Generated using the official AMS LATEX template v5.0 1 Subsidence Warming in the Tropical Cyclogenesis of Cindy (2017): CPEX 2 observations and Coupled Modeling ∗ 3 Edoardo Mazza and Shuyi S. Chen 4 Department of Atmospheric Sciences, University of Washington, Seattle Manuscript ∗ 5 Corresponding author: [email protected] 1 ABSTRACT 6 The formation of tropical cyclones (TC) in unfavorable large-scale environments remains a chal- 7 lenge for TC forecasting. Tropical Storm (TS) Cindy (2017) formed at 1800 UTC 20 June in 8 the Gulf of Mexico despite strong vertical wind shear, low mid-tropospheric relative humidity, 9 and poorly organized convection. A key to TC genesis is the initial development of a warm 10 core within an emergent cyclonic vortex, a process which occurs on small spatial scales and is 11 often difficult to observe. TS Cindy was observed during the Convective Processes Experiment 12 (CPEX) field campaign in 2017 by the NASA DC-8 aircraft, equipped with a Doppler wind lidar, 13 Manuscript precipitation radar, and GPS dropsondes. This study combines CPEX observations and a cloud- 14 resolving, fully-coupled atmosphere-wave-ocean numerical simulation to investigate the formation 15 of TS Cindy. Prior to TC genesis, a shallow cyclonic circulation was embedded in a deep layer 16 of west-southwesterly flow associated with an upper-level trough. Within the disturbance, a warm 17 and dry anomaly was observed by dropsondes near the center of the cyclonic circulation, with 18 a maximum at about the 2.5 km level. The temperature perturbation reaches 5°C along with a 19 dew point temperature depression of 8°C in the coupled model simulation. Backward trajectory 20 analysis shows that subsidence is primarily associated with a thermally indirect circulation along 21 the western flank of the storm. Air parcels descend more than 1000 m towards the lower tropo- 22 sphere while warming up by 9-12°C. The subsidence-induced virtual temperature perturbation in 23 the 1.5-3.5 km layer accounts for 50 % of the sea-level pressure depression. Subsidence warming 24 therefore played a key role in the genesis of TS Cindy. 2 25 1. Introduction 26 The genesis of tropical cyclones is a multiscale process that involves the transformation of a 27 precursor disturbance into a warm core, low-pressure system with a closed surface circulation. 28 Precursor disturbances can be tropical waves in the tropical basins (Frank and Roundy 2006), long- 29 lasting mesoscale convective systems (MCSs) or cloud clusters (Kerns and Chen 2013), monsoon 30 lows, Central-American gyres (Papin et al. 2017) or have an extra-tropical origin (Davis and Bosart 31 2003). TC genesis is facilitated when a set of large-scale conditions are met, such as low vertical 32 wind shear, a moist mid-troposphere, a vorticity-rich low troposphere, a deep and warm ocean 33 Manuscript mixed layer (Gray 1968; McBride and Zehr 1981) and a large thermodynamic disequilibrium 34 between the tropopause and the sea surface (McTaggart-Cowan et al. 2015). TC genesis involves 35 two fundamental processes: the amplification and organization of cyclonic vorticity, and the 36 formation of a warm core vortex. Pre-existent, low-level cyclonic vorticity can be amplified and 37 axisymmetrized by vortical hot towers (Hendricks et al. 2004; Montgomery et al. 2006). Mid- 38 tropospheric vortices can instead result from diabatic heating in the stratiform region of long-lived 39 MCSs (Chen and Frank 1993) or from evaporative cooling in the precipitating region of MCSs 40 (Bister and Emanuel 1997). The formation of the warm core is supported by diabatic heating in 41 the convective and stratiform cloud region (Chen and Frank 1993; Dolling and Barnes 2012a). An 42 observational study by Kerns and Chen (2015) showed that subsidence warming associated with 43 MCSs can contribute directly to development of warm core vortex in TC genesis. 44 About 60 % of all TC genesis events in the North Atlantic from 1948 to 2010 involved a varying 45 degree of baroclinicity (McTaggart-Cowan et al. 2013). In the Western Caribbean Basin and 46 Gulf of Mexico, TC genesis often occurs in unfavorable environments, with upper-tropospheric 47 disturbances enhancing the vertical wind shear and promoting mid-tropospheric dry air intrusions 3 48 (Gray 1968). Bracken and Bosart (2000) found that a pronounced upper-tropospheric trough-ridge 49 pattern is often associated with TC genesis events in the Bahamas and Gulf of Mexico. Strong 50 vertical wind shear is considered to be unfavorable for TC genesis as observed in the Atlantic and 51 the west Pacific (McBride and Zehr 1981; Kerns and Chen 2013), while weak-to-moderate westerly 52 shear can instead assist TC genesis (Bracken and Bosart 2000; Nolan and McGauley 2012; Reasor 53 and Montgomery 2001). Wind shear induces significant structural changes in TCs, which can 54 hinder their development or intensification: idealized