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Peter T. May and Deepak K. Rajopadhyaya
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Peter T. May and Deepak K. Rajopadhyaya

Abstract

Continuous vertical velocity measurements using a 50-MHz wind profiler located at Darwin in northern Australia during periods of active convection have been analyzed. This dataset is dominated by continental-type convection. Numerous examples of shallow, deep, and decaying convection were seen and it is shown that only the deep systems have substantial tilts to the draft structure. The most intense updrafts occur above the freezing level, but shallow convection also produces large-amplitude vertical motions. The strength of these updrafts in this dataset is very similar to other tropical, oceanic data. That observation is consistent with the idea that the magnitude of the updrafts is much less in the Tropics than for intense midlatitude convection because the convective available potential energy is distributed over a much deeper layer in the Tropics, although more intense updrafts may be present at other tropical locations, such as the Tiwi Islands north of Darwin. The size of the cores, however, is significantly greater here than with oceanic data and is similar to midlatitude results, thus supporting the suggestion that boundary layer depth is important in determining the horizontal scale. There is a net detrainment in the upward cores above the freezing level occurring at all space scales. The mass flux in intense updrafts is almost constant with height below the freezing level but is almost cancelled by downdrafts and the immediate surrounding environment. Two populations of downdrafts are seen, one a dynamical response associated with intense updrafts at all heights and a second driven by precipitation processes below the freezing level. The core size, intensity, and mass flux are all approximately lognormally distributed. It is shown that a wide range of velocity and size scales contribute to the upward mass flux.

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Peter T. May and Deepak K. Rajopadhyaya

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Data from a wind profiler located at Darwin, Australia, have been used to examine the vertical motions and precipitation microphysics in a well-developed squall line. Both a mature and developing convective cell are well sampled. The vertical motions within the mature cell are dominated by the effect of glaciation and a convective downdraft feeding a cold pool. The strong updrafts are accompanied by supercooled water as much as 2 km above the freezing level. The two cells are separated by a narrow region of deep descent. The developing cell has a low-level maximum in upward motion coinciding with high radar reflectivity below 3 km, suggesting warm rain processes. There is a large transition region with deep descent and a stratiform region with a classic up- and downdraft circulation. The precipitation characteristics show the aggregation of ice particles as they descend in the stratiform region. Over half of the rain is seen to evaporate between 4 and 2 km. The cooling implied by this and the heating by the growth of ice particulates above the melting level balance the mesoscale circulation in the stratiform region. The Q 1, heating profile is consistent with previous studies above 4 km but shows a net cooling below this. This may in part be due to the storm being sampled when the system was mature with extensive convective downdrafts.

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Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

Abstract

Recently, the authors observed significant echoes from precipitating ice particles above the freezing level in the stratiform region of tropical squall lines with a 50-MHz wind-profiling radar. A technique is described that allows ice particle size distributions to be obtained from the 50-MHz wind-profiling radar spectra. A model ice echo is developed in which it is assumed the number density of ice particles is exponentially distributed. A composite spectrum of dendrites, plates, columns, and bullets is assumed, and good fits to the observed spectra are obtained. To test the reliability and stability of the retrieval technique, simulated data with realistic statistical properties were generated and the shape of the model size distribution varied. Only two types of ice particles are considered. It is shown that the population spectra are recovered well for a wide range of exponential slopes. Relative precision of about 10% is obtained when the clear-air spectral width is 0.1 m s−1, and is about 30% when the spectral width is 0.3 m s−1. However, when the spectral width is large, such as 0.5 m s−1, the relative error can exceed 100%. Spectral widths of about 0.3 m s−1 are typically observed in the trailing stratiform region of tropical squall lines.

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Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

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A technique is described that allows estimates of the raindrop size distribution to be obtained from the Doppler spectra measured by wind-profiling radars. The method makes no a priori assumptions regarding the shape of the drop size distributions. To test the accuracy of the technique, artificial data with realistic statistical properties have been generated and the shape of the model drop size distribution varied, The analysis technique obtains an accuracy of around 10% in the drop size range between 1 and 4 mm for data consistent with typical 50-MHz observations averaged over 5–10 min. There are limitations outside this range and the physical reasons for these are discussed. Simulations with multiple-peaked distributions show that the technique can also well resolve complicated distributions

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Deepak K. Rajopadhyaya, Susan K. Avery, Peter T. May, and Robert C. Cifelli

