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1. Genesis of an East Pacific Easterly Wave from a Panama Bight MCS: A Case Study Analysis from June 2012

   https://doi.org/10.1175/JAS-D-20-0032.1  

   Whitaker, Justin W. ; Maloney, Eric D. ( October 2020 , Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract This study investigates the transition of a Panama Bight mesoscale convective system (MCS) into the easterly wave (EW) that became Hurricane Carlotta (2012). Reanalysis, observations, and a convective-permitting Weather Research and Forecasting (WRF) Model simulation are used to analyze the processes contributing to EW genesis. A vorticity budget analysis shows that convective coupling and vortex stretching are very important to the transition in this case, while horizontal advection is mostly responsible for the propagation of the system. In the model, the disturbance is dominated by stratiform vertical motion profiles and a midlevel vortex, while the system is less top-heavy and is characterized by more prominent low-level vorticity later in the transition in reanalysis. The developing disturbance starts its evolution as a mesoscale convective system in the Bight of Panama. Leading up to MCS formation the Chocó jet intensifies, and during the MCS-to-EW transition the Papagayo jet strengthens. Differences in the vertical structure of the system between reanalysis and the model suggest that the relatively more bottom-heavy disturbance in reanalysis may have stronger interactions with the Papagayo jet. Field observations like those collected during the Organization of Tropical East Pacific Convection (OTREC) campaign are needed to further our understanding of this east Pacific EW genesis pathway and the factors that influence it, including the important role for the vertical structure of the developing disturbances in the context of the vorticity budget. 

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2. Tropical Cyclogenesis From Self‐Aggregated Convection in Numerical Simulations of Rotating Radiative‐Convective Equilibrium

   https://doi.org/10.1029/2019MS002020  

   Carstens, Jacob D. ; Wing, Allison A. ( May 2020 , Journal of Advances in Modeling Earth Systems)

   In a modeled environment of rotating radiative‐convective equilibrium (RCE), convective self‐aggregation may take the form of spontaneous tropical cyclogenesis. We investigate the processes leading to tropical cyclogenesis in idealized simulations with a three‐dimensional cloud‐permitting model configured in rotating RCE, in which the background planetary vorticity is varied acrossf‐plane cases to represent a range of deep tropical and near‐equatorial environments. Convection is initialized randomly in an otherwise homogeneous environment, with no background wind, precursor disturbance, or other synoptic‐scale forcing. We examine the dynamic and thermodynamic evolution of cyclogenesis in these experiments and compare the physical mechanisms to current theories. All simulations with planetary vorticity corresponding to latitudes from 10°–20° generate intense tropical cyclones, with maximum wind speeds of 80 m s⁻¹or above. Time to genesis varies widely, even within a five‐member ensemble of 20° simulations, indicating large stochastic variability. Shared across the 10°–20° group is the emergence of a midlevel vortex in the days leading to genesis, which has dynamic and thermodynamic implications on its environment that facilitate the spin‐up of a low‐level vortex. Tropical cyclogenesis is possible in this model at values of Coriolis parameter as low as that representative of 1°. In these experiments, convection self‐aggregates into a quasicircular cluster, which then begins to rotate and gradually strengthen into a tropical storm, aided by strong near‐surface inflow that is already established days prior. Other experiments at these lower Coriolis parameters instead self‐aggregate into a nonrotating elongated band and fail to undergo cyclogenesis over the 100‐day simulation.

