2D Video Disdrometer Research Articles

Raindrop size distributions of convective rain over equatorial Indonesia during the first CPEA campaign

As part of the Coupling Processes in the Equatorial Atmosphere (CPEA) campaign, precipitation was observed at Kototabang (0.20°S, 100.32°E), Indonesia, by a 2D-Video Disdrometer (2DVD), a 1.3-GHz wind profiler, an Equatorial Atmosphere Radar (EAR) and a Mobile Automatic Weather Station (MAWS). The campaign period is from April 10 to May 9, 2004. This paper presents a comprehensive follow-up of the previous study on the intraseasonal variation (ISV) of raindrop size distribution (DSD) during this campaign. The data were partitioned into stratiform, mixed stratiform/convective, deep convective and shallow convective. Besides the DSD parameters of gamma distribution, Δ Z MP defined as the difference between a measured radar reflectivity and that from Marshall and Palmer's model was also analyzed. Intraseasonal variation of Δ Z MP was only found at heavy rain. Consistent with the previous study, during the inactive phase, Δ Z MP s were generally positive (broad DSD) and decrease toward negative values (narrow DSD) as the phase of ISV shifts to active ones. We found that a broad DSD and skewed towards higher drop diameters during the inactive ISV phase came mainly from deep convective, while a narrow DSD during the active phase came mainly from shallow convective events. Like surface DSD, mass-weighted mean diameter ( D m ) revealed from the EAR measurement was also larger during the inactive phase (deep convective) than that for the active phase (shallow convective). In general, we found that vertical profile of reflectivity gradient (VPRG) from 1.3-GHz wind profiler observations during the inactive phase was larger than for the active phase. This phenomenon may be due to significant coalescence process during the inactive phase.

Characteristics of the Raindrop Size Distribution and Drop Shape Relation in Typhoon Systems in the Western Pacific from the 2D Video Disdrometer and NCU C-Band Polarimetric Radar

Abstract The drop size distribution (DSD) and drop shape relation (DSR) characteristics that were observed by a ground-based 2D video disdrometer and retrieved from a C-band polarimetric radar in the typhoon systems during landfall in the western Pacific, near northern Taiwan, were analyzed. The evolution of the DSD and its relation with the vertical development of the reflectivity of two rainband cases are fully illustrated. Three different types of precipitation systems were classified—weak stratiform, stratiform, and convective—according to characteristics of the mass-weighted diameter Dm, the maximum diameter, and the vertical structure of reflectivity. Further study of the relationship between the height H of the 15-dBZ contour of the vertical reflectivity profile, surface reflectivity Z, and the mass-weighted diameter Dm showed that Dm increased with a corresponding increase in the system depth H and reflectivity Z. An analysis of DSDs retrieved from the National Central University (NCU) C-band polarimetric radar and disdrometer in typhoon cases indicates that the DSDs from the typhoon systems on the ocean were mainly a maritime convective type. However, the DSDs collected over land tended to uniquely locate in between the continental and maritime clusters. The average mass-weighted diameter Dm was about 2 mm and the average logarithmic normalized intercept Nw was about 3.8 log10 mm−1 m−3 in typhoon cases. The unique terrain-influenced deep convective systems embedded in typhoons in northern Taiwan might be the reason for these characteristics. The “effective DSR” of typhoon systems had an axis ratio similar to that found by E. A. Brandes et al. when the raindrops were less than 1.5 mm. Nevertheless, the axis ratio tended to be more spherical with drops greater than 1.5 mm and under higher horizontal winds (maximum wind speed less than 8 m s−1). A fourth-order fitting DSR was derived for typhoon systems and the value was also very close to the estimated DSR from the polarimetric measurements in Typhoon Saomai (2006).

Experimental results on rainfall estimation in complex terrain with a mobile X-band polarimetric weather radar

In this work the capability of reliable rainfall measurements with small weather radars in complex terrain for flood forecasting purposes is examined. Rain measurements were carried out during winter–spring 2007 with a mobile X-band dual-polarization radar in the northwestern mountainous part of the island of Crete in Greece. In this area a 2D-video disdrometer and a network of raingauges was installed for radar calibration and evaluation of rainfall measurements, respectively. The complex terrain of the experimental site may significantly reduce the performance of rain measurements due to ground clutter and partial or total beam blockage. A beam blockage algorithm using high resolution terrain data was applied in order to find areas with significant terrain effects and estimate correction of the radar measurements. Rain attenuation correction was based on modern sophisticated algorithms using differential phase measurements. The accuracy of rainfall estimation from standard or polarimetric algorithms at plan position indicator (PPI) scans was examined for high-temporal resolution (1 min) rainfall rates and accumulated rainfall values for winter and spring time rain events. For high elevation measurements, which were required in order to avoid terrain effects in large areas of interest, a correction for the vertical-profile-of-reflectivity (VPR) was also applied to the radar data. An average VPR model used in the corresponding correction of reflectivity was constructed based on range–height indicator (RHI) scans. It was concluded that quantitative high resolution X-band radar observations of rainfall in complex terrain is possible after careful application of all the above processing steps.

