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Abstract
Dual pulse repetition frequency (PRF) is a commonly used technique, in operational Doppler weather radar networks, that extends the unambiguous Doppler velocity. The technique requires the measurement of the radial velocity at two different PRFs and it assumes that the data is collected from the same velocity. In practice, the data are collected from adjacent alternating PRF radials as the antenna rotates. However, high azimuthal shear or statistical errors due to uncertainty in the measurements can create dealiasing errors. This paper proposes two algorithms to correct these dealiasing errors and analyzes the results. Simulations and real cases are presented to illustrate the benefits and limitations.
Abstract
Dual pulse repetition frequency (PRF) is a commonly used technique, in operational Doppler weather radar networks, that extends the unambiguous Doppler velocity. The technique requires the measurement of the radial velocity at two different PRFs and it assumes that the data is collected from the same velocity. In practice, the data are collected from adjacent alternating PRF radials as the antenna rotates. However, high azimuthal shear or statistical errors due to uncertainty in the measurements can create dealiasing errors. This paper proposes two algorithms to correct these dealiasing errors and analyzes the results. Simulations and real cases are presented to illustrate the benefits and limitations.
Abstract
The effect of ship motion on shipborne polarimetric radar measurements is considered at C band. Calculations are carried out by (i) varying the “effective” mean canting angle and (ii) separately examining the elevation dependence. Scattering from a single oblate hydrometeor is considered at first. Equations are derived (i) to convert the measured differential reflectivity for nonzero mean canting angles to those for zero mean canting angle and (ii) to do the corresponding corrections for nonzero elevation angles. Scattering calculations are also performed using the T-matrix method with measured drop size distributions as input. Dependence on mean volume diameter is examined as well as variations of the four main polarimetric parameters. The results show that as long as the ship movement is limited to a roll of less than about 10°–15°, the effects are tolerable. Furthermore, the results from the scattering simulations have been used to provide equations for correction factors that can be applied to compensate for the “apparent” nonzero canting angles and nonzero elevation angles, so that drop size distribution parameters and rainfall rates can be estimated without any bias.
Abstract
The effect of ship motion on shipborne polarimetric radar measurements is considered at C band. Calculations are carried out by (i) varying the “effective” mean canting angle and (ii) separately examining the elevation dependence. Scattering from a single oblate hydrometeor is considered at first. Equations are derived (i) to convert the measured differential reflectivity for nonzero mean canting angles to those for zero mean canting angle and (ii) to do the corresponding corrections for nonzero elevation angles. Scattering calculations are also performed using the T-matrix method with measured drop size distributions as input. Dependence on mean volume diameter is examined as well as variations of the four main polarimetric parameters. The results show that as long as the ship movement is limited to a roll of less than about 10°–15°, the effects are tolerable. Furthermore, the results from the scattering simulations have been used to provide equations for correction factors that can be applied to compensate for the “apparent” nonzero canting angles and nonzero elevation angles, so that drop size distribution parameters and rainfall rates can be estimated without any bias.
Abstract
The characteristics of radar echoes for 12 thunderstorm days in the vicinity of Sydney, Australia, in the summer of 1995/96 have been examined using an objective methodology for storm identification and tracking. The spatial distribution of identified storms shows a maximum in frequency and intensity along the east side of the mountains that lie inland from the coast. Characteristics such as storm volume, area, and height are shown to have a lognormal frequency distribution. Reflectivity also has a skewed frequency distribution with a prevalence of lower reflectivity storms. Both the maximum reflectivity and storm height are shown to be correlated with the logarithm of storm volume. Although small storms predominate, the bulk of precipitation flux comes from the relatively few large-scale storms. It is also shown that storms generally move or propagate in a direction slightly to the left of the mass-weighted mean wind for the surface-to-300-hPa layer at a speed slightly less than the mean speed. Furthermore the deviation of the storm to the left of the mean layer wind increases and the standard deviation decreases as the storm size increases.
Abstract
The characteristics of radar echoes for 12 thunderstorm days in the vicinity of Sydney, Australia, in the summer of 1995/96 have been examined using an objective methodology for storm identification and tracking. The spatial distribution of identified storms shows a maximum in frequency and intensity along the east side of the mountains that lie inland from the coast. Characteristics such as storm volume, area, and height are shown to have a lognormal frequency distribution. Reflectivity also has a skewed frequency distribution with a prevalence of lower reflectivity storms. Both the maximum reflectivity and storm height are shown to be correlated with the logarithm of storm volume. Although small storms predominate, the bulk of precipitation flux comes from the relatively few large-scale storms. It is also shown that storms generally move or propagate in a direction slightly to the left of the mass-weighted mean wind for the surface-to-300-hPa layer at a speed slightly less than the mean speed. Furthermore the deviation of the storm to the left of the mean layer wind increases and the standard deviation decreases as the storm size increases.
