Profiles Of Raindrop Size Distributions Research Articles

Comparison of vertical profile of raindrop size distribution from micro rain radar with global precipitation measurement over Western Java Island

The NASA-JAXA Global Precipitation Measurement (GPM) mission core satellite, launched in February 2014, carries the Dual-Frequency Precipitation Radar (DPR), which retrieves the three-dimensional structure of precipitation from space, including raindrop size distribution (DSD) parameters: mass-weighted mean diameter (Dm, in mm) and normalized DSD scaling parameter for concentration (Nw, in mm−1 m−3). This study compares DSD's vertical structure from the observations of GPM and ground-based radar (24 GHz Micro Rain Radar MRR) for the first time in Indonesia, precisely in Serpong (6.359°S, 106.673°E). The vertical profile of radar reflectivity, Dm, and Nw gradient was used to express the vertical gradient of DSD in rain columns (0.75–3 km). The gradient was calculated using linear regression as a function of parameters and height. Negative values indicated a downward decrease (DD) of parameters toward the surface; while, positive rates indicated a downward increase (DI) pattern. The DI of radar reflectivity from MRR was found more dominant than that of GPM Ku-band and Ka-band, with DI/DD ratios for MRR, Ku-band, and Ka-band at 7.2, 1.1, and 2.2, respectively. The three observations showed that the DI/DD ratio for convective rain was larger than that of stratiform rain. Most of the rain profiles from GPM showed a DI gradient for Nw, with a DI/DD ratio of 1.9, different from the MRR with the ratio of 0.6 only, indicating much more dominantion of DD. Furthermore, the DI/DD ratio for Dm from GPM observation was 0.8, while for MRR it was 4.55. Thus, the vertical structure of DSD from MRR was different from the one obtained by GPM, especially in convective rain. Besides the difference in profile trends, the Nw value of MRR is much larger than GPM; on the other hand, the Dm value of MRR was found much lower. Although there were some differences in DSD's vertical structure between GPM and MRR, both instruments were able to capture seasonal and diurnal variations of the DSD parameters.

Stochastic simulation experiment to assess radar rainfall retrieval uncertainties associated with attenuation and its correction

Abstract. As rainfall constitutes the main source of water for the terrestrial hydrological processes, accurate and reliable measurement and prediction of its spatial and temporal distribution over a wide range of scales is an important goal for hydrology. We investigate the potential of ground-based weather radar to provide such measurements through a theoretical analysis of some of the associated observation uncertainties. A stochastic model of range profiles of raindrop size distributions is employed in a Monte Carlo simulation experiment to investigate the rainfall retrieval uncertainties associated with weather radars operating at X-, C-, and S-band. We focus in particular on the errors and uncertainties associated with rain-induced signal attenuation and its correction for incoherent, non-polarimetric, single-frequency, operational weather radars. The performance of two attenuation correction schemes, the (forward) Hitschfeld-Bordan algorithm and the (backward) Marzoug-Amayenc algorithm, is analyzed for both moderate (assuming a 50 km path length) and intense Mediterranean rainfall (for a 30 km path). A comparison shows that the backward correction algorithm is more stable and accurate than the forward algorithm (with a bias in the order of a few percent for the former, compared to tens of percent for the latter), provided reliable estimates of the total path-integrated attenuation are available. Moreover, the bias and root mean square error associated with each algorithm are quantified as a function of path-averaged rain rate and distance from the radar in order to provide a plausible order of magnitude for the uncertainty in radar-retrieved rain rates for hydrological applications.

DEVELOPMENT OF A METHOD TO ESTIMATE OF DROP SIZE DISTRIBUTION ABOVE THE MELTING LAYER BY USING 400MHz-BAND WIND PROFILER AND COBRA

The vertical profile of raindrop size distribution (DSD) is important for understanding rain processes. To improve the accuracy of rainfall intensity observed by precipitation radar and microwave radiometers carried on satellites, the information on the vertical profile of DSD is also important. The purpose of this study is to develop a database of DSD for the satellites precipitation retrieval by using a 400MHz-band wind profiler and COBRA (CRL Okinawa Bistatic polarimetric Radar) for various kinds of hydrometer (rain, snow, and mix phase). A new algorithm is proposed above the melting layer. As a result, in the case that the spectrum peak of air motion is difficult to be found from the observed Doppler spectrum above the melting layer, it is possible that DSD can be estimated by the proposed algorithm.