Drop Shapes and Axis Ratio Distributions: Comparison between 2D Video Disdrometer and Wind-Tunnel Measurements

Abstract Comparisons of drop shapes between measurements made using 2D video disdrometer (2DVD) and wind-tunnel experiments are presented. Comparisons are made in terms of the mean drop shapes and the axis ratio distributions. Very close agreement of the mean shapes is seen between the two sets of measurements; the same applies to the mean axis ratio versus drop diameter. Also, in both sets of measurements, an increase in the oscillation amplitudes with increasing drop diameter is observed. In the case of the 2DVD, a small increase in the skewness was also detected. Given that the two sets of measurements were conducted in very different conditions, the agreement between the two sets of data implies a certain “robustness” in the mean shape of oscillating drops that may be extended to natural raindrop oscillations, at least in steady rainfall and above the surface layer.

On the Possible Use of Copolar Correlation Coefficient for Improving the Drop Size Distribution Estimates at C Band

Abstract A decrease in copolar correlation coefficient (ρco) at C band has been observed for several rain events with broad drop size distributions (DSDs). Observational evidence comes from simultaneous measurements with a C-band dual-polarization radar and a 2D video disdrometer. The possibility of utilizing the ρco decrease for DSD retrievals is discussed. A preliminary method using the copolar reflectivity, differential reflectivity, and ρco is given for estimating the DSD parameters. Validation is carried out by deriving the differential propagation phase (Φdp) from the estimated DSD parameters and comparing against measurements. The method presented here shows potential but needs to be further assessed in different rain climatologies.

Orientation Angle Distributions of Drops after an 80-m Fall Using a 2D Video Disdrometer

Abstract This note reports on the use of a 2D video disdrometer to estimate the orientation of drops (>2 mm) that were generated artificially and allowed to fall 80 m from a bridge with no obstruction and under calm conditions. This experimental setup enabled a large number of drops to be generated, up to 10 mm in horizontal dimension. The distribution of the canting angles for all drops >2 mm was found to be nearly symmetric about 0° with standard deviation between 7° and 8°. From the canting angle distributions derived from the two orthogonal camera view planes, the distributions of the polar (θ) and azimuth (ϕ) angles were deduced; these two angles describe the 2D orientation of the symmetry axis. The azimuthal angle distribution was found to be nearly uniform in the range (0, 2π), whereas the distribution of pΩ(θ) = p(θ) sinθ was similar in shape to a special form of the Fisher distribution that is valid for describing the statistics on a spherical surface. The standard deviation of pΩ(θ) showed that larger drops are more stably oriented than smaller ones. This is in agreement with previous radar-based results of standard deviation of the canting angle decreasing with increasing Zdr.

Characterization of the precipitation in southwestern part of Greece with X-band Doppler radar, 2-D video disdrometer and rain gauges

Abstract. We document precipitation in the southwestern part of Greece with the National Observatory of Athens (NOA) X-band radar, NOA 2D video disdrometer and a network of rain gauges. The observations were collected between February and April 2004. Time evolution of the drop size distribution (DSD) is presented for the 9 March 2004 case where rain rate (computed on 1-min period) was measured up to 80 mm/h and reflectivity at the location of the disdrometer exceeded 40 dBZ. We then present the differences of DSD as function of the rain rate for the studied case as well as for the entire observations of the field experiment. It shows that higher the rain rate is, larger the range of the DSD and higher the concentration of the raindrops are.

Rainfall Estimation from C-Band Polarimetric Radar in Okinawa, Japan: Comparisons with 2D-Video Disdrometer and 400 MHz Wind Profiler

This paper presents an inter-comparison of rainfall parameters (median volume diameter and rain rate) using C-band polarimetric radar, a 2D-video disdrometer and a 400 MHz profiler for the Baiu front event of 8-9 June 2005 in Okinawa, Japan. These instruments are part of the Okinawa Sub-Tropical Environment Remote Sensing Center, operated by the National Institute of Information and Communications Technology (NICT). The 2D-video disdrometer is used to derive the mean axis ratio of raindrops versus drop diameter, as well as the drop size distribution for the Baiu event. The data are then used to simulate various relations between polarimetric scattering parameters such as: specific attenuation (Ah), and specific differential attenuation (Adp), versus specific differential phase (Kdp) which are required to correct the measured reflectivity at horizontal polarization (Zh), and the differential reflectivity (Zdr) for rain attenuation. The 2D-video disdrometer data are also used to arrive at retrieval formulas for median volume diameter (D0) from radar Zdr and rain rate from radar Kdp.The intense Baiu event of 8-9 June 2005 was composed of heavy convective rain cells embedded in large areas of stratiform rain. The inter-comparison of D0 and rain rate (R) between instruments was conducted for 12 hours (03:00-07:00, 11:00-19:00 UTC on 8th June 2004). The C-band radar retrievals were found to be in excellent agreement with the 2D-video disdrometer for the entire period. The 400 MHz profiler retrievals of D0 and R were in good agreement with 2D-video disdrometer during the more steady rain periods, with more scatter observed during the heavier convective rain periods. These inter-comparisons demonstrate the accuracy of C-band polarimetric radar to retrieve important rainfall parameters, as well as the accurate correction for rain attenuation using differential propagation phase.