experiments (Jones 1995; Frank and Ritchie 55 1999, 2001) and observational studies (Rodgers et al. 1994; Black et al. 2002; Chen et al. 2006; 56 Corbosiero and MolinariManuscript 2002; Reasor et al. 2013) indicate that the structure of sheared TCs is 57 highly asymmetric, as the deepest convection focuses in the downshear quadrants. Another factor 58 that greatly affects the formation of TCs is mid-tropospheric humidity (Malkus 1958; Gray 1975; 59 McBride and Zehr 1981). TC genesis is favored by high relative humidity in the mid-levels, whereas 60 intrusions of dry air can delay or suppress TC development (e.g. Dunion and Velden 2004; Wang 61 2012; Kerns and Chen 2013). Ventilation of dry and cool air into the developing TC disrupts its 62 thermodynamic structure and suppresses convection by reducing the updrafts buoyancy (Simpson 63 and Riehl 1958; Shelton and Molinari 2009; Riemer and Montgomery 2011; Tang and Emanuel 64 2012; Ge et al. 2013). Importantly, how TCs develop in relatively unfavorable environmental 65 conditions over the Gulf of Mexico remains an active area of research. 66 Subsidence-driven temperature anomalies associated with oceanic convective systems have been 67 documented since the GATE field campaign (Houze 1977; Zipser 1977). Simpson et al. (1997) 68 argue that subsidence is the only viable process for maintaining a low-level warm anomaly in clear 69 air. Descent is typically observed in the form of unsaturated downdrafts underneath the anvil canopy 70 of long-lived MCSs. Chen and Frank (1993) showed the presence of a wake low in a region of 71 low-tropospheric subsidence on the edge of a simulated MCS. In mature tropical cyclones, instead, 4 72 observed subsidence-induced warm anomalies are often attributed to vortex tilting (Halverson 73 et al. 2006; Heymsfield et al. 2006; Shelton and Molinari 2009). The relationship between wind 74 shear and asymmetric warm anomalies in TCs has been investigated by Tao and Zhang (2019), 75 which describe how the alignment of a mid-tropospheric and upper-tropospheric warm anomalies 76 is often seen as the storm approaches the onset of rapid intensification. A small number of studies 77 focuses on the role of subsidence during TC genesis and almost exclusively investigated the role of 78 subsidence within MCSs. Dolling and Barnes (2012b) and Dolling and Barnes (2012a) describe 79 how subsidence helped the formation of a lower-tropospheric warm core in TS Humberto (2001) 80 by inducing a hydrostaticManuscript pressure drop and by capping the boundary layer, allowing for a buildup 81 of high equivalent potential temperature air, which was later ingested in the nascent eyewall. 82 Stossmeister and Barnes (1992) document the formation of a second circulation center in TS Isabel 83 (1982) underneath a region of mesoscale subsidence. Finally, subsidence warming during TC 84 genesis was also captured by dropsondes released in typhoons Megi and Fanapi during the Impact 85 of Typhoons on Ocean in Pacific (ITOP) field campaign (Kerns and Chen 2015). 86 Is the presence of long-lived MCSs the only pathway to low-level subsidence warming during 87 TC genesis? In this study, we focus on the genesis of TS Cindy (2017) to describe how subsidence- 88 induced warm anomalies in the lower troposphere can support TC formation in a very different 89 dynamic and thermodynamic context. TS Cindy developed from a poorly-organized, broad cy- 90 clonic disturbance embedded in a high-shear, low mid-tropospheric relative humidity environment. 91 Airborne observations collected during the NASA Convective Processes Experiment (CPEX) field 92 campaign reveal a low-level warm anomaly formed within a shallow cyclonic circulation in a 93 cloud-free region, well removed from convective clusters and their associated anvils. Using a com- 94 bination of aircraft observations and a convection-resolving, fully-coupled atmosphere-wave-ocean 95 simulation, we address three main questions: a) what are the spatial and temporal characteristics 5 96 of the subsidence-induced temperature perturbation, b) does the subsidence-induced warming pro- 97 duce a significant pressure perturbation during TC genesis? and c) what mechanisms drive the 98 descent? The thermodynamic and kinematic properties of the disturbance are investigated using 99 dropsonde and airborne wind lidar retrievals, complemented by a high-resolution model simulation 100 to overcome their limited spatial and temporal sampling. The model simulation is then employed 101 to perform backward trajectory analysis and to diagnose the driving mechanisms of subsidence. 102 The paper is organized as follows: section 2 is an overview of the meteorological evolution of TS 103 Cindy, the data and methodology employed in the study are described in section 3. The results are 104 presented in section 4,Manuscript 5, 6, 7 and 8. A discussion of the results and their implications is included 105 in section 9.