Showing papers on "Disdrometer published in 1999"

Systematic variation of drop size and radar-rainfall relations

Abstract: Time histories of the characteristics of the drop size distribution of surface disdrometer measurements collected at Kapingamarangi Atoll were partitioned for several storms using rain rate R, reflectivity factor Z, and median diameter of the distribution of water content D0. This partitioning produced physically based systematic variations of the drop size distribution (DSD) and Z-R relations in accord with the precipitation types viewed simultaneously by a collocated radar wind profiler. These variations encompass the complete range of scatter around the mean Z-R relations previously reported by Tokay and Short [1996] for convective and stratiform rain and demonstrate that the scatter is not random. The systematic time or space variations are also consistent with the structure of mesoscale convective complexes with a sequence of convective, transition, and stratiform rain described by various authors. There is a distinct inverse relation between the coefficient A and the exponent of the Z-R relations which has been obscured in prior work because of the lack of proper discrimination of the rain types. Contrary to previous practice it is evident that there is also a distinct difference in the DSD and the Z-R relations between the initial convective and the trailing transition zones. The previously reported Z-R relation for convective rain is primarily representative of the transition rain that was included in the convective class. The failure of present algorithms to distinguish between the initial convective and the trailing transition rains causes an erroneous apportionment of the diabatic heating and cooling and defeats the primary intent of discriminating stratiform from convective rains.

Tropical rainfall associated with convective and stratiform clouds : Intercomparison of disdrometer and profiler measurements

Abstract: The motivation for this research is to move in the direction of improved algorithms for the remote sensing of rainfall, which are crucial for meso- and large-scale circulation studies and climate applications through better determinations of precipitation type and latent heating profiles. Toward this end a comparison between two independent techniques, designed to classify precipitation type from 1) a disdrometer and 2) a 915-MHz wind profiler, is presented, based on simultaneous measurements collected at the same site during the Intensive Observing Period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment. Disdrometer-derived quantities such as differences in drop size distribution parameters, particularly the intercept parameter N0 and rainfall rate, were used to classify rainfall as stratiform or convective. At the same time, profiler-derived quantities, namely, Doppler velocity, equivalent reflectivity, and spectral width, from Doppler spectra were used to classify precipitation type in four categories: shallow convective, deep convective, mixed convective‐stratiform, and stratiform. Overall agreement between the two algorithms is found to be reasonable. Given the disdrometer stratiform classification, the mean profile of reflectivity shows a distinct bright band and associated large vertical gradient in Doppler velocity, both indicators of stratiform rain. For the disdrometer convective classification the mean profile of reflectivity lacks a bright band, while the vertical gradient in Doppler velocity below the melting level is opposite to the stratiform case. Given the profiler classifications, in the order shallow‐deep‐mixed‐stratiform, the composite raindrop spectra for a rainfall rate of 5 mm h 21 show an increase in D0, the median volume diameter, consistent with the dominant microphysical processes responsible for drop formation. Nevertheless, the intercomparison does reveal some limitations in the classification methodology utilizing the disdrometer or profiler algorithms in isolation. In particular, 1) the disdrometer stratiform classification includes individual cases in which the vertical profiles appear convective, but these usually occur at times when the disdrometer classification is highly variable; 2) the profiler classification scheme also appears to classify precipitation too frequently as stratiform by including cases that have small vertical Doppler velocity gradients at the melting level but no bright band; and 3) the profiler classification scheme includes a category of mixed (stratiform‐convective) precipitation that has some features in common with deep convection (e.g., enhanced spectral width above the melting level) but other features in common with stratiform precipitation (e.g., well-developed melting layer signature). Comparison of the profiler-derived vertical structure with disdrometer-determined rain rates reveals that almost all cases of rain rates greater than 10 mm h 21 are convective. For rain rates less than 5 mm h21 all four profiler-determined precipitation classes are well represented.

On the Performance of a Low-Cost K-Band Doppler Radar for Quantitative Rain Measurements

Abstract: This paper deals with the ability of a vertically pointing, FM–CW Doppler radar to measure quantitatively raindrop size distributions and rainfall intensity. The wavelength of the radar is 12.5 mm (K band). To improve estimates of the rainfall intensity, radar-received noise, Mie scatter, and radar calibration corrections are applied to the radar data. The electronic noise correction rendered the radar particle size retrievals below 0.7-mm drop diameters invalid; exponential extrapolation of the spectrum below drop diameters of 0.7 mm decreased the rain intensity up to 20%. Proper Mie corrections to the particle backscatter cross sections decreased the rain intensity between 5% and 30%. Calibration of the radar against a Joss–Waldvogel disdrometer led to a decrease in rain intensity of 49%. The electronic noise, Mie scatter, and calibration corrections yielded a correlation coefficient of 0.94 in a comparison of the radar and disdrometer data for a case study. The daily rain sums over half a year...