Abstract
This note builds on prior technique development related to the classification of rain types utilizing C-band polarimetric (CPOL) radar measurements. While the prior work was preliminary and limited in scope, the authors elaborate here on the basis of the drop size distribution (DSD)-based indexing technique for rain-type classification (convective/stratiform/mixed), and place it on firmer footing by testing the methodology against texture- and disdrometer-based methods as applied to Darwin datasets. A microphysical-based methodology is attractive as it links more directly to the underlying rainfall physical processes.
Statistics of the DSD parameters, namely, histograms of log10(Nw ) and D 0, for convective and stratiform rain types across the premonsoon buildup and monsoon regimes were derived and further separated for over land and over ocean regions. The maximum value for mean D 0 (1.64 mm) and the largest histogram standard deviation (0.32 mm) occurred for convective rain over land during the buildup regime. The largest differences in D 0 and NW histograms were found to be for convective rain between the buildup and monsoon regimes (independent of land or ocean areas). Stratiform rain histograms were found to be very similar during the buildup regime with little land–ocean differences. However, somewhat larger land–ocean differences were found for the monsoon stratiform rain. The main histogram characteristics of the “mixed” or “uncertain” rain type were closer to the convective rain type than to stratiform, across both regimes and land–ocean areas. Additionally, the Nw versus D 0 cluster of points (mean ±1σ) for convective rain agrees very well with the previously published range of values for maritime convective (equilibrium-like) DSDs.
Abstract
This note builds on prior technique development related to the classification of rain types utilizing C-band polarimetric (CPOL) radar measurements. While the prior work was preliminary and limited in scope, the authors elaborate here on the basis of the drop size distribution (DSD)-based indexing technique for rain-type classification (convective/stratiform/mixed), and place it on firmer footing by testing the methodology against texture- and disdrometer-based methods as applied to Darwin datasets. A microphysical-based methodology is attractive as it links more directly to the underlying rainfall physical processes.
Statistics of the DSD parameters, namely, histograms of log10(Nw ) and D 0, for convective and stratiform rain types across the premonsoon buildup and monsoon regimes were derived and further separated for over land and over ocean regions. The maximum value for mean D 0 (1.64 mm) and the largest histogram standard deviation (0.32 mm) occurred for convective rain over land during the buildup regime. The largest differences in D 0 and NW histograms were found to be for convective rain between the buildup and monsoon regimes (independent of land or ocean areas). Stratiform rain histograms were found to be very similar during the buildup regime with little land–ocean differences. However, somewhat larger land–ocean differences were found for the monsoon stratiform rain. The main histogram characteristics of the “mixed” or “uncertain” rain type were closer to the convective rain type than to stratiform, across both regimes and land–ocean areas. Additionally, the Nw versus D 0 cluster of points (mean ±1σ) for convective rain agrees very well with the previously published range of values for maritime convective (equilibrium-like) DSDs.
Abstract
Temperature measurements obtained using radiosondes and Radio Acoustic Sounding Systems (RASS) are compared to assess the utility of the RASS technique for meteorological studies. The agreement is generally excellent; rms temperature differences are about 1.0°C for comparisons during a variety of meteorological conditions. Observations taken under ideal circumstances indicate that a precision of about 0.2°C is achievable with the RASS technique. A processor being designed for RASS should allow routine temperature measurements approaching this precision.
Abstract
Temperature measurements obtained using radiosondes and Radio Acoustic Sounding Systems (RASS) are compared to assess the utility of the RASS technique for meteorological studies. The agreement is generally excellent; rms temperature differences are about 1.0°C for comparisons during a variety of meteorological conditions. Observations taken under ideal circumstances indicate that a precision of about 0.2°C is achievable with the RASS technique. A processor being designed for RASS should allow routine temperature measurements approaching this precision.
Abstract
Rain drop size distributions retrieved from polarimetric radar measurements over regularly occurring thunderstorms over the islands north of Darwin, Australia, are used to test if aerosol contributions to the probability distributions of the drop size distribution parameters (median volume diameter and normalized intercept parameter) are detectable. The observations reported herein are such that differences in cloud properties arising from thermodynamic differences are minimized but even so may be a factor. However, there is a clear signature that high aerosol concentrations are correlated with smaller number concentrations and larger drops. This may be associated with enhanced ice multiplication processes for low aerosol concentration storms or other processes such as invigoration of the updrafts.
