A Summary of Precipitation Characteristics from the 2006–11 Northern Australian Wet Seasons as Revealed by ARM Disdrometer Research Facilities (Darwin, Australia)

AbstractThe variability of rainfall and drop size distributions (DSDs) as a function of large-scale atmospheric conditions and storm characteristics is investigated using measurements from the Atmospheric Radiation Measurement Program (ARM) facility at Darwin, Australia. Observations are obtained from an impact disdrometer with a near continuous record of operation over five consecutive wet seasons (2006–11). Bulk rainfall characteristics are partitioned according to diurnal accumulation, convective and stratiform precipitation classifications, objective monsoonal regime, and MJO phase. Findings support previous Darwin studies suggesting a significant diurnal and DSD parameter signal associated with both convective–stratiform and wet season monsoonal regime classification. Negligible MJO phase influence is determined for cumulative disdrometric statistics over the Darwin location.

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abstract: AbstractTropical cyclone (TC) rainfall over the Australian continent is studied using observations from 41 TC seasons 1969/70 to 2009/10. A total of 318 storms, whose centers either crossed the coastline or were located within 500 km of the coast, are considered in this study. Mean seasonal (November/April) contributions by TCs to the total rainfall are largest along the northern coastline from 120°–150°E. However, the percentage contributions by TCs are greatest west of 125°E, with mean coastal values of 20%–40% and inland values of approximately 20%. Farther east, percentages near the coast are only around 10%, and even lower inland. Inland penetration by TC rainfall is generally greatest over western portions of the continent, associated with greater inland penetration of TC tracks. During the peak of the TC season (January–March), TCs contribute around 40% to the rainfall total of coastal regions west of 120°E, while during December, TCs contribute approximately 60%–70% to the total rainfall west of 1...

title: Rainfall Characteristics Associated with the Remnants of Tropical Storm Helene in Upstate South Carolina
abstract: A description is given of rainfall characteristics associated with the remnants of Tropical Storm Helene, which made landfall on the panhandle of Florida and moved through parts of Georgia, South Carolina, and North Carolina during 22–23 September 2000. Also included in the description are rainfall events in advance of the approach of Helene to South Carolina. Emphasis is placed on the implications of these characteristics regarding the ability of the Weather Surveillance Radar-1988 Doppler (WSR-88D) to measure rainfall accurately for this system when employing reflectivity factor–rainfall rate (Z–R) algorithms as defined by the National Weather Service. The results are based on analysis of raindrop size spectra collected with a disdrometer in upstate South Carolina at the Clemson University Atmospheric Research Laboratory. It was found that there were pronounced variations in the Z–R relationship from day to day and within a day that would limit the accuracy of radar measurement of rainfall. A d...

title: Stratiform precipitation in regions of convection : A meteorological paradox ?
abstract: It was once generally thought that stratiform precipitation was something occurring primarily, if not exclusively, in middle latitudes—in baroclinic cyclones and fronts. Early radar observations in the Tropics, however, showed large radar echoes composed of convective rain alongside stratiform precipitation, with the stratiform echoes covering great areas and accounting for a large portion of the tropical rainfall. These observations seemed paradoxical, since stratiform precipitation should not have been occurring in the Tropics, where baroclinic cyclones do not occur. Instead it was falling from convection-generated clouds, generally thought to be too violent to be compatible with the layered, gently settling behavior of stratiform precipitation. In meteorology, convection is a dynamic concept; specifically, it is the rapid, efficient, vigorous overturning of the atmosphere required to neutralize an unstable vertical distribution of moist static energy. Most clouds in the Tropics are convection-generated...

title: Kinematics and Microphysics of the Transition Zone of the 10–11 June 1985 Squall Line
abstract: A high-resolution composite analysis covering the entire breadth of the northern portion of a mature leading-line, trailing stratiform squall-line system reveals that mean subsidence observed in the transition zone consisted of two different types of average downdraft: one at upper levels that was mechanically forced and one at lower levels that was microphysically forced. Both the upper-level and lower-level mean downdrafts in the transition zone appeared to be the average effect of convective-scale vertical drafts associated with convective structures that moved relative to the front edge of the convective line. The structure of individual upper-level convective-scale downdrafts suggested that they may have been partially composed of gravity waves excited by the interaction of the penetrative convective updrafts of the mature and dissipating convective cells with the stable ambient flow. The lower-level mean downdraft extended from midlevels to near the surface but was maximum near the melting ...

title: Three-Dimensional Kinematic and Microphysical Evolution of Florida Cumulonimbus. Part I: Spatial Distribution of Updrafts, Downdrafts, and Precipitation
abstract: This paper is the first in a three-part study that examines the kinematic and microphysical evolution of Florida cumulonimbus and focuses on the convective-to-stratiform transition of the storm. This first paper lays the groundwork for the subsequent papers by defining the problem under study, delineating the setting for the storm, and describing the spatial distribution of updrafts, downdrafts, and precipitation. High-resolution radar data of a typical line of storms associated with the Florida sea breeze is the centerpiece of this study. The high-resolution data reveal details of the internal structure of the squall line that were beyond the resolution of previous squall-line studies. Radar reflectivity filled in between cells at upper levels as the storm evolved. Reflectivity values were only weakly associated with updraft and downdraft magnitude. The updrafts and downdrafts in the storm tended to be irregular in their three-dimensional shape and less than 5 km in horizontal extent. At any giv...

