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The mirror image (MI) rain echo, received through the double reflection of the radar pulse from the surface, may provide useful information in estimating the rainfall rate from airborne and spaceborne weather radars. However, because of the complicated scattering mechanisms involving the surface and rain and the relatively small amount of measured data, studies of the MI effect have been few. In this paper, a more rigorous model of the MI return power has been constructed that yields the co- and cross-polarized components of the MI and bistatic returns as a functions of the radar parameters (antenna beamwidth and radar altitude) and the scattering properties of the rain and surface. As a test of the model, the mirror image return, as estimated from theory, is compared with the measured MI range profile, and reasonably good agreement is obtained. For meteorological applications, algorithms for estimation of the rain path attenuation are developed based on the difference of the direct and MI returns at the same distance from the surface. The accuracies of the algorithms are analyzed and compared with those of the surface reference technique (SRT) through the simulations for airborne and spaceborne [the case of the Tropical Rain Measuring Mission (TRMM)] geometries. Index Terms - Air/space-borne radars, attenuation, rain, scattering, TRMM.

This study investigates the characteristics of system noise sampled by the Precipitation Radar (PR) onboard the Tropical Rainfall Measuring Mission satellite. First, we examine the long-term trend in system noise. The result shows that the system noise level was quite stable for a long time period except for a few sudden change events. The averaged system noise shows a periodic change that relates to the temperature of the PR. This change corresponds to the changes in the solar beta angle, and the amplitude of the fluctuation is about 0.15 dB. The contribution of surface emission from the ocean to the system noise is also examined by averaging the system noise data under a no-rain condition over a long period (e.g., one month) to remove the effect of the temperature fluctuation related to the solar beta angle. The resulting amplitude is less than 0.1 dB, and the spatial distribution almost corresponds to the pattern of the sea surface temperature. These results confirm that the system noise of the PR is quite stable, indicating that the fluctuation of the sampled system noise is caused mainly by the fading effect. The characteristics of the system noise of three rain categories-"rain certain," "rain possible," and "no-rain"-are examined by taking a histogram. Because the PR's system noise is supposed to be stable, broadening the system noise level of about 2.5 dB almost corresponds to the fading effect. The histograms of rain-certain and of no-rain show a similar shape with a difference of a few tenths of decibels. The higher noise levels in rain-certain conditions are caused by the emission from raindrops. In contrast, the rain-possible histogram shows a widespread and skewed shape. It can be explained by saying that the rain-possible pixels are obtained when the sampled noise level is made accidentally small by the fading effect. Therefore, a fixed noise level threshold is introduced to the rain/no-rain classification for a more reliable rain-possible classification. The result shows that the rain area increases by about 15%, and the amount of rainfall increases by about 1% by adding reliable rain-possible pixels to the current standard product. [ABSTRACT FROM AUTHOR]

Assessments on the performance of dual-frequency (13.6/35.5 GHz) Precipitation Radar (DPR) rain retrieval techniques are performed by means other than relying on the surface reference technique. A DPR inversion technique (DPR-IT) with an independent estimate of the differential attenuation, i.e., the difference of attenuation differences between two frequencies over a certain range, is introduced as an alternative to surface reference or iterative methods for resolving the path-integrated attenuation (PTA) information. Retrieval performance of the proposed method is tested to some vertical rain profiles synthesized with arbitrarily defined and disdrometer-measured raindrop-size distribution data. Retrievals of two other DPR-IT-type methods, namely the DPR-IT with sets of preselected surface PIAs at two frequencies from wide ranges and the DPR-IT iterative algorithm, are also considered for the analysis. The comparison of the simulated results obtained from these three methods is presented and discussed. [ABSTRACT FROM AUTHOR]

To clarify the scattering characteristics of the ground surface for Ka band electromagnetic waves, which is necessary for evaluation of the effect of the Ka band rain radar on board the Global Precipitation Measurement Mission core satellite, the dependence of the normalized radar cross section (backscattering coefficient) σ0 of the soil on the surface roughness and the water content in the soil was measured with a Ka band polarimetric scatterometer. The results show that the dependence of the backscatter coefficient σ0 on the surface roughness, the moisture sensitivity, and the angle of incidence is obtained for the Ka band waves. For an angle of incidence of 10°,σ0 does not depend much on the surface roughness and increases in proportion to the water content. σ0 is calculated by the theoretical equation and the empirical equation, and is found accurate by comparison with the experiment. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 90(2): 1– 11, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20339 [ABSTRACT FROM AUTHOR]

