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JUNE 2015 Z H U E T A L . 2267 Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes LIN ZHU Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China, and Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland DA-LIN ZHANG AND STEFAN F. CECELSKI Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland XINYONG SHEN Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China (Manuscript received 24 July 2014, in final form 15 March 2015) ABSTRACT The ‘‘bottom up’’ generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel ‘‘marsupial pouch’’ associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs’ merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to ‘‘escape’’ from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby’s genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is trans- formed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided. 1. Introduction vertical wind shear (VWS; Gray 1968, 1985). What pro- cesses are responsible for TCG within African easterly Tropical cyclogenesis (TCG) remains one of the waves (AEWs) after moving offshore over the Atlantic challenging topics in tropical meteorology today, de- Ocean remain elusive, given the lack of high-resolution spite recent advances made by observational and mod- observations over the genesis regions. These waves are eling studies. The climatological and synoptic-scale typically bounded latitudinally within 108–208Nand conditions associated with TCG are well known, ranging propagate off the West African coast with diurnally active from warm sea surface temperatures (SSTs) to weak deep convection (Frank 1970; Avila and Pasch 1992; Hopsch et al. 2007, 2010; Goldenberg and Shapiro 1996). Several field campaigns have been conducted in the past Corresponding author address: Dr. Da-Lin Zhang, Department of Atmospheric and Oceanic Science, University of Maryland, few years, such as the NASA African Monsoon Multi- College Park, 2419 CSS Building, College Park, MD 20742. disciplinary Activities (NAMMA) field experiment in 2006 E-mail: [email protected] (Zipser et al. 2009) and the Pre-Depression Investigation DOI: 10.1175/JAS-D-14-0217.1 Ó 2015 American Meteorological Society 2268 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 of Cloud Systems in the Tropics (PREDICT) in 2010 occurred during the NAMMA campaign (Chiao and (Montgomery et al. 2012), because more than half of Jenkins 2010; Sippel et al. 2011; Lin et al. 2013). Lin et al. North Atlantic tropical storms (TSs) form within AEWs. (2013) found that the origin of a pre-Debby mesoscale The relationship between AEWs and the associated convective system (MCS) could be traced back to an processes leading to TCG has been the focal point area to the southwest of the Arabian Peninsula, Asir of numerous observational and modeling studies. Mountains, and Ethiopian highlands. Chiao and Jenkins Thorncroft and Hodges (2001), Hopsch et al. (2007), and (2010) showed the importance of the Guinea highlands Ross and Krishnamurti (2007) have shown two pre- in the formation of the AEW by modulating the im- dominant tracks for AEWs: one on the northern side pinging westerly flow to enhance the cyclonic circula- and the other on the southern side of an African easterly tion. Sippel et al. (2011) used ensemble simulations to jet (AEJ), typically located near 158N, both of which reveal that dry air in the SAL slowed Debby’s in- have implications for TCG (Chen et al. 2008; Zawislak tensification during the pre-TD and TD stages, while and Zipser 2010). Ross et al. (2009) and Cornforth et al. cold SSTs and large VWS were responsible for Debby’s (2009) showed the important roles of barotropic energy dissipation thereafter. Despite the previous studies of conversion and diabatic heating in the growth of AEWs, the case, it still remains unclear what physical processes which in turn preconditions an environment for TCG. have led to the genesis of Debby. Emphasizing the role of AEWs in TCG, Dunkerton Thus, the objectives of this study are (i) to document et al. (2009) postulated that mesovortices and deep the evolution of an AEW in which Debby was spawned convection tend to be generated near the AEW critical and the environmental conditions that were favorable latitude. Later, Montgomery