Subsidence Warming as an Underappreciated Ingredient in Tropical Cyclogenesis. Part I: Aircraft Observations

The formation of tropical cyclones (TCs) in unfavorable large-scale environments remains a challenge for TC forecasting. Tropical Storm (TS) Cindy (2017) formed at 1800 UTC 20 June 2017 in the Gulf of Mexico despite strong vertical wind shear, low midtropospheric relative humidity, and poorly organized convection. A key to TC genesis is the initial development of a warm core within an emergent cyclonic vortex, a process that occurs on small spatial scales and is often difficult to observe. TS Cindy was observed during the Convective Processes Experiment (CPEX) field campaign in 2017 by the NASA DC-8 aircraft, equipped with a Doppler wind lidar, precipitation radar, and GPS dropsondes. This study combines CPEX observations and a cloud-resolving, fully coupled atmosphere–wave–ocean numerical simulation to investigate the formation of TS Cindy. Prior to TC genesis, a shallow cyclonic circulation was embedded in a deep layer of west-southwesterly flow associated with an upper-level trough. Within the disturbance, a warm and dry anomaly was observed by dropsondes near the center of the cyclonic circulation, with a maximum at about the 2.5-km level. In the coupled model simulation, the temperature perturbation reached 5°C along with a dewpoint temperature depression of 8°C. Backward trajectory analysis shows that subsidence is primarily associated with a thermally indirect circulation along the western flank of the storm. Air parcels descend more than 1000 m toward the lower troposphere while warming up by 9°–12°C. The subsidence-induced virtual temperature perturbation in the 1.5–3.5-km layer accounts for 50% of the sea level pressure depression. Subsidence warming, therefore, played a key role in the genesis of TS Cindy.

Precursor synoptic disturbance characteristics associated with 35 tropical cyclone (TC) genesis events in the South China Sea (SCS) during 2000–2011 were examined. All genesis events occurred between May and December. Six types of precursor synoptic disturbances are identified, and their low-level composite patterns are constructed. They are synoptic-scale wave train (29%), TC energy dispersion (14%), Pacific easterly wave (9%), Borneo vortex (11%), TC southwesterly shear induced vortex (23%), and others (14%). In 13 (22) of 35 genesis cases, precursor perturbations originated within (outside of) the SCS. The satellite IR image data are used to analyse mesoscale convective activity prior to TC genesis. It is found that mesoscale convective systems were observed at 83% of the genesis cases, whereas 54% of the genesis cases experienced multiple mesoscale convective system development at a single time. Most of synoptic disturbances associated with the tropical cyclogenesis occurred in the region where atmospheric quasi-biweekly oscillation (QBW) and Madden-Julian oscillation (MJO) modes are in an active phase. The synoptic disturbances-associated TC genesis events in the SCS are strongly modulated by atmospheric low-frequency oscillations (especially the MJO). Meanwhile, favourable precursor sea surface temperature signals are observed, primarily on the 10- to 20-day and 20- to 90-day timescales.

A modeling study on the effects of MJO and equatorial Rossby waves on tropical cyclone genesis over the western North Pacific in June 2004