Range profile data from the surface echo of the Precipitation Radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite enable the assessment of the nonuniformity of rainfall. The surface echo profile can be converted to a horizontal pattern of surface echo strength within the PR's footprint at off-nadir angles. Then, using the no-rain surface echo profile over ocean as a reference, the horizontal pattern of the path-integrated attenuation (PIA) can be estimated under moderate to heavy rainfall. Based on the horizontal pattern of the PIA, the nonuniform beam-filling parameter is estimated. However, this technique is applicable only to off-nadir angles and over ocean. Comparisons using ground-based radar data obtained simultaneously with TRMM data verify the effectiveness of this technique. [ABSTRACT FROM AUTHOR]

Use of dual-wavelength radar, with properly chosen wavelengths, will significantly lessen the ambiguities in the retrieval of microphysical properties of hydrometeors. In this paper, a dual-wavelength algorithm is described to estimate the characteristic parameters of the snow size distributions. An analysis of the computational results, made at X and Ka bands (T-39 airborne radar) and at S and X bands (CP-2 ground-based radar), indicates that valid estimates of the median volume diameter of snow particles, D0, should be possible if one of the two wavelengths of the radar operates in the non-Rayleigh scattering region. However, the accuracy may be affected to some extent if the shape factors of the gamma distribution used for describing the particle distribution are chosen far from the true values or if cloud water attenuation is significant. To examine the validity and accuracy of the dual-wavelength radar algorithms, the algorithms are applied to the data taken from the Convective and Precipitation-Electrification Experiment (CaPE) in 1991, in which the dual-wavelength airborne radar was coordinated with in situ aircraft particle observations and ground-based radar measurements. Having carefully coregistered the data obtained from the different platforms, the airborne radar-derived size distributions are then compared with the in situ measurements and ground-based radar. Good agreement is found for these comparisons despite the uncertainties resulting from mismatches of the sample volumes among the different sensors as well as spatial and temporal offsets. [ABSTRACT FROM AUTHOR]

One of the goals of the Global Precipitation Measurement project, the successor to the Tropical Rainfall Measuring Mission (TRMM), is to produce a 3-hourly global rainfall map using several spaceborne microwave radiometers. It is important, although often difficult, to classify radiometer observations over land as either “rain” or “no rain” because background land surface conditions change significantly with time and location. In this study, a no-rain brightness temperature database was created to infer land surface conditions using simultaneous observations by TRMM Microwave Imager (TMI) and precipitation radar (PR) with a resolution of 1 month and 1° latitude × 1° longitude. This paper proposes new rain/no-rain classification (RNC) methods that use the database to determine the background brightness temperature. The proposed RNC methods and the RNC method developed for the Goddard profiling algorithm (GPROF; the standard rain-rate retrieval algorithm for TMI) are applied to all TMI observations for the entire year of 2000, and the results are evaluated against the RNC made by PR as the “truth.” The first method (M1) simply uses the average brightness temperature at 85-GHz vertical polarization [denoted as TB (85 V)] under no-rain conditions as the background brightness temperature at 85-GHz vertical polarization [denoted as TBe (85 V)]. The second method (M2) uses a regression equation between TB (85 V) and TB (22 V) under no-rain conditions from the database. Here, TBe (85 V) is calculated by substituting the observed TB (22 V) into the regression equation. The ratio of accurate rain detection by GPROF to all rain occurrences detected by PR was 59%. This ratio was 57% for M1 and 63% for M2. The ratio with the weight of the rain rate was 81% for M1 and 86% for M2; it was 80% for GPROF. These comparisons were made by setting a threshold using a constant coefficient k0 to make the ratio of false rain detection to all no-rain occurrences detected by PR almost the same (approximately 0.85%) for all three methods. Further comparisons among the methods are made, and the reasons for the differences are investigated herein. [ABSTRACT FROM AUTHOR]

