A study of evening convective clouds over Kochi and neighbourhood

Radar echoes of 0900 and 1100 UTC over Kochi and 200 km around were studied from 1996 to 1999 along with SST of southeast Arabian Sea and Kochi. The following results are obtained : Monsoon convective cloud tops were lower than Pre-monsoon and Post-monsoon convective cloud tops. (ii) In the mean, monsoon cloud tops gradually increased from 1996 to 1998 and then decreased. (iii) Very large convective activity existed during August 1997 to June 1998 compared to other periods of this study. Seasonally the higher the SST, the higher is convective cloud top. (v) Interannually, large positive SST anomaly coincided with high convective activity and this may be related to then prevailing El Nino.

Diurnal variation of cloudiness during southwest monsoon season using I NSAT-IB radiance data

An attempt has been made to study the diurnal variation of convective clouds. For this study 3 hourly full resolution infrared data of INSAT-IB have been used for the monsoon season (Jun-Sep) of 1987-89. The area of study extends from 35°N to 25°S and 40oE to l00oE, which is subdivided into small areas of 2.5x 2.5 Lat./Long. Mean temperature and the fractional area covered by clouds colder than a given threshold temperature over each sub area are the parameters used for this study. Two threshold temperatures. namely 265°K & 235oK are chosen to represent convective clouds and deep convective clouds respectively. Using the three hourly observations, times of maximum and minimum convective activity are also obtained. Maximum convective activity is observed over head Bay of Bengal at about noon and this maximum migrates westward onto land till midnight and swings back to oceanic area by morning. This eastwest oscillation is less over equatorial regions (open ocean).

Observational study on boundary-layer moist static energy budget over the tropical western Pacific and its variability associated with boreal summer intraseasonal oscillation

Moist static energy (MSE) in the atmospheric boundary layer (BL) is one of the essential parameters determining convective activity over tropical oceanic areas. It is thus important to quantitatively understand BL MSE budget processes and their variability. Among these processes, only few studies have evaluated contributions of entrainment across the BL top and convective downdraft. This study aims to estimate these contributions by analyzing upper-air and surface meteorological observations obtained using Research Vessel Mirai over the tropical western Pacific in June 2008. Daily-mean downward mass fluxes due to the two processes are calculated using BL dry static energy and moisture budget equations under the BL quasi-equilibrium approximation. Estimated mass fluxes are consistent with convective activity observed by a shipborne weather radar and a ceilometer. This study further examines how the mass fluxes and budget processes are modulated when a convectively active phase of boreal summer intraseasonal oscillation arrives at the observation area in the second half of the month. It is found that, while the contribution of the entrainment does not change significantly, the convective downdraft mass flux and the resultant BL MSE export increase 5 times and 3 times, respectively, in the convectively active period compared with those in the pre-active period. Furthermore, ~1/4 of the increase in the convective downdraft mass flux is attributable to the increase in MSE of convective downdraft air associated with mid-tropospheric moistening.

Using Ensemble Data Assimilation to Explore the Environmental Controls on the Initiation and Predictability of Moist Convection

Atmospheric deep moist convection has emerged as one of the most challenging topics for numerical weather prediction, due to its chaotic process of development and multi-scale physical interactions. This study examines the dynamics and predictability of a weakly organized linear convective system using convection permitting EnKF analysis and forecasts with assimilating all-sky satellite radiances from a water vapor sensitive band of the Advanced Baseline Imager on GOES-16. The case chosen occurred over the Gulf of Mexico on 11 June 2017 during the NASA Convective Processes Experiment (CPEX) field campaign. Analysis of the water vapor and dynamic ensemble covariance structures revealed that meso-α (2000-200 km) and meso-β (200-20 km) scale initial features helped to constrain the general location of convection with a few hours of lead time, contributing to enhancing convective activity, but meso-γ (20-2 km) or even smaller scale features with less than 30-minute lead time were identified to be essential for capturing individual convective storms. The impacts of meso-α scale initial features on the prediction of particular individual convective cells were found to be classified into two regimes; in a relatively dry regime, the meso-α scale environment needs to be moist enough to support the development of the convection of interest, but in a relatively wet regime, a drier meso-α scale environment is preferable to suppress the surrounding convective activity. This study highlights the importance of high-resolution initialization of moisture fields for the prediction of a quasi-linear tropical convective system, as well as demonstrating the accuracy that may be necessary to predict convection exactly when and where it occurs.

