges. The relative error is nonetheless sensitive to the rain rate, with maximum errors at the low and high ends of the rain intensities (+60% and −30%, respectively) and a minimum error between 1 and 7 mm h−1. The retrieval method is optimized to exhibit a low total bias for climatological purposes and thus shows a high standard deviation on point-to-point comparisons. The algorithm is applied to the case of Hurricane Bret (1999). The retrieved rain field is analyzed in terms of structure and intensity and is then compared with the TRMM PR original rain field. The results show that the mesoscale structures are indeed well reproduced even if the retrieved rain misses the highest peaks of precipitation. Nevertheless, the mesoscale asymmetries are well reproduced and the maximum rain is found in the correct quadrant. Once again, the total bias is low, which allows for future calculation of the heat sources/sinks associated with precipitation production and evaporation.

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Nicolas Viltard, Estelle Obligis, Virginie Marecal, and Claude Klapisz

Abstract

The aim of this paper is to report on the retrieval of the vertically averaged liquid cloud water content and vertically averaged precipitation rates (rain and ice) from microwave airborne radiometric observations in a two-plane parallel layer atmosphere. The approach is based on the inversion of a simple radiative transfer model in which a raindrop size distribution derived from microphysical measurements is introduced. The microwave data (18.7, 21, 37, and 92 GHz) used were acquired by the Airborne Multichannel Microwave Radiometer and Advanced Microwave Moisture Sounder on board NASA DC8 within a mesoscale convective system on 6 February 1993 during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment.

Before interpreting the results, the quality of the inversion is checked. The fit between the measured and the model-retrieved brightness temperatures is good when compared to the model and measurements uncertainties. Doppler radar data from three other aircraft help the result’s interpretation, providing reflectivity and wind fields. The cloud liquid content seems to be difficult to retrieve. The ice and liquid rain rates are consistent with the other data sources: order of magnitude for convective and stratiform regions, presence of ice and liquid precipitation correlated with cell structure, and presence of cloud particles in the lighter precipitating regions.

A quantitative comparison is done between the radiometric rainfall rates and those derived from the Airborne Rain Mapping Radar observations (also on board NASA DC8). There is a good agreement between the two from the statistical point of view (mean and standard deviation values). Moreover, the finescale rain structures that appear in radar results are rather well reproduced in the radiometric results. The importance of the new drop size distribution introduced in the radiative transfer model is emphasized by this last comparison.

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Nicolas Viltard, Christian Kummerow, William S. Olson, and Ye Hong

Abstract

A combination of passive microwave and radar observations from the Tropical Rainfall Measuring Mission (TRMM) satellite is used to investigate the consistency between the two sensors. Rather than relying on some absolute “truth” to verify retrievals, this paper focuses on one assumption—namely, the drop size distribution (DSD)—and how different DSDs lead to improved or reduced consistency. Results from a case in the central Pacific suggest that a crude consistency may be achieved if a different drop size is used for the radiometer and the radar. In this particular case, a Marshall–Palmer or a gamma distribution with the shape parameters properly set leads to similar results. Although this study offers no independent validation of its conclusions, it does demonstrate that rainfall validation need not be confined to surface rainfall measurements, which are only loosely related to the volumetric observations made by most sensors. As mean size distributions of raindrops are measured in the TRMM field experiments by disdrometers, profilers, multiparameter radars, and direct aircraft observations, the technique presented in this paper can be applied on a storm-by-storm basis, and conclusions can be verified directly.

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Thomas Garot, Hélène Brogniez, Renaud Fallourd, and Nicolas Viltard

Abstract

The spatial and temporal distribution of upper-tropospheric humidity (UTH) observed by the Sounder for Atmospheric Profiling of Humidity in the Intertropics by Radiometry (SAPHIR)/Megha-Tropiques radiometer is analyzed over two subregions of the Indian Ocean during October–December over 2011–14. The properties of the distribution of UTH were studied with regard to the phase of the Madden–Julian oscillation (active or suppressed) and large-scale advection versus local production of moisture. To address these topics, first, a Lagrangian back-trajectory transport model was used to assess the role of the large-scale transport of air masses in the intraseasonal variability of UTH. Second, the temporal evolution of the distribution of UTH is analyzed using the computation of the higher moments of its probability distribution function (PDF) defined for each time step over the domain. The results highlight significant differences in the PDF of UTH depending on the phase of the MJO. The modeled trajectories ending in the considered domain originate from an area that strongly varies depending on the phases of the MJO: during the active phases, the air masses are spatially constrained within the tropical Indian Ocean domain, whereas a distinct upper-tropospheric (200–150 hPa) westerly flow guides the intraseasonal variability of UTH during the suppressed phases. Statistical relationships between the cloud fractions and the UTH PDF moments of are found to be very similar regardless of the convective activity. However, the occurrence of thin cirrus clouds is associated with a drying of the upper troposphere (enhanced during suppressed phases), whereas the occurrence of thick cirrus anvil clouds appears to be significantly related to a moistening of the upper troposphere.