The Tropical Rainfall Measuring Mission (TRMM) satellite changed its altitude from 350 to 402.5 km in August 2001. As a result, the level-1 algorithm for the new orbit of the Precipitation Radar (PR) onboard the TRMM has to correct the "beam mismatch" resulting from the altitude change. Since the PR uses fixed transmission-reception timing, an attitude change of 50 km corresponds to a delay of return signals of 1 pulse repetition interval (PRI). This is not a serious problem if the angle bin of the next pulse is the same as the angle bin of the current pulse. Otherwise, return signals arrive at the PR when the antenna direction shifts to the next angle bin. This is called a "beam mismatch." It affects one pulse sample out of 32 averaged pulse samples. In other words, one "beam mismatch" pulse sample and 31 normal pulse samples are averaged at the onboard processor of the PR. A new algorithm was added to the PR's level-1 algorithm, 1B21, to eliminate this mismatch sample at the 402.5-km altitude. In this paper, the effect of beam mismatch is estimated for both rain echo and surface echo in terms of the received power and the incident angle dependency. The basic function of the beam mismatch correction algorithm is to estimate the received power of the mismatched pulse. Theoretically, the effective round-trip antenna pattern of a mismatched pulse has a peak right in the middle of the transmission and reception directions with a gain reduction of 6 dB. The new 1B21 algorithm uses the average of the received power of successive angle bins with a 6-dB gain reduction as the power from the mismatched pulse. The effectiveness of the correction algorithm was evaluated using high angular resolution data obtained during external calibration observations and the statistics of the normalized radar cross section of the earth's surface (σ°), which is thought to be unchanged. The estimated error was less than 0.2 dB for the rain echo, and a large error of up to 0.5 dB was found at the boundary of the surface based on the error estimation using high angular resolution data. The difference in σ° and its angle dependency is explained using a simple surface model. The model results indicate the correction error reaches up to 0.8 dB at the skirts of the surface echo. [ABSTRACT FROM AUTHOR]

A method to estimate the drop size distribution (DSD) parameters for dual-frequency radar observations that does not rely on the surface echoes is proposed. The method adjusts the estimates of attenuation factor at the surface iteratively until these estimates retrieve the reflectivity factors that are equal to the original measured reflectivity factors. In this iteration method, the backward-recursion equations are used as a main routine to calculate the DSD parameters for each range bin. The estimates of attenuation factor at the surface are obtained from these DSD parameters. The method is applied to the airborne radar data from the CaPE experiment in 1991. The comparison between the results derived from the surface reference technique and those derived from the proposed method is presented and discussed. [ABSTRACT FROM AUTHOR]

In developing the upcoming Global Precipitation Measurement (GPM) mission, a dual-frequency Ku-Ka-band radar system will be used to measure rainfall in such a fashion that the reflectivity ratio intrinsic to the measurement will be sensitive to underlying variations in the drop size distribution (DSD) of rain. This will enable improved techniques for retrieving rain rates, which are dependent upon several key properties of the DSD. This study examines this problem by considering a three-parameter set defined by liquid water content (W), DSD effective radius (re), and DSD effective variance (ye). Using radiative transfer simulations, this parameter set is shown to be related to a radar reflectivity factor and specific attenuation in such a fashion that details of the DSDs are immaterial under constant W, and thus effectively represent important variations in DSD that affect rain rate but with a minimal number of parameters. The analysis also examines the effectiveness of including some measure of mean Doppler fall velocity of raindrops (&vmacr;), given that the fundamental properties of a given precipitation situation are uniquely defined by a combination of a drop mass spectrum and drop vertical velocity spectrum. The results of this study have bearing on how future dual-frequency precipitation retrieval algorithms could be formulated to optimize the sensitivity to underlying DSD variability, a problem that has greatly upheld past progress in radar rain retrieval. [ABSTRACT FROM AUTHOR]

Proposes several variable pulse repetition frequency operations for the global precipitation measurement project. Formulation of the constraints of pulse repetition frequency on spaceborne weather radars that involve cross-track beam scans.

Numerical simulations of two heavy rainfall cases in the Changjiang-Huaihe River basin are performed with TRMM/PR (precipitation radar) data incorporated into the PSU/NCAR meso scale model MM5. The mixing ratio of rainwater q r is obtained from the R −q r relation ( R is the rainfall rate), and the mixing ratio of water vapor q v in the model is replaced by q 1′v = q v+ q r. Then, TRMM/PR data are used to modify humidity analysis obtained from conventional radiosonde data, and sensitivity experiments (STE) are performed and compared to control experiments (CTL). Results show that both the heavy rainfall distribution and its maximum amounts from STE are improved compared with those from CTL. [ABSTRACT FROM AUTHOR]

