On forecasting tracks of tropical disturbances using ATOVS data over Bay of Bengal

lkj &,d m".kdfVca/kh; vonkc ds thou pØ ds vkadMs+ rFkk nks m".kdfVca/kh; pØokrh rwQkuksa ds o"kZ 2002&03 dh vof/k ds vkadMs+ mPp Vh- vks- oh- ,l- ¼,- Vh- vks- oh- ,l-½ /kzqod{kh; mixzgksa ,u- vks- ,- , 15 rFkk 16] ftuesa mPp lw{e rjaxh; ifjKkiu bdkbZ ¼,- ,e- ,l- ;w½ yxh gqbZ gaS ls izkIr fd, x, gSa ftudk fo’ys"k.k bu rwQkuksa ds ekxZ dk iwokZuqeku djus ds fy, fd;k x;k gSA bu ekSle fo{kksHkksa ds 700&400 gsDVkikLdy ¼gs-ik-½ Lrj esa e/; {kksHkeaMyh; m".krk e/; Lrjh ckfgokZg ds dkj.k gksrh gS tks rwQku ds 200&700 fd-eh- vkxs rd foLrkfjr gksrh gS rFkk fo{kksHkksa dh xfr’khyrk dk djhc 6 ls 24 ?kaVs igys iwokZuqeku djus esa iwoZ ladsrd dk dk;Z djrh gSA ;g fo{kksHk yxHkx mlh v{k dks vuqxeu djrk gS tks e/; {kksHkeaMy esa foLrkfjr ¼vkxs c<s+ gq,½ ftg~okdkj m".k {ks= dks dsUnz ls tksM+rk gSA e/;e rhozrk okys nks HkweaMyh; pØokrksa dh fLFkfr esa tc 7º ls 13º lsfYl;l rkieku dk m"edksj Åijh {kksHkeaMyh; Lrj ¼250&200 gs-ik-½ ds djhc dsafnzr jgk ml le; vonkc dh fLFkfr esa fdlh fo’ks"k m".krk dk irk ugha pyk gSA Advanced TOVS (ATOVS), comprising the Advanced Microwave Sounding Unit (AMSU), data obtained from polar orbiting satellites NOAA 15 and 16 during the life cycle of a tropical depression and two tropical cyclonic storms during 2002-03 have been analysed to predict the track of these disturbances. The mid-tropospheric warming due to altostratus outflow from these weather disturbances in the layer 700 – 400 hPa which protrudes about 200 -700 km ahead the storm acts as a pre-cursor to predict the movement of the disturbances with a lead time of about 6 to 24 hours. The disturbance almost follows the axis connecting the centre with the warm tongue that protrudes ahead of the disturbance in the mid-troposphere. While warm core of 7 to 13° C is centered around the upper tropospheric level (250 – 200 hPa) in the case the two moderate intensity tropical cyclones, no significant warmness could be seen in the depression stage.

An Initiation Process of Tropical Depression-type Disturbances under the Influence of Upper-level Troughs

In this study, the statistical relationship between tropical upper-tropospheric troughs (TUTTs) and the initiation of summertime tropical-depression type disturbances (TDDs) over the western and central North Pacific is investigated. By applying a spatiotemporal filter to the 34-year record of brightness temperature and using JRA-55 reanalysis products, TDD-event initiations are detected and classified as trough-related (TR) or non-trough-related (non-TR). The conventional understanding is that TDDs originate primarily in the lower-troposphere; our results refine this view by revealing that approximately 30% of TDDs in the 10°N-20°N latitude ranges are generated under the influence of TUTTs. Lead-lag composite analysis of both TR- and non-TR-TDDs clarifies that TR-TDDs occur under relatively dry and less convergent large-scale conditions in the lower-troposphere. This result suggests that TR-TDDs can form in a relatively unfavorable low-level environment. The three-dimensional structure of the wave activity flux reveals southward and downward propagation of wave energy in the upper troposphere that converges at the mid-troposphere around the region where TR-TDDs occur, suggesting the existence of extratropical forcing. Further, the role of dynamic forcing associated with the TUTT on the TR-TDD-initiation is analyzed using the quasi-geostrophic omega equation. The result reveals that moistening in the mid-to-upper troposphere takes place in association with the sustained dynamical ascent at the southeast side of the TUTT, which precedes the occurrence of deep convective heating. Along with a higher convective available potential energy due to the destabilizing effect of TUTTs, the moistening in the mid-to-upper troposphere also helps to prepare the environment favorable to TDDs initiation.

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.

An Implicit – Not Explicit – Understanding of Hurricane Storms

The goal of this research was to examine students’ risk perception of hurricanes and hurricane-related storms to address a critical gap in the literature. Participants were asked to rate their perceptions of a tropical storm, tropical depression, and category 1 through 5 hurricanes on five dimensions and define the storms based on wind speed. Lastly, individual differences in sex and growing up on the coast were examined to determine whether they explain differences in risk perceptions. Findings showed that participants’ perceptions of category 1 through 5 hurricanes followed a linear pattern and each pair was perceived to be significantly different. However, participants rated tropical storm and tropical depression as more severe than a category 1 hurricane and were unable to define any of the storms based on wind speed. In fact, coastal natives were less accurate at defining the storms and believed the low-tier storms to be less severe than non-coastal natives. This research is the first to show that people implicitly understand the Saffir-Simpson Scale that defines category 1 through category 5 hurricanes, but not the lesser-tiered storms. The present work demonstrates a need for enhanced education of hurricanes, as students do not make important distinctions at the lower-end of the hurricane scale.