A diagnostic study of interannual variability of Indian summer monsoon using outgoing longwave radiation (OLR) data

ABSTRACT. Using the monthly outgoing longwave radiation (OLR) data obtained from NOAA polar orbiting satellites, during the period 1979-92, composite OLR anomalies in respect of good monsoon years (1983 and 1988), bad monsoon years (1982 and 1987 for the case associated with ENSO and 1979 and 1986 separately for the case without ENSO) and normal monsoon years (1980, 1981, 1984, 1985, 1989, 1990, 1991 & 1992) were examined. The computation has been performed over the global tropics (30°N-30°S) bounded between the longitudes 50°E and 130°W (through date line) on 5° longitude × 5° latitude grid. There are significant differences in the spatial distributions of composite OLR anomalies between these four cases from the month of April to September indicating spatial and temporal changes in the organized convective pattern. For the good monsoon years persistent negative anomalies indicating enhanced convective activity were observed over the Indonesian regions, whereas large positive anomalies indicating depressed convective activity were observed over equatorial Pacific just west of date line. During the bad monsoon years above normal convection was observed over Pacific region (ENSO case) and over equatorial Indian Ocean (Non ENSO case). During normal monsoon years the spatial patterns of OLR anomalies were similar to that of good monsoon years, but with weaker anomalies. These observations can be explained through the relative interaction between tropical convergence zone (TCZ) over the Indian sub-continent and that over the north Indian Ocean and Pacific. The eastward shift of the convective activity during El-Nino years can be attributed to shift/reversal of Walker circulation. There are strong signals of OLR anomalies during pre-monsoon months which may be useful in inferring the nature of the subsequent monsoon activity.

Utilisation of ‘Aerostat’ Doppler Weather Radar in nowcasting of convective phenomena

‘Aerostat’ system is a part of the air defence radar network, adopted by the Indian Air Force. Many meteorological instruments have been integrated with this system, including Doppler Weather Radar (DWR). The ground-based DWR has a maximum range of 300 NM, however, it generally uses 150 NM range on scan mode. The scan mode images are provided at half an hour interval, which are being utilised very effectively for nowcasting of thunderstorms at various IAF bases. In the present study, utilisation of DWR images for nowcasting of thunderstorms / dust storms is discussed over NW India with the help of a few case studies during pre-monsoon and SW monsoon seasons of 2008. Further, products generated through operational meso-scale NWP model runs have been studied in order to obtain indications / guidance for expected convective activity over the area at least 24-36 hours in advance. Thus, short-range weather forecasts through NWP models can be used as an advance indication for careful monitoring of DWR images in near real time. It has been found that the DWR is a very good tool to track the movement of significant weather echoes around the airfields, which can be very helpful in issuing appropriate warnings / advisories with sufficient lead time. Meso-scale NWP models are capable of generating reliable indications for expected convective activity at least 24-36 hours in advance. The integration of both the inputs can increase the accuracy and reliability of location and time specific prediction of convective activity.

Ozone variations pertaining to dry and wet monsoon seasons over Indian region

’kq"d vkSj vknzZ ekulwu o"kkZsa ds nkSjku Vh-lh-vks- forj.k dk v/;;u djus ds fy, Hkkjrh; {ks+= esa o"kZ 1982]1983]1987 ,oa 1988 ds dqy dkWye vkstksu ¼Vh-lh-vks-½ ds ekfld vkSlr dk mi;ksx fd;k x;k gS A bl ’kks/k&Ik= esa mDr o"kksZa ds Hkkjr ds 13 LVs’kuksa ds Vh-lh-vks- vkadM+ksa dk v/;;u fd;k x;k gSA ’kq"d vkSj vknzZ ekulwu o"kksZa ds nkSjku Vh-lh-vks- forj.k dh rqyuk ls ;g irk pyk gS fd Vh-lh-vks- ds eku vknZz o"kksZa dh rqyuk esa ’kq"d o"kksZa esa vf/kd ik, x, gSaA Vh-lh-vks- esa ifjorZu gksuk ’kq"d ,oa vknzZ o"kksZa ds nkSjku laoguh; xfrfof/k esa fHkUurk dks ekuk tk ldrk gSA ’kq"d ¼vknzZ½ o"kksZa ds nkSjku laogu esa deh ¼o`f)½ Vh-lh-vks- dh ek=k dks c<+krh ?kVkrh gSA ’kq"d ,oa vknzZ o"kksZa ds chp ds ekulwu ds eghuksa ds nkSjku Vh-lh-vks- ds egRo dh tk¡p djus ds fy, lkaf[;dh; Vh--VsLV dk iz;ksx fd;k x;k gSA ;g varj nene dks NksM+dj vU; lHkh LVs’kuksa ds fy, lkaf[;dh; n`f"V ls 5 izfr’kr rd egRoiw.kZ gSA ,slk dgk tk ldrk gS fd Hkkjr esa xzh"edkyhu ekulwu eghuksa ds nkSjku vks-,y-vkj- rFkk Vh-lh-vks- ds chp vPNs laca/k jgs gSa D;ksafd bl vof/k ds nkSjku laogu dkQh izcy jgk gS A Monthly mean total column ozone (TCO) over Indian region for the years 1982, 1983, 1987 and 1988 has been utilized to study the TCO distribution during dry and wet monsoon years. TCO data for 13 Indian stations for the above years have been considered in the study. Comparison of TCO distribution during dry and wet monsoon years suggested that TCO values are found higher during dry years than those in wet years. The changes in TCO may be attributed to difference in convective activity during dry and wet years. The suppressed (enhanced) convection during dry (wet) years may lead to increase (decrease) in TCO. The statistical t-test is applied to test the significance of TCO difference during monsoon months between dry and wet years. The difference is statistically significant at 5% level of confidence for all stations except Dumdum. It can be said that the relation between OLR and TCO holds good during Indian summer monsoon months, as convection is stronger during this period.

