Dynamics associated with the expanding Hadley Circulation (HC) are gaining attention owing to their far-reaching consequences on tropical and subtropical weather and climate. In the present communication, the co-variability of ascending and descending regions of the HC at regional scales is investigated. The results from four decades of ERA-5 reanalysis emphatically show that the ascending and descending regions of the HC move in tandem with profound zonal asymmetry in their degree of co-variability, and their long-term trends show similar signs with notable differences in magnitude. In more than half of the longitudinal sectors, the relative movement between the ascending and descending boundaries of each hemisphere is in unison. At this juncture, where there are debates on the causative mechanism for the observed expansion of HC as well as their zonal asymmetries, the present results provide new insights into the regional HC dynamics by emphasizing the coherent variations of ascending and descending regions of the HC at regional scales for the first time. The results discussed in the study will be useful for identifying differences in the regional forcing of HC expansion and for quantifying the regional expansion rates of both ascending and descending regions of the HC.

Abstract A comprehensive study is carried out to investigate the characteristics of the quasi‐16‐day wave (Q16DW) in the mesosphere lower thermosphere (MLT) over the low and equatorial latitudes using a decade of meteor radar observations over Thumba (8.5°N, 77°E) during 2005–2015 and Kototabang (0.2°S, 100.3°E) during 2003–2012. Daily mean winds from the meteor radars are used to determine the amplitude and phase of Q16DW and their monthly mean climatology. The analysis reveals that Q16DW has relatively large amplitudes in the zonal wind component, with a primary peak during boreal winter solstices and a secondary peak during austral winter solstices over both stations. It is also noted that the boreal winter peak over Thumba is relatively larger than that over Kototabang. The annual cycle of Q16DW in zonal winds shows a semiannual oscillation over both stations, which was not reported in previous studies using limited data sets over Thumba. The vertical structure during boreal winters over both stations agrees very well, and there are significant differences during austral winters. The wavelet spectrum during boreal winter showed a relatively high degree of covariability over the low and equatorial latitudes as compared to that during austral winter. The vertical profiles of the phase are used to infer the vertical wavelengths of Q16DW. The significance of the present study mainly lies in establishing the climatology of Q16DW using a decade of meteor radar observations over the low and equatorial latitudes in the MLT region simultaneously for the first time.

Observations of Dual-frequency Precipitation Radar (DPR) on-board Global Precipitation Measurement (GPM) over an extremely severe cyclonic storm that formed over the Arabian Sea in the month of June 2023 are used to investigate the spatial pattern of microphysical processes of precipitating clouds embedded in the eye-wall, inner and outer rainband regions of the cyclone. The diameter of the eye of the cyclone is observed to be around 50 km with echo tops reaching as high as 15 km near the eye-wall region. The spatial and vertical distribution of radar reflectivity (Ze), median drop diameter (Dm), drop concentration (NW) and latent heating during various stages of the cyclone are explored. The structural and microphysical differences in the eyewall, inner and outer rainbands are investigated using composite analysis. The contour frequency by altitude diagram analysis revealed that the Dm of about 1.75–2 mm dominates at the lower altitude levels (below 5 km) in the inner rainband region of the cyclone whereas Dm of about 1.5–1.75 mm (1.25–1.5 mm) found to be dominant in the eye wall (outer rainband) regions. It is also noted that the precipitation clouds embedded in the eye-wall region has relatively large drop concentration followed by inner and outer rainbands. Results indicate that the maximum deviation of weighted number concentration is 4(5)

The robust expansion of Hadley circulation (HC) reported during the past few decades stands as a testimony for response of large scale circulations to the changing climate. The consequences of HC expansion on the precipitation distribution, tropical cyclone formation latitudes, mid-latitude cyclone tracks and shifting of subtropical Jet are well reported in the recent past. In the present communication, the consequences of HC expansion on the distribution of low-level clouds such as subtropical marine stratocumulus clouds (SC), which are not reported hitherto, are discussed. Due to the large-scale descent of HC and other favourable conditions over the sub-tropics, large decks of SC are found to be squeezed within the boundary layer especially over the coastal regions of the subtropical north-east Pacific, south-east Pacific and south-east Atlantic. Cloud fraction dataset along with zonal and meridional winds during the years 1981–2021 from the state-of-the art ERA5 reanalysis are employed for the study. The zonally resolved HC boundaries are retrieved over each 10 degree longitude sector, using the zonally resolved meridional mass stream function computed from Helmholtz decomposed horizontal winds. The results have emphatically shown that the low-level clouds over all the three study regions migrated towards poles with expansion rates of 31.5, 70.38 and 68.8 km/decade over the north-east Pacific, south-east Pacific and south-east Atlantic, respectively. The observed rates are significant at 95