Relative Performance of Automatic Rain Gauges under Different Rainfall Conditions

Abstract: Six different types of automatic rain gauges, including tipping bucket, weighing, capacitance, optical, disdrometer, and acoustical sensors, were deployed for 17 months (September 1993–January 1995) at the NOAA Atlantic Oceanographic and Meteorological Laboratory in Miami, Florida. Different rainfall conditions encountered during the experiment included wintertime stratiform frontal rainfall, intense springtime convective systems with extremely high rainfall rates (over 100 mm h−1), summertime convective storms, mesoscale convective systems in the rainy season (September–October), and one tropical storm (Tropical Storm Gordon). Overall, all of the rain gauges performed well, with intercorrelations of order 0.9 or better using 1-min rainfall rates and biases of less than 10%; however, each showed limitations under different rainfall situations. In particular, under extremely heavy rainfall rates (over 100 mm h−1), the disdrometer and tipping bucket rain gauges biased low, while the optical rain ga...

Fluctuation Properties of Precipitation. Part IV: Finescale Clustering of Drops in Variable Rain

Abstract: In recent studies it is shown that in variable rain the spatial distribution of drops is not Poissonian. However, these past studies were limited to 1-min drop counts, which likely correspond to spatial scales of a few hundred to several hundreds of meters. In this work results based on 1-s drop counts using a video disdrometer are reported. It is shown that the clustering of raindrops previously found during intervals of 1 min also occurs during 1 s as well in convective rain. These latter temporal scales likely correspond to spatial features having dimensions from only a few to tens of meters. Combined with the authors’ earlier results, these findings suggest that clustering of raindrops and meteorological variability span the range of scales from at least as small as a few meters to several hundreds of meters in convective precipitating systems. Consequently, non-Poissonian clustering reported in previous work (analyzing data accumulated over hours using 1-min drop counts) cannot be dismissed ...

Towards a stochastic model of rainfall for radar hydrology : testing the poisson homogeneity hypothesis

Abstract: In order to investigate to what extent rainfall fluctuations observed with different types of instruments reflect the properties of the rainfall process itself and to what extent they are merely instrumental artefacts we are in the process of developing a stochastic model of rainfall. The starting point for the development of the model has been the notion that at the spatial and temporal scales associated with many types of surface rainfall measurements, rainfall is a discrete process describing the arrival of raindrops of different sizes at the ground. A fundamental question is whether this raindrop arrival process can be considered a homogeneus (Poisson) process or whether it behaves as a clustering (or possibly even scaling) process, as has recently been proposed in the litereture. We have tested the classical Poisson homogeneity hypothesis in rainfall on a 35 min time series of 10 s raindrop size spectra collected with a 50 cm2 disdrometer. The rain rates calculated from the spectra indicated roughly uncorrelated fluctuations around a constant mean rain rate of about 3.5 mm h−1. Two types of analysis of the drop counts were carried out, a global analysis taking into account all drops regardless of their size and an analysis considering the drop counts in the 16 0.21 mm diameter intervals separately. The first type of analysis revealed that evene for the more or less stationary time series under consideration the total raindrop arrival rate was overdispersed with respect to the homogeneous Poisson process. The second type of analysis demonstrated that this rejection of the homogeneity hypothesis could be attributed entirely to raindrops with diameters smaller than 1.14 mm. Although these drops account for 66% of the raindrop concentration in the air and 55% of the raindrop arrival rate at the ground, they only account for 14% of the rain rate and 2% of the radar reflectivity factor (on the basis of the mean drop size distribution during the experiment). In order words, although clustering may be a significant phenomenon for the smallest raindrops, the analyzed data seem to indicate that for moderate rain rates the arrival rate fluctuations of the raindrops which contribute most to the rain rate and radar reflectivity factor behave according to Poisson statistics.

High-Resolution Measurement of a Hail Region by Vertically Pointing Doppler Radar

Abstract: The precipitation and structure of a hail-producing region embedded within a severe squall line are investigated by combining data simultaneously measured by vertically pointing and volume-scanning Doppler radar. These data are complemented by surface measurements made, at the location of the vertically pointing radar, with a Joss–Waldvogel disdrometer. The vertically pointing radar measured the standard radar data fields (Z, υυ, συ) and the power spectrum of the vertical Doppler velocities. Once the timescale is converted to a spatial scale, based on an estimate of the propagation speed and direction of the storm, the horizontal resolution of the data is less than ∼100 m for Z, υυ, and συ, and less than ∼200 m for power spectra. Spatial resolution in the vertical direction is 250 m for the Z, υυ, συ data and 2.25 km for the power spectra data. The vertical scan measurements were made directly within the weak echo region associated with the hailfall observed at the radar site. These high-resoluti...