Extreme rainfall and tropical cyclones (TCs) are of great interest to the meteorological community. Their influences extend to many different industries such as agriculture, airlines and insurance, and they keep challenging forecasters in making their decisions. A great deal of effort has been put into improving our understanding of the extreme rainfall and tropical cyclone genesis, as well as increasing the forecast accuracy in the numerical weather prediction models and operational forecast systems. The research presented here examines extreme rain events and tropical cyclone genesis from a potettial vorticity (PV) viewpoint using mainly European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim (ERAI) reanalysis. PV is used as a diagnostic of the atmospheric flow in this research and the main meteorological field to investigate the synoptic conditions during extreme rain events and the formation of tropical cyclones. The objective tracking technique introduced by Berry et al. (2012) is used to track cyclonic PV centres on three isentropic surfaces (315 K, 330 K, and 350 K) and construct a global climatology of the coherent cyclonic PV centres density and motion. It is found that the coherent cyclonic PV centres lie predominantly on the equatorward flank and commonly travel around the periphery of the mean subtropical anticyclones. This motion implies the transport of cyclonic PV centres from the midlatitudes to the tropics on the eastern side of the anticyclones. One area in which the meridional transport of the PV from the extratropics toward the tropics is large is the east coast of Australia during December-January-February (DJF). Extreme rainfall is defined by the 95th percentile of the daily rainfall time series for particular season from 1997 to 2009 from Global Precipitation Climate Project (GPCP). It is shown that 95% and 82% of the extreme rain days in the Australian and north African tropics, respectively, are associated with a cyclonic PV centre. The composites based on extreme rain days in the two regions also show a coherent cyclonic PV centre upstream of the precipitating region. The vertical structures are broadly the same, although the amplitude is a little larger in the Australian tropics. The vertical wind structures reveal that the synoptic pattern in the Australian tropics is similar to a mesoscale convective system (MCS), whereas in the north African tropics it is closer to an easterly wave. It is also shown that after extreme rain events in the Australian tropics, the large-scale circulation dramatically changes with the positive rainfall anomaly regions and the monsoon trough moving far southward, and with rainfall over west Australia and monsoon trough frequency in the extratropics increasing significantly. Tropical cyclones are usually associated with coherent cyclonic PV centres in the lower and mid-troposphere. It is found that there are approximately 3 - 4 TCs per year in North Atlantic which have associated coherent cyclonic PV centres originating in the extratropics (i.e. poleward of 23.5◦), whereas in South Pacific, approximately 10 TCs are found over 30 years (1980 - 2009) with associated coherent cyclonic PV centres originating in the extratropics. Tropical cyclone Larry (March 2006) formed in the Coral Sea and was associated with a 330 K isentropic coherent cyclonic PV centre originating in the extratropics is investigated here. PV filamentation along the east coast of Australia plays a role in the generation of TC Larry, and Larry is the only TC in March-April-May season that has an associated coherent PV maximum detected first in the extratropics. It is also found that Rossby waves breaking (RWB) characterised by an elongated upper-level PV trough over the Coral Sea intensifies a tropical disturbance into a tropical depression before a second RWB event associated with a cold surge, combines with small vertical wind shear, to intensify the low-level vorticity eventually forming TC Larry.

This is the first of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. The primary TC-LAPS vortex enhancement mechanism is presented in Part I, the entire genesis process is illustrated in Part II using a single TC-LAPS simulation, and in Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. The primary vortex enhancement mechanism in TC-LAPS is found to be convergence/stretching and vertical advection of absolute vorticity in deep intense updrafts, which result in deep vortex cores of 60–100 km in diameter (the minimum resolvable scale is limited by the 0.15° horizontal grid spacing). On the basis of the results presented, it is hypothesized that updrafts of this scale adequately represent mean vertical motions in real TC genesis convective regions, and perhaps that explicitly resolving the individual convective processes may not be necessary for qualitative TC genesis forecasts. Although observations of sufficient spatial and temporal resolution do not currently exist to support or refute this proposition, relatively large-scale (30 km and greater), lower- to midlevel tropospheric convergent regions have been observed in tropical oceanic environments during the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE), the Equatorial Mesoscale Experiment (EMEX), and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), and regions of extreme convection of the order of 50 km are often (remotely) observed in TC genesis environments. These vortex cores are fundamental for genesis in TC-LAPS. They interact to form larger cores, and provide net heating that drives the system-scale secondary circulation, which enhances vorticity on the system scale akin to the classical Eliassen problem of a balanced vortex driven by heat sources. These secondary vortex enhancement mechanisms are documented in Part II. In some recent TC genesis theories featured in the literature, vortex enhancement in deep convective regions of mesoscale convective systems (MCSs) has largely been ignored. Instead, they focus on the stratiform regions. While it is recognized that vortex enhancement through midlevel convergence into the stratiform precipitation deck can greatly enhance midtropospheric cyclonic vorticity, it is suggested here that this mechanism only increases the potential for genesis, whereas vortex enhancement through low- to midlevel convergence into deep convective regions is necessary for genesis.

Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale Model (MM5). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite-derived cloud-top black body temperature (T bb ) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was associated with intense upward motion in the MCS with speed up to 80 cm s−1. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front.

