Testing Vertical Wind Shear and Nonlinear MJO/ENSO Interactions as Predictors for Subseasonal Atlantic Tropical Cyclone Forecasts

Hansen et al. (2020) found patterns of vertical wind shear, relative humidity (RH) and non-linear interactions between the Madden-Julian Oscillation and El Niño-Southern Oscillation that impact subseasonal Atlantic TC activity. We test whether these patterns can be used to improve subseasonal predictions. To do this we build a statistical-dynamical hybrid model using Navy-ESPC reforecasts as a part of the SUBX project. By adding and removing Navy-ESPC reforecasted values of predictors from a logistic regression model, we assess the contribution of skill from each predictor. We find that Atlantic SSTs and the MJO are the most important factors governing subseasonal Atlantic TC activity. RH contributes little to subseasonal TC predictions, however, shear predictors improve forecast skill at 5-10 day lead times, before forecast shear errors become too large. Non-linear MJO/ENSO interactions did not improve skill compared to separate linear considerations of these factors but did improve the reliability of predictions for high-probability active TC periods. Both non-linear MJO/ENSO interactions and the subseasonal shear signal appear linked to PV streamer activity. This study suggests that correcting model shear biases and improving representation of Rossby wave-breaking is the most efficient way to improve subseasonal Atlantic TC forecasts.

Tropical cyclone Genesis Potential Parameter (GPP) and it’s application over the north Indian Sea

bl 'kks/k i= esa mRrjh fgUn egklkxj esa m".kdfVca/kh; pØokr ds cuus ds foHko izkpy ¼th- ih- ih-½ dk fo’ys"k.k fd;k x;k gSA dksVy }kjk fodflr ¼2009½ pØokr cuus ds foHko izkpy dk vkdyu pkj ifjofrZrkvksa ds vk/kkj ij fd;k x;k gS tks bl izdkj gS % 850 gSDVkikLdy ij Hkzfeyrk] e/; {kksHkeaMyh; lkisf{kd vknzZrk] e/; {kksHkeaMyh; vfLFkjrk vkSj ml LFkku ds lHkh fxzM IokbaVksa ij m/okZ/kj iou vi:i.kA bu fLFkfr;ksa esa fxzM IokbaV ij th-ih-ih- ij ;g fopkj fd;k x;k fd lHkh ifjorhZ Hkzfeyrk] e/; {kksHkeaMyh; lkisf{kd vknzZrk] e/; {kksHkeaMyh; fLFkjrk vkSj m/okZ/kj iou vi:i.k 'kwU; ls cM+k gS vkSj ;g ekuk x;k gS fd tc buesa ls dksbZ Hkh ifjorhZ 'kwU; ls de ;k cjkcj gks rks og 'kwU; gh ekuk tk,xkA ;wjksih; e/;kof/k ekSle iwokZuqeku dsUnz ¼b-lh-,e-MCY;w-,Q-½ fun’kZ vk¡dM+ksa dk mi;ksx djrs gq, bu ifjofrZrkvksa dk vkdyu fd;k x;k gSaA b- lh- ,e- MCY;w- ,Q- fun’kZ dh lwpukvksa ¼http://www.imd.gov.in/section/nhac/dynamic/analysis.htm ij miyC/k½ dk okLrfod le; dk mi;ksx djrs gq, lkr fnuksa rd ds fy, tsusfll izkpy ds iwokZuqeku Hkh rS;kj fd, x,A ml {ks= esa th-ih-ih- ds mPprj ekuksa ls ml LFkku ds tsfufll ds mPprj foHko dk irk pyk gSA ml LFkku ij th-ih-ih- ds eku 30 ds cjkcj vFkok vf/kd gksus dh fLFkfr esa pØokr mRifRr ds fy, mPp foHko {ks= ik;k x;k gSA izkpy ds fo’ys"k.k vkSj 2010 esa pØokrh fo{kksHkksa ds nkSjku budh izHkko’khyrk ls mRrjh fgUn egklkxj esa pØokr mRifÙk ds fy, iwokZuqeku lwpd flaxuy ¼4&5 fnu igys½ ds :i esa vkSj fodkl dh vkjafHkd voLFkkvksa esa fodflr vkSj xSj&fodflr iz.kkfy;ksa ds rzhohdj.k ds fy, foHko dk fu/kkZj.k gsrq budh mi;ksfxrk dh iqf"V gqbZ gSA An analysis of tropical cyclone genesis potential parameter (GPP) for the North Indian Sea is carried out. The genesis potential parameter developed by Kotal et al. (2009) is computed based on the product of four variables, namely: vorticity at 850 hPa, middle tropospheric relative humidity, middle tropospheric instability and the inverse of vertical wind shear at all grid points over the area. The GPP at a grid point is considered under the conditions that all the variables vorticity, middle tropospheric relative humidity, middle tropospheric instability and the vertical wind shear are greater than zero and it is taken as zero when any one of these variables is less or equal to zero. The variables are computed using the European Centre for Medium Range Weather Forecast (ECMWF) model data. Forecast of the genesis parameter up to seven days is also generated on real time using the ECMWF model output (available at http://www.imd.gov.in/section/nhac/dynamic/Analysis.htm). Higher value of the GPP over a region indicates higher potential of genesis over the region. Region with GPP value equal or greater than 30 is found to be high potential zone for cyclogenesis. The analysis of the parameter and its effectiveness during cyclonic disturbances in 2010 affirm its usefulness as a predictive signal (4-5 days in advance) for cyclogenesis over the North Indian Sea and for determining potential for intensification of developing and non-developing systems at the early stages of development.

