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Taoufik Tani and Paul Amayenc

Abstract

Rain measurements of the airborne Tropical Rainfall Measurement Mission (TRMM) radar simulator Airborne Rain-Mapping Radar in a typical event of Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are used to compare near-nadir rain-rate profiles retrieved from a set of deterministic attenuation-compensating algorithms. This set includes generic algorithms such as the Hitschfeld–Bordan (HB) estimate and the surface reference (SR) methods constraining the total path-integrated attenuation (PIA), and two hybrid algorithms of which one uses the same principle as the “standard” TRMM radar-profiling algorithm. In absence of reference rain data for validating the retrievals, the study is based upon an intercomparison of the results and analyzes their features in relation with theoreretical predictions. The rain data sequence is the same as used previously in a companion paper, which allowed the authors to get rain relations better adjusted to the observed rain system than those associated to a Marshall–Palmer (MP) drop size distribution. These two sets of rain relations are used here to study subsequent changes in the various rain retrievals. How comparing the results among the generic and/or the hybrid algorithms may help identify the physical assumptions and error sources in the various methods is pointed out.

With the MP relations, all methods provide almost similar retrievals in stratiform light rain. In convective heavy rain, the retrievals are largely scattered; erroneous HB-derived PIA estimates are responsible for a downward collapsing effect in the HB and the TRMM-like radar algorithms, which prevents them from recovering a credible cell-like structure. With the adjusted relations, the rain estimates from all methods are generally increased and in much better agreement. A mirror image algorithm performing PIA profiling from the surface is also exploited. The direct estimate of the surface backscatter coefficient, obtainable below very light rain only, agrees with the value measured in clear air. The one-way PIAs, immune to errors in the radar calibration and the rain relations, are found to be well correlated with those derived from SR algorithms over a 5-dB range, provided that a bulk correction factor involving these two error types be adjusted in the latter case.

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Paul Amayenc, Jean Philippe Diguet, Mongi Marzoug, and Taoufik Tani

Abstract

In Part I, four single-frequency (SF) algorithms and a dual-frequency (DF) algorithm for range profiling of the rain rate from a spaceborne radar were described and tested from numerical simulations. In Part II, performances of these algorithms are studied using data from a DF (X and Ka bands) near-nadir-pointing airborne radar. The data, gathered over ocean near Wallops Island in 1988, mimic future spaceborne radar measurements.

Rain retrievals are performed for isolated and series of contiguous rain measurements within stratiform and convective rain regions overflown by the aircraft. General features and aspects specific to the experiment conditions are pointed out. The SF algorithms provide more or less scattered results according to their own sensitivities to uncorrected scaling errors. Improvement of the algorithms stability by constraining the total path-integrated attenuation from surface echo measurements is confirmed. The correlation between attenuation coefficients at both frequencies, which forms the theoretical basis of the DF algorithm, is verified from the data. Results from DF algorithm are likely more reliable than SF counterparts since they are globally corrected for scaling errors. Rain-rate thresholds above which “attenuation” algorithms should relay ZR methods to avoid negative bias in rain-rate estimates are found near 12 mm h−1 at X band and 1 mm h−1 at Ka band for a 2.5-km rain depth. Coherent spatial structures of the rain rate within a vertical cross section of the storm are recovered from the “attenuation” algorithms.

The data obtained with a cross-range resolution L 0 ≅ 1 km are also used to perform 2D simulations of beam-averaging, and nonuniform beam-filling (NUBF) effects are used for cross-range resolutions L = 2, 3, and 4 km. Degrading the resolution produces a smoothing of small-scale rain-rate structures and a lowering of the rain-rate estimation. Bias due to NUBF depend on the involved “attenuation” algorithm. It increases with L but remains below 15%, up to L = 4 km for mean rain-rate estimates at large horizontal scale (≈100 km). The ZR methods are weakly affected by the NUBF.

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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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