Abstract
Rain drop size distributions retrieved from polarimetric radar measurements over regularly occurring thunderstorms over the islands north of Darwin, Australia, are used to test if aerosol contributions to the probability distributions of the drop size distribution parameters (median volume diameter and normalized intercept parameter) are detectable. The observations reported herein are such that differences in cloud properties arising from thermodynamic differences are minimized but even so may be a factor. However, there is a clear signature that high aerosol concentrations are correlated with smaller number concentrations and larger drops. This may be associated with enhanced ice multiplication processes for low aerosol concentration storms or other processes such as invigoration of the updrafts.
Abstract
The sensitivity of polarimetric variables at a 5-cm wavelength to raindrop size and axial ratio is examined using T-matrix modeling of the scattering process for gamma raindrop size distributions fitted to tropical rainfall collected at Darwin, Australia. These simulations demonstrate that, while specific differential phase (KDP)–based estimates of rainfall, attenuation (AH), and differential attenuation are less affected by drop size distribution (DSD) variations, large drop occurrence can have significant impacts. Attenuation is sensitive to the occurrence of large drops, which can produce anomalously high values associated with resonance effect scattering. The polarimetric variables are sensitive to the relation between the equivolume diameter and axial ratio. Variations in the assumed form of the raindrop axial ratio can result in significant biases in rainfall and attenuation. Combined rainfall estimators, which include differential reflectivity (ZDR), such as R(KDP, ZDR) and R(AH, ZDR) are more robust to both DSD and raindrop axial ratio variations. The results also demonstrate that polarimetric techniques employed to classify the phase of hydrometeors are sensitive to the assumed raindrop axial ratio.
Abstract
The sensitivity of polarimetric variables at a 5-cm wavelength to raindrop size and axial ratio is examined using T-matrix modeling of the scattering process for gamma raindrop size distributions fitted to tropical rainfall collected at Darwin, Australia. These simulations demonstrate that, while specific differential phase (KDP)–based estimates of rainfall, attenuation (AH), and differential attenuation are less affected by drop size distribution (DSD) variations, large drop occurrence can have significant impacts. Attenuation is sensitive to the occurrence of large drops, which can produce anomalously high values associated with resonance effect scattering. The polarimetric variables are sensitive to the relation between the equivolume diameter and axial ratio. Variations in the assumed form of the raindrop axial ratio can result in significant biases in rainfall and attenuation. Combined rainfall estimators, which include differential reflectivity (ZDR), such as R(KDP, ZDR) and R(AH, ZDR) are more robust to both DSD and raindrop axial ratio variations. The results also demonstrate that polarimetric techniques employed to classify the phase of hydrometeors are sensitive to the assumed raindrop axial ratio.
Abstract
A 5-cm wavelength (C band) polarimetric radar was deployed during the MCTEX (Maritime Continent Thunderstorm Experiment) field program. This paper investigates the use of the C-band data for quantitative rainfall measurements with particular emphasis on specific differential phase (K DP) and traditional reflectivity-based rain-rate estimates in moderate to high rain rates (10–200 mm h−1). Large values of backscatter differential phase shift are occasionally seen in these data, thus resonance scattering effects are important. A consensus algorithm for K DP estimation in these cases is described. The rain-rate estimates are compared with the data from a d-scale rain gauge network. The K DP estimates are shown to produce the highest quality data, although variations in drop size distribution characteristics have a significant effect on the rain estimates. When corrections are applied for beam blockage and attenuation, good agreement can also be obtained with Z–R-based estimates. The attenuation corrections were made using a polarimetric variable, total differential phase, which provides an estimate of the total water content along the path. The polarimetric estimates of total accumulation also show excellent agreement.
Abstract
A 5-cm wavelength (C band) polarimetric radar was deployed during the MCTEX (Maritime Continent Thunderstorm Experiment) field program. This paper investigates the use of the C-band data for quantitative rainfall measurements with particular emphasis on specific differential phase (K DP) and traditional reflectivity-based rain-rate estimates in moderate to high rain rates (10–200 mm h−1). Large values of backscatter differential phase shift are occasionally seen in these data, thus resonance scattering effects are important. A consensus algorithm for K DP estimation in these cases is described. The rain-rate estimates are compared with the data from a d-scale rain gauge network. The K DP estimates are shown to produce the highest quality data, although variations in drop size distribution characteristics have a significant effect on the rain estimates. When corrections are applied for beam blockage and attenuation, good agreement can also be obtained with Z–R-based estimates. The attenuation corrections were made using a polarimetric variable, total differential phase, which provides an estimate of the total water content along the path. The polarimetric estimates of total accumulation also show excellent agreement.