Lightning Evolution and VLF Perturbations Associated With Category 5 TC Yasa in the South Pacific Region

In this paper, we present the D-region ionospheric response during the lifespan (10–19 December 2020) of a severe category 5 tropical cyclone (TC) Yasa in the South Pacific by using the very low frequency (VLF, 3-30 kHz) signals from NPM, NLK, and JJI transmitters recorded at Suva, Fiji. Results indicate enhanced lightning and convective activity in all three regions (eyewall, inner rainbands, and outer rainbands) during the TC Yasa that are also linked to the wave sensitive zones of these transmitter-receiver great circle paths. Of the three regions, the outer rainbands showed the maximum lightning occurrence; hence convective activity. Prominent eyewall lightning was observed just before the TC started to weaken following its peak intensity. Analysis of VLF signal amplitudes showed both negative and positive perturbations (amplitudes exceeding ±3σ mark) lasting for more than 2 hours with maximum change in the daytime and nighttime signal amplitudes of -4.9 dB (NPM) and -19.8 dB (NLK), respectively. The signal perturbations were wave-like, exhibiting periods of oscillations between ~2.2-5.5 hours as revealed by the Morlet wavelet analysis. Additionally, the LWPC modeling of the signal perturbations indicated a 10 km increase in daytime D-region reference height, H¢, and a 12 km decrease in nighttime D-region H¢ during TC Yasa. The D-region density gradients (sharpness), b, showed small perturbations of 0.01–0.14 km-1 from its normal values. We suggest that the observed changes to the D-region parameters are due to the enhanced convection during TC Yasa which excites atmospheric gravity waves producing travelling ionospheric disturbances to the D-region.

The effect of vertical wind structure on some Aspects of convective activity at Bombay

The convective activity along the west coast of India in the southwest monsoon season has some characteristic features, the reasons for which could not be given earlier. The observed features of convective weather over Bombay in this season show that they do not fall into the general pattern found in other areas of the tropics. A study of the thermodynamic conditions reveal that these features cannot be explained in terms of the observed instability. On the other hand, these features could be explained in terms of the environmental wind field.

Utilization of weather-radar data to observe the Sea Breeze Front (SBF) over the North Coast of Banten-Jakarta (case study in 2018)

One of the important factors in weather and climate dynamics that can trigger precipitation on the coast and the surrounding area is a sea breeze. Sea breeze occurs because of differences in the surface temperature between land and sea due to solar heating which then forms a pressure gradient that leads to a land called the sea breeze circulation. An important part of sea breeze circulation is the Sea Breeze Front (SBF). SBF is a boundary area where wind from the sea direction meets the wind from the land direction, which is marked by significant changes in temperature, humidity, wind and can trigger convective activity. This study aims to determine the characteristics of the SBF on the north coast of Banten-Jakarta in 2018 which were identified using a Doppler weather radar Plan Position Indicator (PPI) product and convective activity using the Coloumn Maximum (CMAX) product. Qualitative and quantitative methods are used to determine the SBF parameters such as frequency of occurrence, onset time, duration, length, column depth, and SBF penetration, and convective activity during the occurrence of SBF. The results showed that SBF was detected more in the rainy season January, February, and December 2018, and occur between 08:08 LT and 15:20 LT. SBF can trigger the occurrence of convective clouds and affect the temperature and humidity conditions around the SBF.