Medium range prediction of tropical cyclogenesis of intense vortices over Indian Seas by a Global Spectral Model

The performance of a Global Spectral Model (T-80) operational at the National Centre for Medium Range Weather Forecasting (NCMRWF), New Delhi in predicting the cyclogenesis of six tropical cyclones over Indian Seas formed during 1995-96 has been evaluated. It has been found that the model has the capability to predict cyclogenesis in wind field at least 72 hours in advance although the positions of predicted vortices are seen to be displaced from those of analysed ones in some cases. The quantitative estimates of the atmospheric conditions favourable for cyclogenesis also confirm the conclusions drawn from the qualitative analysis of cyclogenesis predictions of the model in terms of appearance of cyclonic circulation. It also follows from this analysis that the predicted circulations at the cyclogenesis stage are in general more intense and stronger as compared to the corresponding analysis in terms of wind and mass fields. On examining the model systematic errors of prediction it is found that the model has a clear bias for predicting more intense vortex during genesis and weakening stages. On the order hand it predicts relatively less intense vortex during intensification process.

The role of random vorticity stretching in tropical depression genesis

Tropical deep convection plays a key role in the tropical depression stage of tropical cyclogenesis by aggregating vorticity, but no existing theory can depict such a stochastic vorticity aggregation process. A vorticity probability distribution function (PDF) is proposed as a tool to predict the horizontal structure and wind speed of the tropical depression. The reason lies in the tendency for a vortex to adjust to an axisymmetric and monotonic vorticity structure. Assuming deep convection as independent and uniformly distributed vortex tube stretching events in the low-mid troposphere, repetitive vortex tube stretching will make the air column area shrink many times and significantly increase vorticity. A theory of the vorticity PDF is established by modelling the random stretching process as a Markov chain. The PDF turns out to be a weighted Poisson distribution, in good agreement with a randomly-forced divergent barotropic model (weak temperature gradient model), and in rough agreement with a cloud-permitting simulation. The result shows that a stronger and sparser deep convective mode tends to produce more high vorticity air columns, which leads to a more compact major vortex with a higher maximum wind. Based on the vorticity PDF theory, a parameterization of the eddy acceleration effect on the tangential flow is proposed.

A theory of spontaneous tropical cyclogenesis from quasi-random convection

How the cumulus clouds organize into a tropical cyclone remains poorly understood. The difficulty lies in that the deep convection is noisy at the kilometer scale, but follows the physical feedbacks at the mesoscale. We build a barotropic numerical model to understand the interaction of the stochastic and deterministic processes in the genesis of a tropical depression. Deep convection is represented as a multitude of isolated convergence forcing. The convection is assigned to distribute randomly at the small scale. At the mesoscale, convection is preferentially seeded to regions with a high spatially-filtered vertical vorticity. The preferential seeding mimics the physical feedbacks, and the filter implicitly represents the nonlocal convective triggering by gravity wave and cold pool. The result shows that the early-stage evolution is dominated by random vortex tube stretching. Subsequently, the regions where repetitive stretching occurs become vortex clusters, and induce more convection around them. The collision and coalescence between vortex clusters lead to a major vortex, which accelerates the growth by the preferential seeding. This physical picture agrees with a cloud-permitting simulation of spontaneous tropical cyclogenesis over uniform sea surface temperature. A theoretical model with approximate analytical solution is presented to depict the full evolution process.

Favorable monsoon environment over eastern Africa for subsequent tropical cyclogenesis of African easterly waves

Eastern Africa is a common region of African easterly wave (AEW) onset and AEW early-life. How the large-scale environment over east Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared to non-developing AEWs. Using a 21-year August-to-September (1990-2010) climatology of AEWs, differences in the large-scale environment between developers and non-developers are identified and are propose to be used as key predictors of subsequent tropical cyclone formation and could informtropical cyclogenesis prediction. TC precursors when compared to non-developing AEWs experience: an anomalously active West African Monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet (AEJ). These large-scale conditions are linked to near-trough attributes of developing AEWs which favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, deeper wave pouch, and arguably more AEW and Mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or west Africa. Developing AEWs are more likely to be southern-track AEWs than non-developing AEWs.

How Does Cyclogenesis Commence Given a Favorable Tropical Environment?

In a series of collaborative Russian–American works (Levina and Montgomery, 2009–2015), we applied the fundamental ideas of self-organization in turbulence with broken mirror symmetry, the so-called “helical” turbulence. In this context, tropical cyclogenesis is considered as a threshold extreme event in the three-dimensional helical moist convective atmospheric turbulence of a vorticity-rich environment of a pre-depression zone. This allowed us to discover a large-scale vortex instability and answer the question “When will cyclogenesis commence given a favorable tropical environment?”. The new instability emerges against the background of seemingly disorganized convection, without a well-defined center of near-surface circulation and noticeably precedes the formation of a tropical depression. This can give the fundamental ground and quantitative substantiation for the term “Potential Tropical Cyclone” as a beginning of TC genesis. In the present work, we explore in detail the crucial role of special convective coherent structures of cloud scales—vortical hot towers (VHTs)—in the formation and maintenance of the secondary circulation and, therefore, of the whole mesoscale vortex system. On this basis, we propose how the onset of large-scale instability, i.e., the beginning of TC genesis, can be diagnosed exactly and distantly with VHTs patterns in the field of temperature (satellite data) and vertical helicity (cloud-resolving numerical analysis). The present research is intended to contribute to a recently initiated development of operational diagnosis of the beginning of TC genesis based on GOES Imagery and supported by cloud-resolving numerical modeling.