The tropical middle atmosphere is characterized by long-period oscillations such as the Quasi Biennial Oscillation and the Semiannual Oscillation which are primarily driven by the interaction of a broad spectrum of atmospheric waves with the background flow. Using reanalysis datasets and independent rocket soundings from a low latitude location, we identified a hitherto unreported variability in the tropical middle atmosphere that appears at a variable interval of 2-5 years in the late 20th century and 7-9 years in the early 21st century. The newly identified variability, Quasi-Periodic Easterly Bursts (QPEBs) as we call them, manifests as enhanced easterlies during the easterly phase of the Stratopause Semiannual Oscillation around May-July. QPEBs are found to have remote influences on the Southern Hemispheric polar vortex as well as residual circulation in the lower mesosphere. A momentum budget analysis reveals that QPEBs are found to be primarily caused by enhanced cross-equatorial advection as well as gravity wave drag. Even though a close association with the Quasi Biennial Oscillation winds is observed, the cause of the observed periodicity remains elusive.

The vertical profiles of specific humidity in the troposphere measured by the millimetre-wave humidity sounder (MHS) onboard of the Earth Observation Satellite (EOS)-07 (Microsoft 2B) are validated using ground-based radiosonde observations during July-August 2023. MHS onboard EOS-07 (hereafter referred as EOS-07/MHS) is a 6-channel cross-track scanning radiometer operating in the 183.31 +/- 16.25 GHz band. The EOS-07 observations are used to retrieve three-dimensional specific humidity profiling from 1000 hPa (surface) to 100 hPa (16 km) atmospheric pressure levels with a spatial resolution of 10 km at nadir. The EOS-07/MHS measurements over India have a high correlation coefficient of 0.98 with the radiosonde observations. The mean relative bias is found to be 0.44 +/- 0.55 g kg(-1) below 350hPa and 2.8 +/- 2.3 g kg(-1) above 350 hPa. It is also found that above the 350 hPa level, MHS seems to be systematically overestimating the radiosonde measurements. After that, the EOS-07/MHS measurements are used to construct the diurnal variation of humidity during the active phases of the Indian summer monsoon. The observations are consistent with the present understanding of the Indian summer monsoon system. The present results are very encouraging and demonstrate the great potential of EOS-07/MHS observations of humidity for meteorological application, especially in understanding the hydrological cycle.

Tropical cloud clusters (TCC) play a vital role in earth’s climate by not only releasing a large amount of latent heat into the atmosphere but also by forming the basis for development of tropical cyclones (TC). However, not all the TCCs can be developed into cyclones and only a few of them develop into TC, selectively. There are large uncertainties in present understanding on why only certain TCC develops into a TC and others don't? The present study employs the global TCC observations generated by GridSat and IBTrACS datasets from 1980 to 2009 to investigate the TCC distributions over the various Oceanic basins such as North Atlantic (NA), South Atlantic (SA), East-West and South Pacific (EP, WP and SP) and North Indian (NI) and South Indian (SI) basins. The central objective of the present study is to characterize the size spectrum of TCCs and to investigate their potential transformation into TC. The TCCs are identified based on the different IR temperature thresholds in each basin. The present results suggest that overall ~ 5.5% of TCCs were developed into TCs per year across the globe and there is an increasing trend in number of TCCs that were grown into TCs during the study period. The size spectrum of TCCs showed a dominant peak at 100-200 km 2 . About 48% of TCCs transform into TCs within 24 hr of being identified. Furthermore, 85% of TCCs develop in to TCs within 84 hr of the first identification and only 5% of TCCs develop into TCs after 84 hr. Further, we have also analysed the background environmental conditions such as low-level wind speed, vorticity, divergence, vertical shear, upper level relative humidly and latent heating (LH) for developing and non-developing TCCs over NI basin. It is noted that the relative humidity in the developing composite is around 10-20% higher than that in non-developing TCCs and LH in developing TCCs is 0.15 K/hr larger than that in non-developing TCCs. The significance of the present study lies in investigating the developing TCCs as function of their size and lifetime including their long-term trends and bringing out the favourable environmental conditions for developing TCCs in the NI Ocean.

