The Hydrometeor Structure of 2-Day Rain Events During DYNAMO

Each large tropical field campaign has extended our understanding of the tropical cloud population and its structure. Mesoscale convective systems were recognized as member of the tropical oceanic cloud population in GATE (Houze and Betts 1980). Kingsmill and Houze (1999a) expanded the conceptual model of the MCS by using Doppler velocity data from TOGA-COARE to characterize the layered kinematic structure of convective and stratiform regions, largely verifying Moncrieff (1992). The polarimetric capabilities of the S-PolKa radar deployed during DYNAMO provide the opportunity to expand these conceptual models by characterizing the organization of hydrometeors in these canonical structures. While Leary and Houze (1979b) utilized reflectivity data from GATE and drop size distributions to theorize the vertical hydrometeor structure of an MCS, DYNAMO provides the first opportunity to investigate their theorized structure using dual-polarimetric radar and the particle identification program developed in Vivekanandan (1999). The purpose of this study is to build upon the conceptual model put forth by Kingsmill and Houze (1999a) by using their canonical kinematic structure to investigate how hydrometeors are distributed throughout MCSs. 

This study focuses on the eleven rain events that produced the largest 24-hour accumulated rain amount during DYNAMO (Oct 2011 – January 16th, 2012), which are identified in Zuluaga and Houze (2013) using the SPolKa radar. In this study, the two RHI sectors (6°-82°, 116°-140°) in the SPolKa domain are manually reviewed for the duration of each rain event and databases of RHI scans that occur within 100 km of the radar and are characterized by the canonical layered airflows identified in Kingsmill and Houze (1999a) are created. Four databases are compiled. Three of the databases are based on the layered airflow with one database containing scans displaying the sloping convective updraft, one containing scans displaying a stratiform mid-level inflow, and one containing scans display very strong near surface winds. The fourth database contains scans that contain a bright band. Each database is created solely using the reflectivity and single-Doppler radial velocity fields; the structure of the polarimetric variables or particle identification program is disregarded while these databases are being constructed. After these databases are created the polarimetric variables and particle identification program is used to map the location of the different types of hydrometeors with reference to these canonical kinematic structures. Finally, the cases in each database are composited with respect their characteristic airflow or bright band structure, respectively, and a conceptual model of the hydrometeor organization in an MCS is developed.

Hydrometeors display a systematic structure within the convective and stratiform portions of mesoscale convective systems. Within the convective regions, the heaviest rain occurs just behind the convective updraft and rain intensity decreases with distance from the convective updraft. The convective updraft core often experiences moderate or light rain. Above the melting level, dry aggregates in the convective core extend almost to echo top and only a thin layer of non-oriented ice exists above the dry aggregates. Additionally, within the most intense convective updraft cores narrow, isolated regions of graupel may extend 1.5-2 km above the melting level.

In stratiform regions, the structure above the melting layer is more layered. The melting layer itself is characterized by a band of wet aggregates. Directly above the melting layer lies a relatively thin layer of dry aggregates. A relatively deep layer of non-oriented ice lies above the dry aggregates and characterizes the top portion of stratiform regions. Finally, a thin layer of horizontally oriented ice may characterize the echo boundaries. This thin layer of horizontally oriented ice is also observed along the echo boundaries of convective regions and in areas experiencing strong entrainment. During DYNAMO, 50-dBZ bright bands were observed quite often. Our results indicate that these regions are characterized with a very shallow layer of graupel just above and below the melting layer. High spectral width is often coincident with these regions, which may just suggest that highly turbulent air motions within these regions may be creating extremely large wet aggregates. This hypothesis might be supported by videosonde data from MISMO, which also displayed a thin layer of wet aggregates just above the melting level in stratiform regions (Suzuki et al., 2006). While rain directly below these 50-dBZ bright bands may be heavy, rain in stratiform regions is usually light or moderate in intensity.