Satellite remote sensing is an indispensable means of measuring and monitoring precipitation on a global scale. The Tropical Rainfall Measuring Mission (TRMM) is continuing to make significant progress in helping the global features of precipitation to be understood, particularly with the help of a pair of spaceborne microwave sensors, the TRMM Microwave Imager (TMI) and precipitation radar (PR). The TRMM version-5 standard products, however, are known to have a systematic inconsistency in mean monthly rainfall. To clarify the origin of this inconsistency, the authors investigate the zonal mean precipitation and the regional trends in the hydrometeor profiles in terms of the precipitation water content (PWC) and the precipitation water path (PWP) derived from the TMI profiling algorithm (2A12) and the PR profile (2A25). An excess of PR over TMI in near-surface PWC is identified in the midlatitudes (especially in winter), whereas PWP exhibits a striking excess of TMI over PR around the tropical rainfall maximum. It is shown that these inconsistencies arise from TMI underestimating the near-surface PWC in midlatitude winter and PR underestimating PWP in the Tropics. This conclusion is supported by the contoured-frequency-by-altitude diagrams as a function of PWC. Correlations between rain rate and PWC/ PWP indicate that the TMI profiling algorithm tends to provide a larger rain rate than the PR profile under a given PWC or PWP, which exaggerates the excess by TMI and cancels the excess by PR through the conversion from precipitation water to rain rate. As a consequence, the disagreement in the rainfall products between TMI and PR is a combined result of the intrinsic bias originating from the different physical principles between TMI and PR measurements and the purely algorithmic bias inherent in the conversion from precipitation water to rain rate. [ABSTRACT FROM AUTHOR]

Describes the Tropical Rainfall Measuring Mission satellite standard algorithm that estimates the vertical profiles of attenuation-corrected radar reflectivity factor and rainfall rate. Derivation of the nonuniform beam-filling correction factors; Drop size distribution models that are consistent for convective rain over ocean.

Discusses the use of the surface reference technique for rain rate estimates from the precipitation radar (PR) aboard the Tropical Rainfall Measuring Mission satellite. Assessment of the reliability of the technique; Path-attenuation estimates and reliability; Characteristics of the surface cross sections.

The dual-frequency ratio of radar reflectivity factors (DFR) has been shown to be a useful quantity as it is independent of the number concentration of the particle size distribution and primarily a function of the mass-weighted particle diameter Dm. A drawback of DFR-related methods for rain estimation, however, is the nonunique relationship between Dm and DFR. At Ku- and Ka-band frequencies, two solutions for Dm exist when DFR is less than zero. This ambiguity generates multiple solutions for the range profiles of the particle size parameters. We investigate characteristics of these solutions for both the initial-value (forward) and final-value (backward) forms of the equations. To choose one among many possible range profiles of Dm, number concentration, and rain rate R, independently measured path attenuations are used. For the backward approach, the possibility exists of dispensing with externally measured path attenuations by achieving consistency between the input and output path attenuations. The methods are tested by means of a simulation based on disdrometer-measured raindrop size distributions and the results are compared with a simplified version of the operational R–Dm method. [ABSTRACT FROM AUTHOR]

With the use of 213,456 one-minute measured data of droplet-size distribution (DSD) of rain collected during several National Aeronautics and Space Administration (NASA)-sponsored field campaigns, the relationships between rainfall rate R, mass-weighted diameter Dm and normalized intercept parameter Nw of the gamma DSD are studied. It is found, based on the simulations of the gamma DSD model, that R, Dm and Nw are closely interrelated, and that the ratio of R to Nw is solely a function of Dm, independent of the shape factor μ of the gamma distribution. Furthermore, the model-produced ratio agrees well with those from the DSD data. When a power-law equation is applied to fit the model data, we have: R = aN w D m b , where a = 1.588 × 10 − 4 , b = 4.706 . Analysis of two-parameter relationships such as R–Dm, Nw–R and Nw–Dm reveals that R and Dm are moderately correlated while Nw and Dm are negatively correlated. Nw and R, however, are uncorrelated. The gamma DSD model also reveals that variation of R–Dm relation is caused primarily by Nw. For the application of the Ku- and Ka-band dual-frequency radar for the retrieval of the DSD bulk parameters as well as the specific radar attenuations, the study is carried out to relate the dual-frequency radar reflectivity factors to the DSD and attenuation parameters. [ABSTRACT FROM AUTHOR]