The genesis of intense cyclonic vorticity in the boundary layer and the transformation of a low-level cold pool to a warm-core anomaly associated with the long-lived mesoscale convective systems (MCSs), which produced the July 1977 Johnstown flash flood and later developed into a tropical storm, are examined using a 90-h real-data simulation of the evolution from a continental MCS/vortex to an oceanic cyclone/storm system. It is shown that the midlevel vortex/trough at the end of the continental MCS's life cycle is characterized by a warm anomaly above and a cold anomaly below. The mesovortex, as it drifts toward the warm Gulf Stream water, plays an important role in initiating and organizing a new MCS and a cyclonic (shear) vorticity band at the southern periphery of the previously dissipated MCS. It is found from the vorticity budget that the vorticity band is amplified through stretching of absolute vorticity as it is wrapped around in a slantwise manner toward the cyclone center. Then, the associated shear vorticity is converted to curvature vorticity near the center, leading to the formation of a “comma-shaped” vortex and the rapid spinup of the surface cyclone to tropical storm intensity. Thermodynamic budgets reveal that the vertical transfer of surface fluxes from the warm ocean and the convectively induced grid-scale transport are responsible for the development of a high-θe tongue, which is wrapped around in a fashion similar to the vorticity band, causing conditional instability and further organization of the convective storm. Because the genesis occurs at the southern periphery of the vortex/trough, the intensifying cyclonic circulation tends to advect the pertinent cold air in the north-to-northwesterly flow into the convective storm and the ambient warmer air into the cyclone center, thereby transforming the low-level cold anomaly to a warm-cored structure near the cyclone core. It is shown that the transformation and the evolution of the surface cyclone are mainly driven by the low-level vorticity concentrations. It is found that many of the cyclogenesis scenarios in the present case are similar to those noted in previous tropical cyclogenesis studies and observed at the early stages of tropical cyclogenesis from MCSs during the Tropical Experiment in Mexico. Therefore, the results have significant implications with regard to tropical cyclogenesis from MCSs.

Numerical simulations of three separate events of tropical cyclogenesis (TC-genesis) off the West African coast between the years of 2006 and 2008 were performed. The purpose of this study was to investigate the processes that take place during the transition of an African easterly wave (AEW) and any associated mesoscale convective systems (MCSs) as they progress from continental West Africa into the maritime environment of the eastern Atlantic Ocean. Three tropical cyclones that were associated with AEWs and related MCSs over continental West Africa that progressed off the coast, later achieving at least tropical storm (TS) strength, were selected to be investigated. The three tropical cyclones were: TS Debby (2006), Hurricane Helene (2006), and TS Josephine (2008). The Weather Research and Forecasting (WRF) model was utilized to conduct numerical model simulations beginning 72 h prior to each system’s AEW being classified as a tropical depression (TD). Results demonstrated that the model was able to recapture the evolution of each MCS in association with AEWs during all three events. The sensitivity experiments of the impact of topography (i.e., Guinea Highlands) suggested that the elevation of the Guinea Highlands plays a significant role in relation to TC-genesis, even though the highest peaks of the Guinea Highlands are only approximately 1,300 m. Simulation results supported that topographical blocking and northwest deflection of strong southwest winds from the Atlantic played an important role in the enhancement of low-level cyclonic circulation. Without the presence of the Highlands, wind speeds associated with each circulation by simulation’s end were either weaker or the simulation failed to generate a circulation completely. As the MCSs developed along the coast, they became phaselocked in the downstream flow of an AEW as it exited the West African coast. The MCS in each event acted as a catalyst for TC-genesis with the associated AEW. Without the Guinea Highlands, the MCS features were either weakened or failed to develop, thus hindering TC-genesis for these three cases.

This is the second of two papers examining the role of equatorial Rossby (ER) waves in tropical cyclone (TC) genesis. Based on results from Part I, it is hypothesized that genesis resulting from the circulation of an ER wave alone is uncommon and that the majority of ER wave–related genesis events occur when a sufficiently intense ER wave interacts with a favorable background flow environment. This paper examines this contention by performing a series of simulations in which ER waves are imposed upon idealized background flows. The background flows are designed to resemble a region of a monsoon trough (MT), a flow feature observed at certain times of the year in all of the TC basins, and most dramatically, in the western North Pacific basin. It is believed that ER wave interactions with the MT may speed up the internal breakdown genesis mechanism of the MT, or even result in genesis when the MT is too weak to breakdown from in situ processes alone. The latter scenario is examined here. When just the MT is simulated without the ER wave anomaly fields, the MT remains quasi-steady and TC genesis does not occur. It is only when the ER wave is imposed on the MT that TC genesis is initiated. The results imply that the ER wave–MT interactions produce more TCs than would otherwise occur if no such interactions took place. Results demonstrate that wave breaking of the ER wave is a mechanism by which vorticity is organized on the scale of a TC. This process features a decrease in the initial horizontal scale of the cyclonic gyre of the ER wave to a scale comparable with a TC. This genesis mechanism is sensitive to the magnitude of the background cyclonic vorticity of the MT, as TC genesis is only initiated when the value of the 850-mb relative vorticity of the MT is larger than 2 × 10−5 s−1. This genesis pathway provides a unique interpretation of TC genesis and is compared with previous theories on TC genesis.