Drop Axis Ratios from a 2D Video Disdrometer

Abstract Results from an experiment to measure the drop shapes using a 2D video disdrometer (2DVD) are reported. Under calm conditions, drops were generated from a hose located on a bridge 80 m above ground, this height being sufficient to allow drop oscillations to reach a steady state. The disdrometer data had to be carefully processed so as to eliminate the drops mismatched by the instrument and to remove the system spreading function. The total number of drops analyzed was around 115 000. Their axis ratio distributions were obtained for diameters ranging from 1.5 to 9 mm. The mean axis ratio decreases with increasing drop diameter, in agreement with the upper bound of the Beard and Chuang equilibrium shape model. The inferred mode of oscillation appears to be dominated by the oblate–prolate axisymmetric mode for the diameter range of 1.5 to 9 mm. The mean axis ratio agrees well with two empirically fitted formulas reported in earlier studies. In addition, a linear fit was applied to the data for radar applications relating to rain retrievals from dual-polarization measurements. The 2DVD data taken in moderate stratiform rain were also analyzed in a similar way and the results agree with the artificially generated drop experiment, at least up to 4 mm. No data for larger diameters were available for stratiform precipitation. Finally, the fall velocity was examined in terms of drop diameter. The results closely follow an empirical formula fitted to the Gunn and Kinzer data as well as the Beard and Pruppacher data including a slight decrease in the terminal velocity with a diameter beyond 7 mm.

Intraseasonal variation of raindrop size distribution at Koto Tabang, West Sumatra, Indonesia

Intraseasonal variation of raindrop size distribution (DSD) in response to Madden Julian Oscillation (MJO) is studied using a 2D video disdrometer (2DVD), a boundary layer radar (BLR) and the Equatorial Atmosphere Radar, operated at Koto Tabang, west Sumatra, as well as GOES‐9 infra‐red brightness temperature. As a parameter of DSD, ΔZMP, which is defined as the difference between a measured radar reflectivity in dB and that from the Marshall‐Palmer (MP) radar reflectivity (Z) ‐ rain rate (R) relationship, Z = 200 R1.6, is used. It is found that in non‐active phase of MJO, 2DVD‐derived ΔZMPs are generally positive, indicating that DSDs are broad, while they decrease toward negative values as the phase of MJO shifts to active ones. Rain‐top height derived from the BLR indicates that the convective processes are more intense in the non‐active MJO phase than in the active phase, which would cause the difference in DSDs.

A Methodology for Estimating the Parameters of a Gamma Raindrop Size Distribution Model from Polarimetric Radar Data: Application to a Squall-Line Event from the TRMM/Brazil Campaign

Abstract A methodology is proposed for estimating the parameters of a gamma raindrop size distribution model from radar measurements of Zh, Zdr, and Kdp at S band. Previously developed algorithms by Gorgucci et al. are extended to cover low rain-rate events where both Zdr and Kdp are noisy. Polarimetric data from the S-band Dual-Polarization Doppler Radar (S-Pol) during the Tropical Rainfall Measuring Mission (TRMM)/Brazil campaign are analyzed; specifically, the gamma parameters are retrieved for samples of convective and trailing stratiform rain during the 15 February 1999 squall-line event. Histograms of Nw and Do are retrieved from radar for each rain type and compared with related statistics reported in the literature. The functional behavior of Nw and Do versus rain rate retrieved from radar is compared against samples of 2D-video and RD-69 disdrometer data obtained during the campaign. The time variation of Nw, Do, and μ averaged over a 5 km × 5 km area (within which a network of gauges and a profi...

Drop Size Distributions Measured by a 2D Video Disdrometer: Comparison with Dual-Polarization Radar Data

An analysis of drop size distributions (DSDs) measured in four very different precipitation regimes is presented and is compared with polarimetric radar measurements. The DSDs are measured by a 2D video disdrometer, which is designed to measure drop size, shape, and fall speed with unprecedented accuracy. The observations indicate that significant DSD variability exists not only from one event to the next, but also within each system. Also, despite having vastly different storm structures and maximum rain rates, large raindrops with diameters greater than 5 mm occurred with each system. By comparing the occurrence of large drops with rainfall intensity, the authors find that the largest median diameters are not always associated with the heaviest rainfall, but are sometimes located either in advance of convective cores or, occasionally, in stratiform regions where rainfall rates are relatively low. Disdrometer and polarimetric radar measurements of radar reflectivity Z, differential reflectivity ZDR, specific differential phase KDP, and R(Z) and R(KDP) rain-rate estimators are compared in detail. Overall agreement is good, but it is found that both R(Z) and R(KDP) underestimate rain rate when the DSD is dominated by small drops and overestimate rain rate when the DSD is dominated by large drops. The results indicate that a classification of different rain types (associated with different DSDs) should be an essential part of polarimetric rainfall estimation. Furthermore, observations suggest that ZDR is a key parameter for making such a distinction. Last, the authors compute and compare maximum and average of gamma shape, slope, and intercept parameters for all four precipitation events. Potential measurement errors with the 2D video disdrometer are also discussed.