Drop Size Distributions and Radar Observations of Convective and Stratiform Rain over the Equatorial Indian and West Pacific Oceans

Two-dimensional video disdrometer (2DVD) data were analyzed from two equatorial Indian (Gan) and west Pacific Ocean (Manus) islands where precipitation is primarily organized by the intertropical convergence zone and theMadden–Julian oscillation (MJO). The 18 (3.5) months of 2DVDdata fromManus (Gan) Island show that 1) the two sites have similar drop size distribution (DSD) spectra of liquid water content, median diameter, rain rate R, radar reflectivity z, normalized gamma number concentration Nw, and other integral rain parameters; 2) there is a robust Nw-based separation between convective (C) and stratiform (S) DSDs at both sites that produces consistent separation in other parameter spaces. The 2DVD data indicate an equatorial, maritime average C/S rainfall accumulation fraction (frequency) of 81/19 (41/59) at these locations. It is hypothesized that convective fraction and frequency estimates are slightly higher than previous radar-based studies, because the ubiquitous weak, shallow convection (,10mmh) characteristic of the tropical warm pool is properly resolved by this high-resolution DSD dataset and identificationmethod. This type of convection accounted for about 30%of all rain events and 15%of total rain volume. These rain statistics were reproduced when newly derived C/SR(z) equations were applied to 2DVD-simulated reflectivity. However, the benefits of using separate C/S R(z) equations are only realizable when C/S partitioning properly classifies each rain type. A single R(z) relationship fit to all 2DVD data yielded accurate total rainfall amounts but overestimated (underestimated) the stratiform (convective) rain fraction by 610% and overestimated (underestimated) stratiform (convective) rain accumulation by 150% (215%). 1. Background and motivation The majority of the world’s rainfall occurs in the tropics, particularly over the warm pool spanning the equatorial Indian and west Pacific Oceans. Attributing rainfall to certain cloud types (i.e., shallow, congestus, or deep convection, stratiform rain, or a mixture thereof) is of critical importance for diagnosing the resulting vertical distribution of latent heating (Johnson et al. 1999; Schumacher et al. 2004), which can drive convergence and vertical motion (Matsuno 1966;Yanai et al. 1973;Zhang andHagos 2009). Toward this end, identifying dominant modes of tropical, oceanic rain variability is important because this is still a major source of uncertainty in ground-based, shipborne, and spaceborne radar rainfall estimation (Munchak et al. 2012). For example, many studies have thoroughly detailed why and how cloud microphysical processes and verticalmotions differ during convective (C) and stratiform (S) rain, which lead to characteristically different drop size distributions (DSDs) in each rain type (Williams et al. 1995; Tokay andShort 1996;Houze 1997; Tokay et al. 1999;Atlas et al. 1999, 2000; Bringi et al. 2003, hereafter BR03; Houze 2004; Bringi et al. 2009, hereafter BR09; Thurai et al. 2010, hereafter TH10; Schumacher et al. 2015; Zhu et al. 2015). There is also a region where (or time period when) active convective updrafts might be decaying into stratiform precipitation (Biggerstaff and Houze 1993; Braun and Houze 1994; Williams et al. 1995; Uijlenhoet et al. 2003; Sharma et al. 2009). These resulting DSDs lie between convective and stratiform. Additionally, marked differences exist between continental and maritime DSDs, both Corresponding author address: Elizabeth J. Thompson, Department of Atmospheric Science, Colorado State University, 1371 Campus Delivery, Fort Collins, CO 80523-1371. E-mail: [email protected] NOVEMBER 2015 THOMPSON ET AL . 