et al. (2010) and Wang for TCG; (ii) to examine the formation and amplifica- et al. (2010) elaborated on the concept of a ‘‘marsupial tion of several meso-b-scale midlevel (700–500 hPa) pouch’’ near the intersection of the AEW trough axis vortices (MVs) and their merging and interaction with and critical latitude. Within this pouch region, TCG is meso-b-scale low-level (below 700 hPa) vortices (LVs; favored as moist air parcels remain within a closed La- Cecelski and Zhang 2013) leading to the genesis of grangian circulation of the AEW and, thus, are pro- Debby; and (iii) to investigate the roles of bottom-up tected from adverse environmental conditions such as growth, wave pouch, and upper-level processes in the dry air in the Saharan air layer (SAL). The wave pouch genesis of Debby. They are achieved using the best- also facilitates vorticity aggregation and convective or- track, satellite image, the National Centers for Envi- ganization leading to TCG (Wang 2012, 2014). ronmental Prediction (NCEP) final analysis, and an 84-h Most of the previous studies investigating TCG have triply nested-grid Weather Research and Forecast mainly focused on the low- to midlevel processes, es- (WRF) Model simulation with the finest grid size of pecially the growth of cyclonic vorticity in the lower 1.33 km. troposphere, the so-called bottom-up growth of meso- The next section provides an overview of the case. b-scale vortices (e.g., Zhang and Bao 1996; Montgomery Section 3 describes the major WRF Model configuration et al. 2012; Wang et al. 2010). The warm-core de- and verification of the WRF simulation against available velopment is a fundamental character of a mature observations. Section 4 presents the physical processes tropical cyclone (TC; Ritchie and Holland 1997; Houze leading to the genesis of Debby, including the de- et al. 2009). The upper-tropospheric warming during velopment of the upper-level warming in relation to TCG just becomes a topic of studies (e.g., Zhang and meso-a-scale MSLP falls, the bottom-up development Zhu 2012; Cecelski and Zhang 2013; Cecelski et al. of some LVs and their ‘‘escape’’ from the pouch region, 2014). Zhang and Zhu (2012) showed that the upper- as well as the importance of merging MVs and sub- level warming by compensating subsidence and the re- sequent coupling with LVs within the AEW pouch in sidual between diabatic heating and adiabatic cooling in TCG. Piecewise potential vorticity (PV) inversion will updraft regions account hydrostatically for meso-a-scale be used to infer the contribution of the MVs to the falls in mean sea level pressure (MSLP) associated surface MSLP fall. Section 5 examines thermodynamic with a developing tropical depression (TD). Cecelski transformation during the genesis of Debby. A sche- and Zhang’s (2013) work further elaborated on the im- matic TCG model for the present case is given in the portance of upper-level processes in TCG, demonstrat- final section. ing the relationship between deep convection, the convectively generated upper-level outflow, and storm- 2. Overview scale warming. Several recent studies have been conducted to in- According to the official report of the National Hur- vestigate the genesis of TS Debby (2006), which ricane Center (NHC), the genesis of Debby occurred JUNE 2015 Z H U E T A L . 2269 FIG. 1. Hovmöller diagram of the meridionally (58–158N) aver- 2 2 aged cyclonic relative vorticity (shaded; 10 5 s 1), superimposed 2 with meridional wind speeds (dashed; interval of 2 m s 1)at 600 hPa during the period from 1200 UTC 16 Aug to 1200 UTC 20 Aug 2006 and longitudinally spanning from 208Wto258E, based on FIG. 2. (a) Horizontal distribution of the cyclonic relative vor- the NCEP final analysis. The thick dashed line represents the 2 2 ticity (shaded; 10 5 s 1), superimposed with the comoving (black) trough axis of the AEW under study. and ground-relative (red) streamlines at 600 hPa; and (b) west–east vertical cross section of the zonal-mean (5–158N) cyclonic relative 2 2 vorticity (shaded; 10 5 s 1) and meridional wind speeds (contours; as a result of ‘‘convective banding’’ within a broad 2 interval of 2 m s 1; solid is southerly; dashed is northerly) at the closed circulation of an AEW.

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