Microphysics of Raindrop Size Spectra: Tropical Continental and Maritime Storms

Abstract

This work uses raindrop size spectra measured at the surface in tropical continental storms to determine the associated parameters of the best-fit gamma distributions. The physical processes responsible for those parameters and their relations to the measurable radar reflectivity Z and differential reflectivity ZDR are then explored. So too are their relations to quantitative measurements of rain. Comparison is then made with corresponding features previously reported in tropical maritime regimes. The storms observed in Brazil and Arecibo, Puerto Rico, have been divided into convective (C), transition (T), and stratiform (S) segments. The raindrop size distribution (DSD) parameters are clearly defined on a gamma parameter diagram (GPD) that shows 1) how median volume drop size D 0 increases from S to T to C segments of the rain while 2) the range of the spectrum breadth parameter increases, and the range of the slope parameter ⌳ decreases in the same sequence of S to C. Drop growth occurs predominantly below the 0°C level by collision, coalescence, and breakup in the C rains. The median volume diameter D 0 grows as more of the water is concentrated near that size and so the DSD narrows; that is, both and ⌳ increase. In both maritime and continental storms the DSD in the convective portion of the storm approaches equilibrium. The coefficient A in the Z ϭ AR b relation increases with D 0 while the exponent b approaches unity. The D 0 and A pair increase with, and appear to be determined largely by, the updraft strength, thus providing a possible means of determining the appropriate algorithms for rainfall measurement. Although the small drop number samples measured by the surface disdrometer relative to the large volumes sampled by a radar tend to truncate the DSD at both small and large drop sizes, narrow distributions with ϭ 5 to 12 cannot be attributed to such an effect. Such narrow DSDs accord with common experience of monodispersed large drops at the beginning of a convective storm. There is also remarkable agreement of the surface-based observations of ZDR-Z-D 0 with the time-space variations from C to T to S rain types observed by radar in England and elsewhere. Because the C region of a storm often accounts for a major share of the rain accumulation despite its shorter duration, it is particularly important to measure that region more accurately. There are distinctive clusters of the generalized number parameter N W versus D 0 between maritime and continental storms. Methods for remote sensing and parameterization must partition the rainstorms into convective, transition, and stratiform segments.

Differences in raindrop size distribution from southwest monsoon to northeast monsoon at Gadanki

“…As shown in Figures 1(a) and (b), at Gadanki the wind flow in the SW monsoon is over the continent, while in the NE monsoon it is over the ocean (Bay of Bengal). Therefore, the rainfall and DSD are mostly continental and oceanic in nature during SW and NE monsoon seasons, respectively, consistent with the observations of Kozu et al (2006) (Tokay et al, 2002;Rosenfeld and Ulbrich, 2003;Bringi et al, 2003;Ulbrich and Atlas, 2007). For instance, during moderate to heavy rain (R > 10 mm hr −1 ), 80% of the D m population at Gadanki is in the range 1.5-2.6 and 1.3-2.2 mm in the SW and NE monsoons, respectively.…”

“…Different DSDs in different seasons will lead to different Z -R relationships ( Figure 9). The prefactor and exponent of the relations are completely different from each other and also from that used at other geographical locations (Rosenfeld and Ulbrich, 2003;Ulbrich and Atlas, 2007;and references therein). Thus, the usage of a single Z -R relation for converting radar reflectivity into R will underestimate in one season and overestimate in the other season.…”

“…Several possible reasons for the observed differences in DSD are examined, including instrumental problems to geophysical mechanisms. From the observed wind field and similarities in the distribution (and the range) of D m during the SW (NE) monsoon and continental (oceanic) DSD elsewhere (Tokay et al, 2002;Bringi et al, 2003;Rosenfeld and Ulbrich, 2003;Ulbrich and Atlas, 2007), it is considered that the rain DSD at Gadanki in the SW and NE monsoons are, respectively, continental and oceanic in nature. Except for the first study mentioned above, all other studies used ensembles of rain DSDs collected from several geographical locations with an assumption that all oceanic (continental) DSD are influenced by similar microphysical processes.…”

A Comparative Study of B-, Γ- and Log-Normal Distributions in a Three-Moment Parameterization for Drop Sedimentation

“…From comparisons with measurements, we know that rain drop spectra are well modeled by Γ-and Log-Normal Distributions [32][33][34][35]. While the Γ-Distribution in one-or two-parameter exponential form (as suggested by Marshall and Palmer [9] or Waldvogel [10]) is suitable for describing rain drop spectra averaged over a long time, the three-parameter Γ-and also the Log-Normal Distribution are both able to model spectra from a short rain event [32].…”

Tropical Rain Classification and Estimation of Rain From Z-R (Reflectivity-Rain Rate) Relationships

“…Atlas et al in [7] and Ulbrich and Atlas in [12] studied the DSDs during the three regimes (stratiform, convective and transition) and determined the Z-R relations for each of these regimes. They pointed out that there is a systematic variation of the Z-R relations for these three types of rain [12].…”