Weather forecasting is predicting the state of the atmosphere for a given location and time, and it depends on many meteorological parameters, which is a challenging task. In recent years, the use of artificial intelligence (AI) technology has grown leaps and bounds in understating data-driven Earth systems. The main objective of the present paper is to predict the zonal wind using machine learning (ML) techniques. Traditionally, numerical weather prediction (NWP) models are used to predict the weather, but they have their constraints and limitations. Rapid advances in AI- and ML-based models could learn quickly to predict high-impact weather events directly from the observed data. Their computations are also faster than the conventional models. In the present study, we have exploited Extreme Gradient Boosting (XGBoost), and Random Forest Regression approaches on a long time series of data to predict zonal winds at different pressure levels. The data of monthly mean values of zonal wind on different pressure levels for a period of 20 years from 2001 to 2020 over Thiruvananthapuram (8.5° N; 76.9° E), is used. The model results are validated with the observations from the unseen data. The models have proven to provide satisfactory results with XGBoost and Random Forest Regression with a mean absolute error of 1.84 and 2.62, respectively. Based on the results obtained, the ML-based models can be sufficiently reliable in predicting the vertical structure of zonal winds on monthly scales. Evaluated the performance of these models and also indicates the XGBoost is the best fit model compared to Random Forest Regression for predicting the vertical structure of zonal wind throughout the year over a given region. Thus the significance of the present study lies in the ability to predict the zonal wind speed at different altitude levels, which is an essential factor for sustainable wind power generation along the western coast of India. Also useful for future climate projections are the Indian summer monsoon low-level jet and tropical-easterly jet at 850 and 200 hPa levels.

It is known that the Hadley circulation (HC) is responsible for the typical wet climate of the tropics and the dry climate of the subtropics. Previous studies have shown that the HC exhibits a poleward expansion of ~ 0.5–1° latitude per decade with significant regional and seasonal variability. Owing to its pivotal role in controlling the climate over tropics and subtropics, it is important to predict the evolution of HC in a future warming scenario from the perspective of formulation of adaptation strategies. In this regard, the current study employs the climate model simulations from the Indian Institute of Tropical Meteorology-Earth System Model (IITM-ESM) archived in the latest Coupled Model Inter-comparison Project 6 (CMIP6) to identify the long-term changes and future projections in the width of the ascending and descending branches of the HC, after validating it against the latest generation ERA5 reanalysis. Results show that the model is able to capture the observed changes in the total width of the HC and its ascending regions. Analysis of trends in the future projection of the width of the HC ascending and descending regions brings out results that are consistent with earlier reports using multi-model simulations archived in CMIP6. The future projections of HC intensity show weakening tendencies in both NH and SH. The trends in the model’s future projection of zonal mean precipitation under two high forcing scenarios show hemispherical asymmetry with SH exhibiting relatively strong trends in both ascending and descending regions of the HC. The results are discussed in the light of the present understanding on HC dynamics. The significance of the present study lies in evaluating IITM-ESM, which is the first model from India to participate in CMIP, using ERA5 reanalysis and discussing the future projections of HC dynamics and its implications on long-term trends in precipitation under high forcing scenarios.

The vertical structure and dynamics of deep convective clouds plays an important role during the Indian summer monsoon period. The characteristics of deep convective cells (DCC) and their mi-crophysics remain unclear, especially during the Indian summer monsoon (ISM). Further, the in-tensity of convection is an important element, which plays a key role in the progression of the monsoon. The present study focuses on characterizing DCC over a coastal location Thumba (8.50 N, 770 E) during the Indian summer monsoon periods of 2017, 2018 and 2019 using C-band polarimetric Doppler Weather Radar (DWR) measurements in terms of their intensity, vertical ex -tent, top heights and their microphysical properties. A method is devised to identify the DCC from the DWR observed reflectivity cross-sections. The results showed that the occurrence of reflectivi-ties in the range of 25-35 dBZ dominates below 6 km and the reflectivities in the range of 10-20 dBZ dominates above 6 km. The diurnal evolution of 40 dBZ depth radar echoes in DCC were analysed for June-September months, which showed peaks at preferential timings. The mean di-urnal evolution of DCC also been investigated and it is found that these systems peak at three lo-cal time intervals viz., 00-05, 13-16 and 19-23 h during ISM period. Further, a separate analysis has been carried out for land and oceanic region for 40 dBZ depth radar echoes in DCC. The analysis show that the occurrence of 40 dBZ depth radar echoes peak at early-morning and late -night over the ocean and afternoon and late night over the land regions. The present results pro-vide a quantitative assessment of DCC including their diurnal evolution during the Indian sum-mer monsoon season over Thumba using DWR observations for the first time.