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Virginie Marecal, Taoufik Tani, Paul Amayenc, Claude Klapisz, Estelle Obligis, and Nicolas Viltard

Abstract

The first part of this paper is dedicated to establishing relations among rain-integrated parameters representative of west Pacific precipitation. This is achieved by using airborne microphysical data gathered within a rain event on 6 February 1993 during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The relations between the rain rate R, the reflectivity factor Z, and the attenuation coefficient K are calculated for moderate to heavy precipitation at 13.8 GHz. They give twice as much attenuation for a given Z than the relations obtained for an exponential distribution with N 0 = 8 × 106 m−4. This effect is related to the large number of small size particles observed in TOGA COARE convective systems.

In the second part of the paper, these relations are used to check the reliability of a rain-profiling method applied to ARMAR (airborne radar-mapping radar) observations at 13.8 GHz in the same rain event. This method provides a bulk correction factor that can be interpreted primarily in terms of a change of the initial ZK relation. Then, the algorithm provides modified ZR and KR relations while assuming a gamma or an exponential-shaped distribution for raindrops with a constant N 0. For the selected case study, the adjusted relations agree very well with those derived from the microphysical measurements. An exponential shape model with constant N 0 for the DSD is found to provide results that are consistent with the microphysical measurements. Moreover, the derived N 0 value is close to that inferred from the radar algorithm. The impact of modifying the initial rain relations in the radar algorithm on the rain-rate estimates is also discussed. The retrieved rain rates are not very sensitive to the choice of initial relations except for very high values. Finally, the results are found more representative of convective rain than stratiform precipitation.

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William S. Olson, Peter Bauer, Nicolas F. Viltard, Daniel E. Johnson, Wei-Kuo Tao, Robert Meneghini, and Liang Liao

Abstract

In this study, a 1D steady-state microphysical model that describes the vertical distribution of melting precipitation particles is developed. The model is driven by the ice-phase precipitation distributions just above the freezing level at applicable grid points of “parent” 3D cloud-resolving model (CRM) simulations. It extends these simulations by providing the number density and meltwater fraction of each particle in finely separated size categories through the melting layer. The depth of the modeled melting layer is primarily determined by the initial material density of the ice-phase precipitation. The radiative properties of melting precipitation at microwave frequencies are calculated based upon different methods for describing the dielectric properties of mixed-phase particles. Particle absorption and scattering efficiencies at the Tropical Rainfall Measuring Mission Microwave Imager frequencies (10.65–85.5 GHz) are enhanced greatly for relatively small (∼0.1) meltwater fractions. The relatively large number of partially melted particles just below the freezing level in stratiform regions leads to significant microwave absorption, well exceeding the absorption by rain at the base of the melting layer. Calculated precipitation backscatter efficiencies at the precipitation radar frequency (13.8 GHz) increase with particle meltwater fraction, leading to a “bright band” of enhanced radar reflectivities in agreement with previous studies. The radiative properties of the melting layer are determined by the choice of dielectric models and the initial water contents and material densities of the “seeding” ice-phase precipitation particles. Simulated melting-layer profiles based upon snow described by the Fabry–Szyrmer core-shell dielectric model and graupel described by the Maxwell-Garnett water matrix dielectric model lead to reasonable agreement with radar-derived melting-layer optical depth distributions. Moreover, control profiles that do not contain mixed-phase precipitation particles yield optical depths that are systematically lower than those observed. Therefore, the use of the melting-layer model to extend 3D CRM simulations is likely justified, at least until more-realistic spectral methods for describing melting precipitation in high-resolution, 3D CRMs are implemented.

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