Abstract
Nested cloud-system-resolving model simulations of tropical convective clouds observed during the recent Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are conducted using the Weather Research and Forecasting (WRF) model. The WRF model is configured with a highest-resolving domain that uses 1.3-km grid spacing and is centered over Darwin, Australia. The performance of the model in simulating two different convective regimes observed during TWP-ICE is considered. The first regime is characteristic of the active monsoon, which features widespread cloud cover that is similar to maritime convection. The second regime is a monsoon break, which contains intense localized systems that are representative of diurnally forced continental convection. Many aspects of the model performance are considered, including their sensitivity to physical parameterizations and initialization time, and the spatial statistics of rainfall accumulations and the rain-rate distribution. While the simulations highlight many challenges and difficulties in correctly modeling the convection in the two regimes, they show that provided the mesoscale environment is adequately reproduced by the model, the statistics of the simulated rainfall agrees reasonably well with the observations.
Abstract
Nested cloud-system-resolving model simulations of tropical convective clouds observed during the recent Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are conducted using the Weather Research and Forecasting (WRF) model. The WRF model is configured with a highest-resolving domain that uses 1.3-km grid spacing and is centered over Darwin, Australia. The performance of the model in simulating two different convective regimes observed during TWP-ICE is considered. The first regime is characteristic of the active monsoon, which features widespread cloud cover that is similar to maritime convection. The second regime is a monsoon break, which contains intense localized systems that are representative of diurnally forced continental convection. Many aspects of the model performance are considered, including their sensitivity to physical parameterizations and initialization time, and the spatial statistics of rainfall accumulations and the rain-rate distribution. While the simulations highlight many challenges and difficulties in correctly modeling the convection in the two regimes, they show that provided the mesoscale environment is adequately reproduced by the model, the statistics of the simulated rainfall agrees reasonably well with the observations.
During November and December 2005, two consortia of mainly European groups conducted an aircraft campaign in Darwin, Australia, to measure the composition of the tropical upper-troposphere and tropopause regions, between 12 and 20 km, in order to investigate the transport and transformation in deep convection of water vapor, aerosols, and trace chemicals. The campaign used two high-altitude aircraft—the Russian M55 Geophysica and the Australian Grob 520 Egrett, which can reach 20 and 15 km, respectively—complemented by upward-pointing lidar measurements from the DLR Falcon and low-level aerosol and chemical measurements from the U.K. Dornier-228. The meteorology during the campaign was characterized mainly by premonsoon conditions—isolated afternoon thunderstorms with more organized convective systems in the evening and overnight. At the beginning of November pronounced pollution resulting from widespread biomass burning was measured by the Dornier, giving way gradually to cleaner conditions by December, thus affording the opportunity to study the influence of aerosols on convection. The Egrett was used mainly to sample in and around the outflow from isolated thunderstorms, with a couple of survey missions near the end. The Geophysica–Falcon pair spent about 40% of their flight hours on survey legs, prioritizing remote sensing of water vapor, cirrus, and trace gases, and the remainder on close encounters with storm systems, prioritizing in situ measurements. Two joint missions with all four aircraft were conducted: on 16 November, during the polluted period, sampling a detached anvil from a single-cell storm, and on 30 November, around a much larger multicellular storm.
During November and December 2005, two consortia of mainly European groups conducted an aircraft campaign in Darwin, Australia, to measure the composition of the tropical upper-troposphere and tropopause regions, between 12 and 20 km, in order to investigate the transport and transformation in deep convection of water vapor, aerosols, and trace chemicals. The campaign used two high-altitude aircraft—the Russian M55 Geophysica and the Australian Grob 520 Egrett, which can reach 20 and 15 km, respectively—complemented by upward-pointing lidar measurements from the DLR Falcon and low-level aerosol and chemical measurements from the U.K. Dornier-228. The meteorology during the campaign was characterized mainly by premonsoon conditions—isolated afternoon thunderstorms with more organized convective systems in the evening and overnight. At the beginning of November pronounced pollution resulting from widespread biomass burning was measured by the Dornier, giving way gradually to cleaner conditions by December, thus affording the opportunity to study the influence of aerosols on convection. The Egrett was used mainly to sample in and around the outflow from isolated thunderstorms, with a couple of survey missions near the end. The Geophysica–Falcon pair spent about 40% of their flight hours on survey legs, prioritizing remote sensing of water vapor, cirrus, and trace gases, and the remainder on close encounters with storm systems, prioritizing in situ measurements. Two joint missions with all four aircraft were conducted: on 16 November, during the polluted period, sampling a detached anvil from a single-cell storm, and on 30 November, around a much larger multicellular storm.