title: An algorithm to estimate the heating budget from vertical hydrometeor profiles
abstract: A simple algorithm to estimate the latent heating of cloud systems from their vertical hydrometer profiles is proposed. The derivation as well as the validation of the algorithm is based on output generated by a non-hydrostatic cloud model with parameterized microphysical processes. Mature and decaying stages of a GATE squall-type convective system have been tested. The algorithm-derived heating budget is in reasonable agreement with the budget predicted by the cloud model. The input to the proposed algorithm can be obtained from either a rain retrieval technique based on information from multichannel passive microwave signals or a kinematic cloud model based on information from Doppler radar wind fields and radar reflectivity patterns. Such an application would have significant implications for spaceborne remote sensing and the large-scale weather prediction data assimilation problem.

title: The Tropical Dynamical Response to Latent Heating Estimates Derived from the TRMM Precipitation Radar
abstract: A 3-yr (1998‐2000) climatology of near-surface rainfall and stratiform rain fraction observed by the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) was used to calculate the four-dimensional distribution of tropical latent heating on seasonal-to-annual time scales. The TRMM-derived latent heating was then used to force an idealized primitive equation model using an initial value approach in order to obtain the quasi-steady-state, nonlinear, zonally asymmetric atmospheric response to precipitating tropical cloud systems. In agreement with previous studies, an increase in stratiform rain fraction elevates circulation centers and strengthens the upper-level response. Furthermore, horizontal variations in the vertical heating profile implied by the PR stratiform rain fraction pattern lead to circulation anomalies of varying height and vertical extent that

title: Modeling of Convective-Stratiform Precipitation Processes: Sensitivity to Partitioning Methods
abstract: Six different convective–stratiform separation techniques are compared and evaluated using 2D numerical simulations of a tropical and a midlatitude continental squall line. The techniques used include a texture algorithm applied to surface rainfall, a similar algorithm but with additional criteria applied to vertical velocity and cloud, a texture algorithm applied to vertical velocities below the melting layer, a simple approach that assumes a constant characteristic width for the convective region, a more sophisticated texture algorithm applied to radar reflectivities below the melting layer, and a new technique based on the premise that the fall speed of precipitation particles is large relative to air velocity in regions of stratiform precipitation. Comparisons are made in terms of rainfall, mass fluxes, apparent heating and moistening, hydrometeor contents, reflectivity and vertical-velocity contoured-frequency-with-altitude diagrams (CFAD), microphysics, and latent heating retrieval. Overall...

title: Raindrop Size Distribution and Radar Parameters at Cape Verde
abstract: Precipitation measurement using passive or active microwaves from space- or ground-based radar involves hypotheses about the raindrop size distribution (DSD). A universal knowledge of DSD characteristics is needed. A 4-yr dataset collected with a disdrometer at Dakar, Senegal, on the Atlantic coast of West Africa is used to analyze the DSD at the end of the continental trajectory of Sahelian squall lines. The DSDs are stratified in eight rain-rate classes and are fitted to analytical distributions. The shape of the averaged DSDs is found to be very similar from one year to the next. From rain rates R higher than about 20 mm h−1, the slope of the DSDs tends toward a constant value. The coefficients of the Z–R relation, between the radar reflectivity factor Z and R, are different for convective and stratiform parts of the squall lines. However, because the Z–R relations for convective rain intersect the relation for stratiform rain for high rates, it is suggested that using a single Z–R relation en...

title: Variability of Raindrop Size Distributions in a Squall Line and Implications for Radar Rainfall Estimation
abstract: The intrastorm variability of raindrop size distributions as a source of uncertainty in single-parameter and dual-parameter radar rainfall estimates is studied using time series analyses of disdrometer observations. Two rain-rate (R) estimators are considered: the traditional single-parameter estimator using only the radar reflectivity factor (Z) and a dual-polarization estimator using a combination of radar reflectivity at horizontal polarization (ZH) and differential reflectivity (ZDR). A case study for a squall-line system passing over the Goodwin Creek experimental watershed in northern Mississippi is presented. Microphysically, the leading convective line is characterized by large raindrop concentrations (>500 drops per cubic meter), large mean raindrop sizes (>1 mm), and wide raindrop size distributions (standard deviations >0.5 mm), as compared to the transition region and the trailing stratiform rain. The transition and stratiform phases have similar raindrop concentrations and mean raind...

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

title: Characterization of Vertical Velocity and Drop Size Distribution Parameters in Widespread Precipitation at ARM Facilities
abstract: Extended, high-resolution measurements of vertical air motion and median volume drop diameter D0 in widespread precipitation from three diverse Atmospheric Radiation Measurement Program (ARM) locations[Lamont, Oklahoma,SouthernGreatPlainssite(SGP);Niamey,Niger;andBlackForest,Germany]are presented. The analysis indicates a weak (0‐10 cm 21 ) downward air motion beneath the melting layer for all three regions, a magnitude that is to within the typical uncertainty of the retrieval methods. On average, the hourly estimated standard deviation of the vertical air motion is 0.25 m s 21 with no pronounced vertical structure. Profiles of D0 vary according to region and rainfall rate. The standard deviation of 1-min-averaged D0 profiles for isolated rainfall rate intervals is 0.3‐0.4 mm. Additional insights into the form of the raindrop size distribution are provided using available dual-frequency Doppler velocity observations at SGP. The analysissuggests thatgamma functionsbetter explain pairedvelocity observations and radarretrievalsfor the Oklahoma dataset. This study will be useful in assessing uncertainties introduced in the measurement of precipitation parameters from ground-based and spaceborne remote sensors that are due to small-scale variability.