Identifying the development of a tropical depression into a tropical storm over the South China Sea

Tropical depressions formed over the South China Sea usually produce severe flooding and wind damage when they develop into a storm and make landfall. To provide an early warning, forecasters should know when, and if, a tropical depression will develop into a tropical storm. To better understand and predict such development, we examine the dynamic and thermodynamic variables of 74 tropical depressions over the South China Sea, 52 of which developed into storms, hereafter ‘developing’, the remaining being classified as ‘nondeveloping’. Using NCEP FNL (National Centers for Environmental Prediction Final) data, verified with ECMWF forecast data, we examine the dynamic and thermodynamic statistics that characterize these tropical cyclones. Based on these characteristics, we propose seven criteria to determine whether a tropical depression will develop. Five had been used before, but two new criteria are also found useful. These two are associated with the diabatic heating rate and help to determine whether a tropical cyclone diurnal cycle exists and whether the convection system remains intact in the center: 1) Presence of a regular diurnal variation of the diabatic heating-rate at the center. 2) Occurrence of specific peaks in the radiative-heating profile. We test all seven criteria on all tropical depression cases in 2018–2019 before the system developed or decayed, showing that these criteria can help to operationally identify whether or not a tropical depression develops into a tropical storm with an average lead time of 36.6 h.

Improved Climatology of Tropical Cyclone Precipitation from Satellite Passive Microwave Measurements

Accurate precipitation climatology is presented for tropical depression (TD), tropical storm (TS), and tropical cyclone (TC) over oceans using the recently-released, consistent and high quality precipitation datasets from all passive microwave sensors covering 1998-2012 along with the Automated Rotational Center Hurricane Eye Retrieval (ARCHER)-based TC center positions. Impacts with respect to the direction of both TC movement and the 200-850 hPa wind shear on the spatial distributions of TC precipitation are analyzed. The TC eyewall contraction process during its intensification is noted by a decrease in the radius of maximum rainrate with an increase in TC intensity. For global TCs, the maximum rainrate with respect to the direction of TC movement is located in the down-motion quadrants for TD, TS, and Cat 1-3 TCs, and in a concentric pattern for Cat 4-5 TCs. A consistent maximum TC precipitation with respect to the direction of the 200-850 hPa wind shear is shown in the down shear left quadrant (DSLQ). With respect to direction of TC movement, spatial patterns of TC precipitation vary with basins and show different features for weak and strong storms. The maximum rainrate is always located in DSLQ for all TC categories and basins, except the Southern Hemisphere basin where it is in the down shear right quadrant (DSRQ). This study not only confirms previously published results on TC precipitation distributions relative to vertical wind shear direction, but also provides a detailed distribution for each TC category and TS, while TD storms display an enhanced rainfall rate ahead of the down shear quadrants.

Convective Coupling in Tropical-Depression-Type Waves. Part I: Rainfall Characteristics and Moisture Structure

Tropical-depression (TD)-type waves are synoptic-scale disturbances embedded with deep convection over the western North Pacific. Studies of these disturbances began over six decades ago; however, some properties of these disturbances remain vague, e.g., the coupling mechanism between the deep convection and the waves. This two-part study aims to examine the rainfall progression in TD-type disturbances and associated tropospheric moisture controlling convective rainfall. Part I investigates the rainfall and moisture characteristics of TD-type waves using TRMM-derived rainfall products and the ERA-Interim data during the period of June–October 1998–2013. The rainfall features a north–south asymmetrical pattern with respect to a TD-type disturbance, with enhanced convective and stratiform rainfall occurring in the southern portion. Along with the northwestward propagation, deep convective and stratiform rainfall occur in phase with the TD-type disturbance without significant preceding shallow convective rainfall. Following the deepest convection, shallow convective rainfall increases in the anomalous southerlies. Such a rainfall progression differs from the paradigm from shallow to deep convection, then to stratiform rainfall, which is suggested in other convectively coupled equatorial waves. The rainfall progression and the atmospheric moisture anomaly are phase locked to the TD-type disturbances such that the relative displacements change little when the disturbances propagate northwestward. The latent heat release in deep convection, which is obtained from the TRMM 3G25 dataset, superposes with a broad warm anomaly in the mid- to upper troposphere, suggesting wave growth through the generation of available potential energy from diabatic heating.

Convective Coupling in Tropical-Depression-Type Waves. Part II: Moisture and Moist Static Energy Budgets

The companion of this paper, Part I, discovered the characteristics of the rainfall progression in tropical-depression (TD)-type waves over the western North Pacific. In Part II, the large-scale controls on the convective rainfall progression have been investigated using the ERA-Interim data and the TRMM 3B42 precipitation-rate data during June–October from 1998 to 2013 through budgets of moist static energy (MSE) and moisture. A buildup of column-integrated MSE occurs in advance of deep convection, and an export of MSE occurs following deep convection, which is consistent with the MSE recharge–discharge paradigm. The MSE recharge–discharge is controlled by horizontal processes, whereby horizontal moisture advection causes net MSE import prior to deep convection. Such moistening by horizontal advection creates a moist midtroposphere, which helps destabilize the atmospheric column, leading to the development of deep convective rainfall. Following the heaviest rainfall, negative horizontal moisture advection dries the troposphere, inhibiting convection. Such moistening and drying processes explain why deep convection can develop without preceding shallow convection. The advection of moisture anomalies by the mean horizontal flow controls the tropospheric moistening and drying processes. As the TD-type waves propagate northwestward in coincidence with the northwestward environmental flow, the moisture, or convective rainfall, is phase locked to the waves. The critical role of the MSE import by horizontal advection in modulating the rainfall progression is supported by the anomalous gross moist stability (AGMS), where the lowest AGMS corresponds to the quickest increase in the precipitation rate prior to the rainfall maximum.