Quantitative analysis of X-band weather radar attenuation correction accuracy

Abstract. At short wavelengths, especially C-, X-, and K-band, weather radar signals are attenuated by the precipitation along their paths. This constitutes a major source of error for radar rainfall estimation, in particular for intense precipitation. A recently developed stochastic simulator of range profiles of raindrop size distributions (DSD) provides a controlled experiment framework to investigate the accuracy and robustness of attenuation correction algorithms. The work presented here focuses on the quantification of the influence of uncertainties concerning radar calibration, the parameterization of power law relations between the integral variables (radar reflectivity Z and specific attenuation k), and total path integrated attenuation (PIA) estimates at X-band. The analysis concerns single frequency, incoherent and non-polarimetric radar systems. Two attenuation correction algorithms, based on a forward and a backward implementation respectively, are studied. From DSD range profiles, the corresponding profiles of integral radar variables are derived. Using a Monte Carlo approach, the accuracy and robustness of the two algorithms are quantified for the different sources of error previously mentioned. This framework of realistic DSD variability provides a robust way to confirm that, under realistic assumptions concerning the PIA estimation uncertainty, the forward algorithm outperforms the backward algorithm for PIA values below 10 dB.

DEVELOPMENT OF ESTIMATION OF DROP SIZE DISTRIBUTION BY USING 400MHz-BAND WIND PROFILER ABOVE THE MELTING LAYER

The vertical profile of raindrop size distribution (DSD) is important for understanding rain formation processes. To improve the accuracy of rainfall intensity observed by precipitation radar and microwave radiometers carried on satellites, the information on the vertical profile of DSD is also important. The purpose of this study is to develop a database of DSD form rainfall (below the melting layer) to snowfall (above the melting layer) for the satellites precipitation retrieval by using a 400MHz wind profiler. A new algorithm is proposed above the melting layer. As a result, in the case of the spectrum peak of air motion is difficult to find from the observed Doppler spectrum above the melting layer, it is possible that DSD can be estimated by the proposed algorithm.

Profiles of Raindrop Size Distributions as Retrieved by Microrain Radars

Abstract Data of vertically pointing microrain radars (MRRs), located at various sites around the Baltic Sea, were analyzed for a period of several years. From the Doppler spectra profiles of drop size distributions (DSDs) are obtained. A significant height dependence of the shape of the DSDs—and thus of the Z–R relations—is observed at high rain rates. This implies, for the considered sites, that ground-based Z–R relations lead to underestimation of high rain rates by weather radars.

A stochastic model of range profiles of raindrop size distributions: Application to radar attenuation correction

To analyze the influence of the spatial variability of the raindrop size distribution (DSD) on rainfall estimation using weather radar, a stochastic model is proposed in order to simulate range profiles of DSDs and consequently profiles of rainfall intensity, radar reflectivity and specific attenuation. A first application concerning the accuracy of attenuation correction algorithms is presented. Using a Monte Carlo technique, it is shown that an attenuation correction algorithm based on a forward implementation is more sensitive to the inadequacy of power‐law relations than a backward algorithm, the latter being sensitive to the uncertainty of the path integrated attenuation (PIA). The sensitivity to spatial heterogeneity is limited for both algorithms. The corrected profiles remain uncertain because they are based on deterministic power‐law relations which are not fully consistent with the stochastic nature of the DSD.

Retrieval of Arbitrarily Shaped Raindrop Size Distributions from Wind Profiler Measurements

Abstract An efficient iterative retrieval method for arbitrarily shaped raindrop size distributions (ITRAN) is developed for Doppler spectra measured with a wind profiler. A measured Doppler spectrum is a convolution of the precipitation spectrum and the turbulent spectrum. Deconvolution of the Doppler spectra is achieved through repeated convolutions. The developed method assumes no prior shape of drop size distributions and automatically obtains raindrop size distributions; additionally, it can be applied to large data volumes. Furthermore, it is insensitive to initial values. The method was applied to both simulated and observed spectra. Derived drop size distributions agree with simulated values. Narrower turbulent spectral widths yield better results. Integral values of median volume diameter (D0), liquid water content (LWC), and radar reflectivity factor are estimated with errors of less than 10%. Accurate vertical profiles of raindrop size distributions result when this method is applied to wind profiler data. The technique performed very well with most observed spectra. Some recovered spectra departed from the corresponding measured spectra, for cases in which a clear-air peak could not be accurately reproduced because of uncertainties in the location of the minimum position between the clear-air echo and the precipitation echo. Statistical relationships between LWC and integral rainfall parameters yield interesting features. The median volume diameter is statistically independent of the LWC and is associated with the large variability of the total number of drops, NT, between events. Vertical profiles from one event show a clear inverse relationship between NT and D0