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Vertical Velocity Characteristics of Deep Convection Over Darwin, Australia

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Vertical Velocity Characteristics of Deep Convection Over Darwin, Australia 1056 MONTHLY WEATHER REVIEW VOLUME 127 Vertical Velocity Characteristics of Deep Convection over Darwin, Australia PETER T. M AY Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia DEEPAK K. RAJOPADHYAYA Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado (Manuscript received 26 January 1998, in ®nal form 16 June 1998) ABSTRACT Continuous vertical velocity measurements using a 50-MHz wind pro®ler 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 signi®cantly 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 ¯ux 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 ¯ux are all approximately lognormally distributed. It is shown that a wide range of velocity and size scales contribute to the upward mass ¯ux. 1. Introduction ing level, deep mature cells with extensive convective downdrafts and new cells forming on the cold pool out- The Australian Bureau of Meteorology and the Aer- ¯ows, and cells that are clearly transitioning into a strat- onomy Laboratory of the National Oceanic and At- iform mesoscale up-/downdraft pattern. These data have mospheric Administration have been operating a 50- been analyzed to investigate the statistics of the intense MHz wind pro®ler at Darwin, Australia, since 1989. During the ®rst year of operation it was only measuring cores within convective clouds. This dataset also in- the vertical wind component. Data from the entire 1989± cludes extensive periods of stratiform and decaying pre- 90 wet season have been examined and 29 events of cipitation systems and these will be the focus of further active convection over the pro®ler have been identi®ed. work. This continuous high-resolution dataset lends itself to a Storms in Darwin occur during a wide variety of statistical analysis comparable with and complementing shear/convective available potential energy (CAPE) aircraft-based studies such as LeMone and Zipser conditions (Keenan and Carbone 1992). Darwin expe- (1980), Jorgensen et al. (1985), Lucas et al. (1994), and riences two distinct types of convection during the wet others. The dataset contains examples of essentially all season. During the monsoon period, where there are stages of the growth of convective systems, with ex- low-level westerly winds, the convection is oceanic in amples of shallow convection only a few kilometers character. The storms sampled during this period are deep, more intense isolated cells reaching near the freez- similar to those sampled over the ocean in the Equatorial Mesoscale Experiment (EMEX). In the buildup to the monsoon (transition period) and during break periods, the low-level winds are easterly. Convection during Corresponding author address: Dr. P. T. May, Bureau of Meteo- these periods is more continental in character. These rology Research Centre, GPO Box 1289K, Melbourne, Victoria 3001, Australia. include storms developing on sea-breeze convergence E-mail: [email protected] lines and well-organized intense squall lines that prop- q 1999 American Meteorological Society Unauthenticated | Downloaded 09/30/21 08:07 PM UTC JUNE 1999 MAY AND RAJOPADHYAYA 1057 agate from inland (Keenan and Carbone 1992). Radar One advantage of the pro®ler data is the continuous studies have generally led to the expectation that break height coverage, so that the vertical extent and coher- period storms are more intense; they are generally much ence of the cores can be studied in addition to the sta- more organized. Here we de®ne the monsoon periods tistics at each level. These correlation structures can be as those with an 850-hPa westerly wind component compared with those obtained in tropical cyclones using (Holland 1986), but the separation of cases does not airborne radar by Black et al. (1996). alter for layer-averaged de®nitions (Drosdowsky 1996). Vertical motions within convective systems have been Most of these pro®ler data were collected under break extensively studied on a case study basis (e.g., Heyms- and transition conditions (25 of 29 cases) as the mon- ®eld and Schotz 1985). Yuter and Houze (1995a,b,c) soon that year was very brief (Rutledge et al. 1992). analyzed the statistical characteristics of the vertical mo- Radar wind pro®lers have been used to study vertical tions and re¯ectivity ®eld within a convective complex motions in convective systems in the past (e.g., Balsley during the Convection and Precipitation/Electri®cation et al. 1988). These include the dual-frequency obser- Experiment (CAPE), using dual±Doppler radar analyses vations of Chilson et al. (1993) focusing on the character in detail. This included the temporal evolution of the of precipitation echoes in deep convection and the study ®elds from a developing stage through to a decay phase. of May and Rajopadhyaya (1996, 1997), which exam- The general characteristics of their results will be com- ined the vertical velocity and precipitation character- pared and contrasted with the data over many events istics of a squall line in some detail. This latter study here. also includes diagnostics of heating rates implied by the system vertical motion ®eld and the balance between 2. Observations vertical motion and changes in the precipitation char- acteristics in the stratiform part of a squall line. The The 50-MHz wind pro®ler was operating in a vertical- studies of Cifelli and Rutledge (1994, 1998) analyzed only mode during the 1989±90 wet season at Darwin. a number of storm systems and the characteristics of It was sampling the height region between 1.7 and 20 individual storm case studies and storm composites were km, but the highest quality data were limited to heights related to the precipitation type and the characteristics below 11 km. The height resolution of the observations of storms in the monsoon and break season rainfall re- was 1 km, but data were sampled every 500 m. The gimes. These papers found some signi®cant differences time resolution was 86 s. The resulting Doppler spectra in the mean vertical motion pro®le between monsoon were then stored for reanalysis. and break storms, with evidence of a double peak in The pro®lers measure the radar backscatter from both the vertical motion pro®le associated with warm rain turbulent irregularities (giving an estimate of the vertical (lower peak) and glaciation and a decrease in precipi- wind) and precipitation particles (both rain and ice; e.g., tation loading (upper peak) for break cases, while the Fukao et al. 1985; Larsen and Rottger 1987; Wakasugi monsoon cases had a more uniform pro®le. However, et al. 1986; May 1991; Chilson et al. 1993). The pro®ler the number of cases was relatively small compared with spectra have been reanalyzed to remove the effects of the sample analyzed in this paper. precipitation echoes from the estimates of the mean ver- There have been many aircraft-based studies of con- tical motion. A least squares ®t of a Gaussian function vection. These range from studies of the vertical velocity is ®tted to the ``clear air'' part of the spectrum. Figure characteristics of fair weather cumulus (Malkus 1955) 1 shows a typical example of several of the spectra and to the pioneering work of the Thunderstorm Project the corresponding velocity estimate seen in deep con- (Byers and Braham 1949), investigating deep intense vection. These velocity estimates were then manually convection over the Midwest of the United States and edited and sometimes reanalyzed to ensure that biases Florida, and hurricane vertical motions (Gray 1965). associated with the precipitation echoes have been re- The classic papers of LeMone and Zipser (1980) and moved. Zipser and LeMone (1980) presented a detailed statis- Periods with vertical motions with an amplitude tical study of the draft and core characteristics based on greater than 1.5 m s21 have been identi®ed and are de- numerous aircraft penetrations of convective towers at ®ned here as convective cores. For our purposes, this various heights during the Global Atmospheric Research threshold value provides a signature of convective pre- Program (GARP) Atlantic Tropical Experiment cipitation that has been selected for this analysis. This (GATE). More recent studies have contrasted the GATE somewhat loose de®nition is almost equivalent to the and Thunderstorm Project data with convection over de®nitions of Houghton (1968) and Houze (1981) and other oceanic basins, such as north of Australia and the corresponds to velocities greater than the typical fall Gulf of Carpentaria during EMEX (Jorgensen and speed of ice crystals and aggregates.
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