Ka Band Propagation Experiments of Experimental Communication Payload (ECP) on ROCSAT-1 - Preliminary Results

Abstract: It is recognized that rain attenuation is the primary factor in the degradation of Earth-satellite communication at the Ka band frequency. The beacon signal of the ROCSAT-1 is set at 19.5 GHz for downlink and 28.5 GHz for uplink. ROCSAT-1 is the low earth orbit (LEO) satellite with a circular orbit at the altitude of 600 km and 35° inclination angle and scheduled to be launched at the beginning of 1999. Given the extremely high frequency of the beacon, impairment of ROCSAT-1 communications due to rain attenuation should be seriously considered. In this paper, the ground-based instruments for the Ka band propagation experiments of ROCSAT-1, including Chung-Li VHF radar, 19.5 GHz radiometer, optical rain gauge, automatic weather station, and disdrometer, are introduced. The spatial distribution of the long-term statistics of rainfall rate is analyzed in this paper on the basis of 8 years (1988-1995) rainfall rate data at one-minute time resolution, recorded by more than 70 tipping bucket rain gauges distributed over Taiwan island. It shows a pronounced latitudinal variation in the percentage of time that the rainfall rate exceeds a specified level, indicating that more severe rain attenuation will be encountered in the southern part than that in the northern part of Taiwan. In addition, the sky noise temperature at the frequency of 19.5 GHz is measured by using a radiometer, both in the conditions of clear-air and precipitation. The observed sky noise temperature in the case of clear-air at the elevation angles of 90°, 60°, 30°, and 15° are respectively 50K, 80K, 100K, and 130K, corresponding to the attenuations of 0.7dB, 0.78dB, 1.2dB, and 2.7dB. Data analysis indicates that the observed clear-air sky noise temperature increasing exponentially with the decrease of the zenith angle is in perfect agreement with our theoretical prediction. The sky noise temperature in the case of precipitation is also investigated. A comparison between observed precipitation sky noise temperature and surface rainfall rate shows that a salient time shift in the two is seen, implying that great caution should be taken in establishing an empirical relationship between precipitation sky noise temperature and surface rainfall rate.

Dependence of Z-R Relations on the Rain Type Classification Scheme

Abstract: The TRMM Global Validation Program is giving us a unique opportunity to compare radar datasets from different sites since they are analyzed in a relatively uniform procedure. Monthly Ze-R relations for four different sites (i.e, Melbourne Florida, Houston Texas, Darwin Australia and Kwajalein Atoll) were derived. The relations were obtained using the Window Probability Matching Method (WPMM). This version of the PMM relies on matching unconditional probabilities of rain rates, R, and radar reflectivity, Ze, using rain gauge and radar data, respectively. This procedure was done separately for convective and stratiform rain type using the Steiner classification procedure. The radar and gauge data from all sites were quality controlled using the same algorithms, which include also an automatic procedure to filter unreliable rain gauge data upon comparison to radar data. An adjusted power law Z-R for each rain type was also derived by comparing the radar-gauge coincident pairs in order to adjust the total monthly rainfall to match the gauges. The obtained PMM based Ze-R relations are found to be curved lines in log-log space rather than any straight line power law. While the PMM based Ze-R curves were always distinctly different between the convective and stratiform rain, the power law based Z-R, in few cases, was found to be the same for both types. In general, a given reflectivity was matched to a much lower rain intensity in the convective rainfall as compared to that in stratiform rainfall. These findings are inherently contradictory to previous findings based on disdrometer data and suggest some precaution for using the latter Z-R relations on radar data when the partition of stratiform and convective rainfall amount is in concern. The inverse trends in the relations might be caused by effects such as partial beam fillings, the use of different classification schemes, as well as having a distinct difference in the Z-R relations between the initial convective and the trailing transition regions as suggested by recent findings.

NOTES AND CORRESPONDENCE On the Performance of a Low-Cost K-Band Doppler Radar for Quantitative Rain Measurements

Abstract: This paper deals with the ability of a vertically pointing, FM‐CW Doppler radar to measure quantitatively raindrop size distributions and rainfall intensity. The wavelength of the radar is 12.5 mm (K band). To improve estimates of the rainfall intensity, radar-received noise, Mie scatter, and radar calibration corrections are applied to the radar data. The electronic noise correction rendered the radar particle size retrievals below 0.7-mm drop diameters invalid; exponential extrapolation of the spectrum below drop diameters of 0.7 mm decreased the rain intensity up to 20%. Proper Mie corrections to the particle backscatter cross sections decreased the rain intensity between 5% and 30%. Calibration of the radar against a Joss‐Waldvogel disdrometer led to a decrease in rain intensity of 49%. The electronic noise, Mie scatter, and calibration corrections yielded a correlation coefficient of 0.94 in a comparison of the radar and disdrometer data for a case study. The daily rain sums over half a year derived from radar measurements at a height of 100 m above ground were on average 12% higher than measurements with a conventional Hellmann rain gauge.