Low level wind shear over Bomba airport

Significant climatological features based on 329 Low Level Wind Shear (LLWS) reports from 1985 to 1989 at Bombay airport are presented, The monsoon season has the highest frequency of occurrence of LLWS mainly due to thunderstorms and strong gusty winds, Other than monsoon season, occurrence of LLWS is related to sea and land breeze and nocturnal increase of surface temperature during night. The preferred time of occurrence of LLWS is between 0000 to 0600 IST and 1800 to 240J IST. The simultaneous occurrence of strong and severe LL WS, low cloud ceiling and very poor visibility has an adverse effect on aircraft operations at Bombay airport during landing and take-off.

SPC Mesoscale Analysis Compared to Field-Project Soundings: Implications for Supercell Environment Studies

Two-hundred-fifty-seven supercell proximity soundings obtained for field programs over the central U.S. are compared to profiles extracted from the SPC mesoscale analysis system (the SFCOA) to understand how errors in the SFCOA and in its baseline model analysis system – the RUC/RAP – might impact climatological assessments of supercell environments. A primary result is that the SFCOA underestimates the low-level storm-relative winds and wind shear, a clear consequence of the lack of vertical resolution near the ground. The near-ground (≤ 500 m) wind shear is underestimated similarly in near-field, far-field, tornadic, and nontornadic supercell environments. The near-ground storm-relative winds, however, are underestimated the most in the near field and in tornadic supercell environments. Under-prediction of storm-relative winds is therefore a likely contributor to the lack of differences in storm-relative winds between nontornadic and tornadic supercell environments in past studies that use RUC/RAP-based analyses. Furthermore, these storm-relative wind errors could lead to an under emphasis of deep-layer SRH variables relative to shallower SRH in discriminating nontornadic from tornadic supercells. The mean critical angles are 5–15° larger and farther from 90° in the observed soundings than in the SFCOA, particularly in the near field, likely indicating that the ratio of streamwise to crosswise horizontal vorticity is often smaller than that suggested by the SFCOA profiles. Errors in thermodynamic variables are less prevalent, but show low-level CAPE to be too low closer to the storms, a dry bias above the boundary layer, and the absence of shallow near-ground stable layers that are much more prevalent in tornadic supercell environments.

Characterisation and asymmetry analysis of rainfall distribution associated with tropical cyclones over Bay of Bengal : NISHA (2008), LAILA (2010) and JAL (2010)

The precipitation characteristics and spatial rainfall asymmetry in respect of three tropical cyclones (TCs) of Bay of Bengal, viz., NISHA (2008), LAILA (2010) and JAL(2010) that affected coastal Tamil Nadu are studied using TRMM based rain rate data. The analysis is carried out by dividing the life cycle of the TC into various stages of intensification and weakening. Percentage frequency distribution, radial profile and quadrant-wise mean rain rates are determined stage-wise for each TC. Further, spatio-temporal variations in the rainfall asymmetry is studied using Fourier analysis by computing the first order wave number-1 asymmetry around the TC centre. The results indicate a shifting of higher frequency rain rates from higher to lower rain rate side when the TC passes from intensification to weakening stages. The azimuthally averaged mean rain rates indicate a peak rain rate of 4-5 mm/hr over 50-100 km from the TC centre during intensification stages which decreases to a very low rate of about 1 mm/hr during the final stages of weakening. For the same intensity category, the radial profiles of mean rain rates show marked difference between the intensification and weakening stages. The quadrant mean rain rates show large asymmetries in the radial rainfall distribution with more rainfall concentrated in front left quadrant during the stages of intensification. Such TC rainfall asymmetries are shown to be influenced by the environmental vertical wind shear and translational speed of the TC. When the wind shear and storm motion vectors are in the same direction, a dominant down shear left asymmetry is observed. Evolution of wave number-1 asymmetry indicates that, by and large, asymmetry amplitude increases from the centre outwards and a cyclonic (anti-cyclonic) shift during the intensification (weakening) stages of the TCs.

Wind Shear of Low-Level Jets and Their Influence on Manned and Unmanned Fixed-Wing Aircraft during Landing Approach

Wind shear at low altitudes represents a potential hazard to landing aircraft. Based on two wind lidar data sets of one year, the occurrence of low-level jets (LLJs), the vertical wind shear and the rotation of the wind direction were analysed. The lidar system was located at the sites of Braunschweig in the North German Plain, Germany, and Clausthal-Zellerfeld in the low mountain range Harz, Germany. The observed wind shear gradients between the altitude of 40 m and the altitude of the maximum wind speed was in the range of −0.23 s−1 to +0.20 s−1. The rotation of the wind direction with altitude occurred both in clockwise and anticlockwise direction. The ratio of clockwise versus anticlockwise occurrence of directional shear was 4:1 for Braunschweig and 3:1 for Clausthal-Zellerfeld. The observed wind shear gradients were compared to values for hazard potential of different levels for a typical aircraft. Although the LLJ was not hazardous for manned aircraft in any observed case, the awareness of LLJ helps to reduce the pilot’s workload and possible pilot-introduced oscillations caused as a result of the wind shear and aircraft characteristics. In contrast to manned aviation, the value of changes in wind speed and direction during LLJ conditions can cause significant risks for unmanned aerial system operations with less than 25 kg of take-off weight. This is a result of the lower airspeed-wind-speed ratio and the flight control and flight planning.