The development of two small mesoscale convective systems (MCSs) in northeastern Colorado is investigated via dual-Doppler radar analysis. The first system developed from several initially isolated cumulonimbi, which gradually coalesced into a minimal MCS with relatively little stratiform precipitation. The second system developed more rapidly along an axis of convection and generated a more extensive and persistent stratiform echo and MCS cloud shield. In both systems, the volumetric precipitation rate exhibited an early meso-b-scale convective cycle (a maximum and subsequent minimum), followed by reintensification into a modest mature stage. This sequence is similar to that noted previously in the developing stage of larger MCSs by McAnelly and Cotton. They speculated that the early meso-b convective cycle is a characteristic feature of development in many MCSs that is dynamically linked to a rather abrupt transition toward mature stage structure. This study presents kinematic evidence in support of this hypothesis for these cases, as derived from dual-Doppler radar analyses over several-hour periods. Mature stage MCS characteristics such as deepened low- to midlevel convergence and mesoscale descent developed fairly rapidly, about 1 h after the early meso-b convective maximum. The dynamic linkage between the meso-b convective cycle and evolution toward mature structure is examined with a simple analytical model of the linearized atmospheric response to prescribed heating. Heating functions that approximate the temporal and spatial characteristics of the meso-b convective cycle are prescribed. The solutions show that the cycle forces a response within and near the thermally forced region that is consistent with the observed kinematic evolution in the MCSs. The initial response to an intensifying convective ensemble is a self-suppressing mechanism that partially explains the weakening after a meso-b convective maximum. A lagged response then favors reintensification and areal growth of the weakened ensemble. A conceptual model of MCS development is proposed whereby the early meso-b convective cycle and the response to it are hypothesized to act as a generalized forcing‐feedback mechanism that helps explain the upscale growth of a convective ensemble into an organized MCS.

Future changes in tropical cyclone (TC) genesis locations and frequency are explored by identifying relationships between TC genesis and dominant daily large-scale patterns, and evaluating the strength of these relationships under a climate change scenario. Self-Organizing Maps (SOMs) are used to characterize the dominant large-scale patterns in reanalysis data and in a regional climate model ensemble simulation of current climate. The main features on the resulting sea level pressure (SLP) SOMs are nodes that resemble both the negative and positive phases of the North Atlantic Oscillation, as well as blocking and ridging regimes. The frequency of the NAO-like nodes is strongly linked to TC genesis frequency and preferred genesis locations. This link is used to develop a statistical relationship between the frequency of large scale SLP patterns and TC genesis. The application of this relationship to an ensemble regional climate simulation under a future climate forcing scenario predicts fewer TCs, which is consistent with the regional climate model that explicitly simulates fewer TCs. This demonstrates the strength of the relationships and their use in assessing future changes in TC genesis locations and frequency.

Tropical cyclone (TC) genesis is initiated by convective precursors or “seeds” and influenced by environmental conditions along the seed-to-TC trajectories. Genesis potential indices (GPIs) provide a simple way to evaluate TC genesis likelihood from environmental conditions but have two limitations that may introduce bias. First, the globally fixed GPIs fail to represent interbasin differences in the relationship between environments and genesis. Second, existing GPIs are only functions of local environmental conditions, whereas nonlocal factors may have a significant impact. We address the first limitation by constructing basin- and time-scale-specific GPIs (local-GPIs) over the eastern North Pacific (ENP) and North Atlantic (NA) using Poisson regression. A sequential feature selection (SFS) algorithm identifies vertical wind shear and a heating condition as leading factors controlling TC genesis in the ENP and the NA, respectively. However, only a slight improvement in performance is achieved, motivating us to tackle the second limitation with a novel trajectory-based GPI (traj-GPI). We merge adjacent nonlocal environments into each grid point based on observed seed trajectory densities. The seed activity, driven mainly by upward motion, and the transition to TCs, controlled primarily by vertical wind shear or heating conditions, are captured simultaneously in the traj-GPI, yielding a better performance than the original GPIs. This study illustrates the importance of seed activity in modeling TC genesis and identifies key environmental factors that influence the process of TC genesis at different stages. Significance Statement The genesis potential index (GPI) is an effective tool for modeling the likelihood of tropical cyclone (TC) genesis for a given time and location. This study reveals that existing GPIs are primarily biased by a lack of information about nonlocal TC seed activity, since they are based only on local large-scale environmental variables. According to our study, upward motion and vertical wind shear are the most influential environmental factors in seed genesis and the transition from seed to TC, respectively. Based on the observed seed trajectories, we build trajectory-based GPIs that include the information from seed activity. Spatiotemporal performances of TC genesis are significantly improved over the original GPIs.