    

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3. On the Creation and Evolution of Small-Scale Low-Level Vorticity Anomalies during Tropical Cyclogenesis

   https://doi.org/10.1175/JAS-D-18-0104.1  

   Smith, Warren P. ; Nicholls, Melville E. ( July 2019 , Journal of the Atmospheric Sciences)

   Recent numerical modeling and observational studies indicate the importance of vortical hot towers (VHTs) in the transformation of a tropical disturbance to a tropical depression. It has recently been recognized that convective-scale downdraft outflows that form within VHTs also preferentially develop positive vertical vorticity around their edges, which is considerably larger in magnitude than ambient values. During a numerical simulation of tropical cyclogenesis it is found that particularly strong low-level convectively induced vorticity anomalies (LCVAs) occasionally form as convection acts on the enhanced vorticity at the edges of cold pools. These features cycle about the larger-scale circulation and are associated with a coincident pressure depression and low-level wind intensification. The LCVAs studied are considerably deeper than the vorticity produced at the edges of VHT cold pool outflows, and their evolution is associated with persistent convection and vortex merger events that act to sustain them. Herein, we highlight the formation and evolution of two representative LCVAs and discuss the environmental parameters that eventually become favorable for one LCVA to reach the center of a larger-scale circulation as tropical cyclogenesis occurs.

    

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4. The Development of Severe Vortices within Simulated High-Shear, Low-CAPE Convection

   https://doi.org/10.1175/MWR-D-18-0246.1  

   Sherburn, Keith D. ; Parker, Matthew D. ( June 2019 , Monthly Weather Review)

   Environments characterized by large values of vertical wind shear and modest convective available potential energy (CAPE) are colloquially referred to as high-shear, low-CAPE (HSLC) environments. Convection within these environments represents a considerable operational forecasting challenge. Generally, it has been determined that large low-level wind shear and steep low-level lapse rates—along with synoptic-scale forcing for ascent—are common ingredients supporting severe HSLC convection. This work studies the specific processes that lead to the development of strong surface vortices in HSLC convection, particularly associated with supercells embedded within a quasi-linear convective system (QLCS), and how these processes are affected by varying low-level shear vector magnitudes and lapse rates. Analysis of a control simulation, conducted with a base state similar to a typical HSLC severe environment, reveals that the key factors in the development of a strong surface vortex in HSLC embedded supercells are (i) a strong low- to midlevel mesocyclone, and (ii) a subsequent strong low-level updraft that results from the intense, upward-pointing dynamic perturbation pressure gradient acceleration. Through a matrix of high-resolution, idealized simulations, it is determined that sufficient low-level shear vector magnitudes are necessary for the development of low- to midlevel vertical vorticity [factor (i)], while steeper low-level lapse rates provide stronger initial low-level updrafts [factor (ii)]. This work shows why increased low-level lapse rates and low-level shear vector magnitudes are important to HSLC convection on the storm scale, while also revealing similarities between surface vortexgenesis in HSLC embedded supercells and higher-CAPE supercells.

    

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5. The Relationship between Madden–Julian Oscillation Moist Convective Circulations and Tropical Cyclone Genesis

   https://doi.org/10.3390/cli11070134  

   Haertel, Patrick ( July 2023 , Climate)

   The Madden–Julian Oscillation (MJO) is a planetary-scale weather system that creates a 30–60 day oscillation in zonal winds and precipitation in the tropics. Its envelope of enhanced rainfall forms over the Indian Ocean and moves slowly eastward before dissipating near the Date Line. The MJO modulates tropical cyclone (TC) genesis, intensity, and landfall in the Indian, Pacific, and Atlantic Oceans. This study examines the mechanisms by which the MJO alters TC genesis. In particular, MJO circulations are partitioned into Kelvin and Rossby waves for each of the developing, mature, and dissipating stages of the convective envelope, and locations of TC genesis are related to these circulations. Throughout the MJO’s convective life cycle, TC genesis is inhibited to the east of the convective envelope, and enhanced just west of the convective envelope. The inhibition of TC genesis to the east of the MJO is largely due to vertical motion associated with the Kelvin wave circulation, as is the enhancement of TC genesis just west of the MJO during the developing stage. During the mature and dissipating stages, the MJO’s Rossby gyres intensify, creating regions of low-level vorticity, favoring TC genesis to its west. Over the 36-year period considered here, the MJO modulation of TC genesis increases due to the intensification of the MJO’s Kelvin wave circulation.

    

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