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The advantages and disadvantages of single-frequency (50 MHz) and dual-frequency (50 and 915 MHz) wind profiler drop size distribution retrievals are discussed by comparing retrievals of median volume drop diameter and rain rates. Simulated data, as well as observational data, show that the median volume diameter estimated from the single-frequency technique is biased higher than what is retrieved using the dual-frequency technique. This result is due to the strong 50-MHz Bragg scatter signal that masks the small drop (low fall velocity) part of the precipitation spectrum. The error in the estimation of the median volume diameter increases markedly with increasing vertical air motion spectral width. The error in the estimation of the median volume diameter is minimum for median volume diameters ranging from 0.5 to about 2.5 mm for the dual-frequency technique and 1.2 to about 2.5 mm for the single-frequency technique. The comparison of retrieved rain rates with rain gauge data shows a very good agreement for both techniques, but it was not always possible to retrieve precipitation information using the single-frequency technique.

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Robert Cifelli, Christopher R. Williams, Deepak K. Rajopadhyaya, Susan K. Avery, Kenneth S. Gage, and P. T. May

Abstract

Drop-size distribution characteristics were retrieved in eight tropical mesoscale convective systems (MCS) using a dual-frequency (UHF and VHF) wind profiler technique. The MCSs occurred near Darwin, Australia, during the 1993/94 wet season and were representative of the monsoon (oceanic) regime. The retrieved drop-size parameters were compared with corresponding rain gauge and disdrometer data, and it was found that there was good agreement between the measurements, lending credence to the profiler retrievals of drop-size distribution parameters. The profiler data for each MCS were partitioned into a three-tier classification scheme (i.e., convective, mixed convective–stratiform, and stratiform) based on a modified version of to isolate the salient microphysical characteristics in different precipitation types. The resulting analysis allowed for an examination of the drop-size distribution parameters in each category for a height range of about 2.1 km in each MCS.

In general, the distributions of all of the retrieved parameters showed the most variability in convection and the least in stratiform, with the mixed convective–stratiform category usually displaying intermediate characteristics. Although there was significant overlap in the range of many of the parameter distributions, the mean profiles were distinct. In the stratiform region, there was minimal vertical structure for all of the drop-size distribution parameters. This result suggests an equilibrium between depletion (e.g., evaporation) and growth (e.g., coalescence) over the height range examined. In contrast, the convective parameter distributions showed a more complicated structure, probably as a consequence of the complex microphysical processes occurring in the convective precipitation category.

Reflectivity–rainfall (Z–R) relations of the form Z = AR B were developed for each precipitation category as a function of height using linear regressions to the profiler retrievals of R and Z in log space. Similar to findings from previous studies, the rainfall decreased for a given reflectivity as the precipitation type changed from convective to stratiform. This result primarily was due to the fact that the coefficient A in the best-fit stratiform Z–R was approximately a factor of 2 greater than the convective A at all heights. The coefficient A generally increased downward with height in each category; the exponent B showed a small decrease (stratiform), almost no change (convective), or a slight increase (mixed convective–stratiform). Consequently, the amount by which convective rain rate exceeded stratiform (for a given reflectivity) varied significantly as a function of height, ranging from about 15% to over 80%.

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Deepak K. Rajopadhyaya, Peter T. May, Robert C. Cifelli, Susan K. Avery, Christopher R. Willams, Warner L. Ecklund, and Kenneth S. Gage

Abstract

Two different frequency radar wind profilers (920 and 50 MHz) were used to retrieve rain rates from a long-lasting rainfall event observed near Darwin, Northern Territory, Australia, during the 1993–94 wet season. In this technique, 50-MHz data are used to derive the vertical air motion parameters (vertical velocity and spectral width); the 920-MHz data are then used to obtain the precipitation characteristics with the vertical air motion corrections. A comparison of the retrieved rain rates with rain gauge measurements shows excellent agreement. A detailed examination of the mean vertical velocity and spectral width corrections in the rain retrieval shows that the error due to an uncorrected mean vertical velocity can be as large as 100%, and the error for an uncorrected spectral width was about 10% for the range of mean vertical velocity and spectral width considered. There was a strong functional dependence between the retrieved mean vertical velocity and percentage difference between observed and retrieved rain rates with and without vertical air motion corrections. The corresponding functional dependence with and without the spectral width corrections was small but significant. An uncorrected upward mean vertical velocity overestimates rain rates, whereas an uncorrected downward mean vertical velocity underestimates rain rates. Uncorrected spectral width estimates have a tendency to overestimate rain rates. There are additional errors in the width correction because of antenna beam mismatching. A method is discussed to quantitatively evaluate this effect, and it is shown to be relatively small compared to the first-order mean vertical velocity correction.

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