The present study describes a method to identify the boundaries of both the ascending and descending regions of Hadley Circulation (HC) at regional scales using the Global Positioning System-Radio Occultation (GPS-RO) measurements on board Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC). COSMIC provides high-quality profiles of temperature and water vapor by using Radio Occultation technique. The bulk static stability (BSS) metric is used for identifying the descending region boundaries of HC whereas relative humidity (RH) measurements are employed to decipher the ascending region boundaries, which coincides with the Inter-tropical Convergence Zone (ITCZ). First, zonally averaged HC boundaries obtained from COSMIC measurements are validated with the meridional mass stream function (MSF) metric estimated using ERA 5 reanalysis. Further, using COSMIC measurements, zonally resolved boundaries are estimated at every 10° longitude. Similarly, the latitudinal distribution of RH over a given longitudinal sector at 5 km altitude is fitted with a Gaussian function to retrieve the ascending region boundaries. After characterizing the regional features in zonally resolved HC, to assess the HC expansion impact, the long-term trends in the precipitation during 1980-2020 are estimated and discussed. The significance of the present study lies in describing a method to simultaneously retrieve zonally resolved HC ascending and descending region boundaries exclusively from COSMIC measurements for the first time and demonstrating its application in assessing the impact of HC expansion.

A rare episode of minor circulation embedded in the Hadley circulation (HC) and processes responsible for its formation is discussed. Using 34 years (1979–2012) of HC climatology derived from reanalysis datasets, a minor circulation centered on ∼35°N and embedded within the northern hemispheric zonal mean HC is observed during the month of July 1993. A longitudinally resolved vertical velocity observations centered on 35°N latitudinal belt revealed that there is an anomalous upwelling over the North American sector possibly associated with "Great Floods of 1993" thus emphasizing the prominence of a regional feature and its impact on the zonally averaged circulation.

Recent investigations have shown a robust signature of poleward migration of the tropical cyclone latitudes using observations and climate model simulations. Most of these studies invoked the role of the Hadley circulation (HC) expansion in the poleward shifting of tropical cyclones. However, none of these studies focused on the dissection of the zonally asymmetric HC into ascending and descending regions at regional scales, which holds the key in establishing the association between these two phenomena. Here, we are reporting the poleward migration of tropical cyclones and their association with ascending region boundaries of the HC at regional scales for the first time. The results emphatically show that the tropical cyclone latitudes as well as latitudes of maximum lifetime intensity vary in tandem with boundaries of the ascending region of the HC as compared to its descending region thus providing a vital clue on processes governing poleward migration of tropical cyclones.

In the present communication, C-band polarimetric Doppler Weather Radar (C-DWR) observations are used to investigate the structure and evolution of organized precipitation bands during the Indian summer monsoon (ISM) of 2017, 2018 and 2019 over Thumba (8.50 N, 770 E), a coastal location in south India. The C-DWR observations show organized high radar reflectivity structures of -50 dBZ that organized into narrow bands of -200 km (North-South) in length and 10-20 km (East-West) in width with vertical extent of -6-8 km. These narrow bands of precipitation structures are observed to be formed over the Arabian Sea and subsequently propagating towards the radar site. The observed differential radar reflectivity (Zdr) and the correlation coefficient values indicate the horizontally oriented droplets and non-uniform hydrometeors. The potential formation mechanisms of the precipitation bands are investigated using reanalysis winds. The results show that blocking effect induced by the Western Ghats around the radar site play a key role in the formation of organized precipitation bands. It is noted that the location of the blocking effect induced convergence (conducive for the formation of precipitating clouds) with respect to the orography is proportional to the intensity of the low-level winds. The significance of the present study lies in understating the structure and evolution of organized precipitation bands and discussing their potential formation mechanisms during the ISM using C-DWR observations.