4091 DOI: 10.1175/JAS-D-14-0206.1 2015 American Meteorological Society of which produce convective and stratiform rain of varying intensities, efficiencies, and integral rain parameters, based on differences in updraft intensity and subcloud processes (Twomey 1977; Ulbrich and Atlas 1978; Zipser and LeMone 1980; Zipser 2003; Ulbrich andAtlas 2007;Minor et al. 2011; Wilson et al. 2013; Kumjian and Prat 2014). The primary goal of this study is to investigate DSDs of equatorial, oceanic rainfall, which are less often studied because of their remote location, despite their contribution to the global hydrologic cycle. To do so, we take advantage of two long-term 2D video disdrometer (2DVD) datasets over the equatorial Indian and west Pacific Oceans, at Gan (3.5-month record) andManus (18-month record) Islands, respectively (Fig. 1). BR03 identified maritime and continental convective DSD ‘‘clusters,’’ as well as a linear variation of stratiform rain in the normalized gamma number concentration and median volume diameter [Nw(D0)] plane, which can be measured by disdrometers or derived from dual-polarization radar data. Their work involved DSD quantities from selected rain events in Florida, coastal Australia, Austria, Puerto Rico, Brazil, Kwajalein, Colorado, Papua New Guinea, and the South China Sea, as well as a mean of many west Pacific warm pool events. A separation line between convective and stratiform rain was determined by BR09 using the Darwin, Australia, datasets. DSDs were considered convective (stratiform) ifNwwas greater (less) than a naturally emerging separator line: log10N w 521.6D01 6.3. This partitioningmethodwas found to be consistent with data from selected rain events in BR03 and with more data from Darwin by TH10 and Penide et al. (2013). TH10 also found agreement between the DSD-based Nw(D0) C/S partitioning method and the widely used Steiner et al. (1995) radar reflectivity-based partitioning algorithm using data fromDarwin. This radar method identifies convective cores based on a reflectivity threshold and whether localized regions of reflectivity standout relative to the smoothed, background reflectivity field, which can be modified for particular regions and radar data resolutions (Yuter and Houze 1997, 1998). The classification and rain attribution of shallow, weak cumulus convection are critical, because this cloud type is ubiquitous across the warm pool (Johnson et al. 1999; Rauber et al. 2007; Jakob and Schumacher 2008; Barnes and Houze 2013), where the atmosphere is conditionally unstable below the equivalent potential temperature ue minimum (Lilly 1960).However, this relatively shallow and weak oceanic convection is not dominant in coastal or continental boundary layers, which likely explains its underrepresentation inBR03,BR09, andTH10,which consist of data mostly from midlatitude and subtropical land locations near oceans. Shallow, maritime, tropical convective clouds moisten the lower troposphere (Nitta and Esbensen 1974; Lin and Johnson 1996; Johnson and Lin 1997; Johnson et al. 1999) and may play an important role in Madden–Julian oscillation (MJO) evolution (KemballCook and Weare 2001; Kiladis et al. 2005; Benedict and Randall 2007; Seo et al. 2014; Ruppert and Johnson 2015; Barnes et al. 2015). However, they are difficult to detect and track because of limited vertical, horizontal, and temporal resolution and the minimum detectable signals of many remote sensing platforms (Schumacher and Houze 2003; Jakob and Schumacher 2008; Funk and Schumacher 2013; Ruppert and Johnson 2015). The ‘‘stretched building block’’ hypothesis by Mapes et al. (2006) explains how stratiform clouds and all three major convective cloud types (shallow, congestus, and deep) are usually present over relatively large areas of the tropics, but some become more dominant than others during certain phases of the MJO.This is also consistentwith recentMJOobservational studies in the equatorial Indian and west Pacific Oceans (Riley et al. 2011; Barnes and Houze 2013; Zuluaga and Houze 2013; Powell and Houze 2013; Rowe and Houze 2014; Xu and Rutledge 2014, 2015; Barnes et al. 2015). Current DSD partitioning methods have not comprehensively considered tropical, oceanic convection. In fact, FIG. 1. DYNAMO northern (NSA) and southern sounding arrays (SSA), TOGA COARE intensive flux (IFA) and large sounding arrays (LSA), and GATE domains. The MISMO domain is a triangle in the same place as theDYNAMONSA, but without the northwest island. Gan Island is within the DYNAMO andMISMO domains, while Manus Island and Kwajalein (diamond) were included in the TOGA COARE array. 4092 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72