Mei-Yu (Baiu in Japan) is a weather and climate phenomenon in the area of Japan, Taiwan, and subtropical China where the seasonal rainfall distribution reaches a peak in late spring and early summer due to the repeated occurrence of the Mei-Yu front. From the satellite pictures, the Mei-Yu front is usually accompanied by a nearly continuous cloud band with organized mesoscale convective systems (MCSs). To the south of the Mei-Yu front, a low-level jet (LLJ) is often observed and is closely related to the formation of MCSs and heavy rainfall events. As the Mei-Yu front approaches Taiwan, the front and the accompanying MCSs and LLJ tend to be affected by the mesoscale topography of Taiwan, the Central Mountain Range (CMR). Besides, the land-sea contrast coupled with island topography produces the land-sea breeze, the mesolow, and the island circulations which are important in modulating the local precipitation.In this paper, an overview of the current understanding of the structure and dynamics of the mesoscale features observed in the Taiwan Mei-Yu season is presented. Research results in the pre-TAMEX era as well as those derived from TAMEX program are discussed for the Mei-Yu front, the LLJ, the MCSs, the mesolow, the land-sea breeze, and the island circulations.

Abstract. In order to investigate the relationship between latent heating (LH) and radiative heating (Qrad), in particular the heating released by mesoscale convective systems (MCSs), we used synergistic satellite-derived data from active instruments. Given the sparse sampling of these observations, we expanded the spectral LH profiles derived from the Tropical Rain Measurement Mission (TRMM-SLH) by applying artificial neural network regressions to the Clouds from InfraRed Sounders (CIRS) data and meteorological reanalyses, following a similar approach as for the expansion of the Qrad profiles. A direct comparison with the collocated TRMM-SLH data shows excellent agreement of the average profiles, but the prediction range is underestimated, in particular between 550 and 900 hPa. Noise related to discrepancies in rain fraction between TRMM and CIRS-ML (machine learning), as well as an underestimation of extremes, can be reduced by averaging over larger areas. The zonal averages of vertically integrated LH (LP) at 01:30 and 13:30 LT align well with those from the full diurnal sampling of TRMM-SLH over the ocean. For upper-tropospheric (UT) clouds with a large amount of latent heat release, the surface temperature has a larger impact on the atmospheric cloud radiative effect (ACRE) in dry environments than in humid ones, while humidity plays a large role in cool rather than in warm environments. In all situations, the cloud height is mostly responsible for the value of ACRE. The distribution of UT clouds in the LP–ACRE plane shows a very large spread in the ACRE for small values of LP, which is gradually reduced towards larger values of LP. The mean ACRE of mature MCSs increases with LP, up to about 115 W m−2. As expected, the shapes of the LH profiles of mature MCSs show that larger, more organized MCSs have a larger contribution of stratiform rain than the smaller MCSs do. Furthermore, convective organization enhances the mean ACRE of mature MCSs on average by about 10 W m−2 over the whole LP range.

The central Great Plains region in North America has a nocturnal maximum in warm-season precipitation. Much of this precipitation comes from organized mesoscale convective systems (MCSs). This nocturnal maximum is counterintuitive in the sense that convective activity over the Great Plains is out of phase with the local generation of CAPE by solar heating of the surface. The lower troposphere in this nocturnal environment is typically characterized by a low-level jet (LLJ) just above a stable boundary layer (SBL), and convective available potential energy (CAPE) values that peak above the SBL, resulting in convection that may be elevated, with source air decoupled from the surface. Nocturnal MCS-induced cold pools often trigger undular bores and solitary waves within the SBL. A full understanding of the nocturnal precipitation maximum remains elusive, although it appears that bore-induced lifting and the LLJ may be instrumental to convection initiation and the maintenance of MCSs at night. To gain insight into nocturnal MCSs, their essential ingredients, and paths toward improving the relatively poor predictive skill of nocturnal convection in weather and climate models, a large, multiagency field campaign called Plains Elevated Convection At Night (PECAN) was conducted in 2015. PECAN employed three research aircraft, an unprecedented coordinated array of nine mobile scanning radars, a fixed S-band radar, a unique mesoscale network of lower-tropospheric profiling systems called the PECAN Integrated Sounding Array (PISA), and numerous mobile-mesonet surface weather stations. The rich PECAN dataset is expected to improve our understanding and prediction of continental nocturnal warm-season precipitation. This article provides a summary of the PECAN field experiment and preliminary findings.