This study was conducted to evaluate the dynamics of superoxide anion radical (O 2) production rate and alterations in DNA content and permeability of cell membranes in etiolated wheat seedlings (Triticum aestivum L. cv. Harmony) as well as those grown under normal daylight regime. The results suggest that the development of etiolated wheat seedlings and those grown under normal daylight is accompanied by the periodic formation of O 2 which leads to alterations in DNA content as well as the permeability of cell membranes. The results also indicate that the first enhancement in the rate of O 2 generation was detected on the sixth and seventh days of development, after which the rate of O 2 ̄ production reduced in etiolated seedlings and those grown under normal daylight regime. The second maximum of the O 2 ̄ producing rate in developing and senescent organs occurred on the ninth day of plant development. The lowest values of all studied parameters, such as O 2 producing rate (37%), total genomic DNA concentration (58%) and electrolyte leakage (EL; 18%) were observed in etiolated wheat seedlings. Towards the end of the study period, DNA concentration and EL in organs of wheat seedlings declined, suggesting possible destruction of cellular organelles and the beginning of apoptotic processes. Overall, these results indicate that O 2 generation is decisive for normal morphogenesis, and it is an indispensable element of synchronous growth and development.

In the present communication, diurnal evolution of vertical structure of precipitating clouds over the southernmost part (7 degrees-11 degrees N) of the Western Ghats (WG) of Indian region during summer and winter monsoon periods is discussed using 17 years of Tropical Rainfall Measuring Mission (TRMM) precipitation radar measurements. This part of the WG experiences both summer and winter monsoons and provides a unique opportunity to study the orographical processes involved in the formation of precipitating clouds at diurnal scales during two contrasting background environments. During summer monsoon, the lower tropospheric winds are dominated by the presence of strong low-level westerly jets, which interact with steep slopes of the WG whereas during winter monsoon, winds are very weak easterlies and encounter the WG from eastern side. The composite longitudinal distribution of vertical structure of precipitating clouds in terms of frequency of occurrence of radar reflectivities >= 23 dBZ is constructed at four local time intervals. The results are discussed in terms of bands of high frequency of occurrence of precipitating clouds and their longitudinal position with respect to the WG. A band of high frequency of occurrence of precipitation clouds is observed to be moving eastwards and their potential formation mechanism is attributed to the forced lifting over the upslope regions and to the upstream blocking over the offshore during the summer monsoon. During winter monsoon, a two band structure is observed one over the southern BOB, which is attributed to the persistent low-pressure system observed over this region and another over the upslope of the WG from eastern side. Thus the present study for the first time brings out the diurnal evolution of the vertical structure of precipitating clouds during summer and winter monsoon over the southernmost part of the WG and discuss the potential physical processes involved in their formation.

In the present communication, first results from an experiment to measure intrinsic frequency spectrum of atmospheric gravity waves using balloon-borne quasiLagrangian frame of reference observations in the mid-stratosphere over a tropical station, Hyderabad (17.4 degrees N, 78.2 degrees E) are discussed. A zero-pressure polyethylene balloon with GPS-sonde payload was drifted at similar to 31 km altitude for a horizontal distance of similar to 100 km for measuring pressure, wind and temperature at 1 sec temporal resolution. The measured altitude of the balloon showed variability within +/- 100 m, thus ensuring a near horizontal drift. These observations are used to estimate the intrinsic frequency spectrum of gravity waves in the mid-stratosphere over an Indian observational site. The successful experiment has opened up a new avenue for studying not only the stratospheric gravity wave dynamics, but also for exploring the horizontal mapping of stratospheric trace gases.

Advance prediction of heavy rainfall days over a given location is of paramount importance as heavy rainfall impacts ecosystems, leads to floods, accounts largely for the total rainfall over the region and its prediction is highly desired for the efficient management of weather-dependent activities. Traditionally, Numerical Weather Prediction models serve the purpose of weather predictions, but they have their constraints and limitations. In this regard, artificial intelligence and machine learning tools have gained popularity in recent years. In the present study, we have employed the Gaussian Process Regression (GPR) approach, one of the machine learning methods, on a long time-series rainfall data for the determination of heavy and light rainfall days. Climatological data of daily rainfall for a period of 116 years from 1901 to 2016 over Sriharikota (13.66°N, 80.23°E), a coastal island location in India, is used for training the GPR model for the identification of the heavy and light category of rainy days. The performance of the GPR model is investigated by predicting the heavy and light rainfall days per year over Sriharikota. K-nearest neighbour, random forest, and decision tree models are also used and results are compared. The validation of GPR results shows that the performance of the proposed model is satisfying (root mean square error = 0.161; mean absolute error = 0.126; mean squared error = 0.026), especially for the heavy rainfall days. Furthermore, GPR model is extended to prediction of spatial distribution of monthly rainfall over the Indian region. Results obtained from the present study encourages the utilization of the GPR model as one of the promising machine learning tools for the prediction of heavy rainfall days over a given location.