The difference in rain drop size distribution (DSD) characteristics between convective and stratiform rain and in monsoon and premonsoon season have been investigated over a tropical location, Kolkata, using seven years of data collected from a Joss–Waldvogel disdrometer (JWD). During the Indian summer monsoon (ISM) and the premonsoon season, Kolkata gets substantial rain. Interannual variability of DSD has been evident. The normalized drop size distributions of DSD show a double peak at 20–50 mm/h rain rate due to drop break up and coalescence process. For convective rain, the concentration of higher drops ($>$3 mm) is more significant compared to stratiform rain. In the premonsoon period, a significant contribution of normalized DSD in the afternoon to late evening has been evident due to nor’westers. The contribution of normalized DSD of higher drops ($>$3 mm) is more in the premonsoon convective rain compared to monsoon convective rain. The larger slope of$\mu $–$\Lambda $relationship has been observed in the premonsoon convective period which corresponds to higher$D_{m}$values. The relationship between radar reflectivity ($Z$) and rain rate ($R$) distinguishes the monsoon and premonsoon seasons. Radiometric measurements have been used to examine the effects of the atmospheric instability parameter convective available potential energy (CAPE) and cloud base height (CBH) on the formation of larger drop sizes. This study will be useful for understanding precipitation microphysics and improving the quantitative precipitation estimation (QPE) algorithm.

The present study addresses the impacts of reduced anthropogenic activities during the lockdown period of COVID-19 pandemic on the aerosol concentration, treated as heat absorbing agent, and on the related atmospheric processes, using ground-based and spaceborne measurements over a highly polluted Indian metropolis, Kolkata. The investigation reveals that reduced aerosol concentrations during the pre-monsoon of 2020, when the lockdown was implemented, decreased atmospheric instability as indicated by low values of the convective available potential energy (CAPE). This hindered the abundance of aerosols above the atmospheric boundary layer. Also, micro rain radar (MRR) observations showed a significant reduction of convective precipitation occurrences over Kolkata during this period. The back trajectory analysis has revealed the absence of continental component toward the wind clusters associated with rain occurrences during pre-monsoon 2020. This resulted in increased occurrences of stratiform rain events during the pre-monsoon of 2020 compared to the same period of previous years.

This letter reveals the prevailing scenario of raindrop size distribution (DSD) in terms of mass-weighted mean drop diameter ($D_{m}$) over a tropical metropolis, Kolkata (22.57°N, 88.37°E), India, in a contrasting aerosol environment that occurred during the COVID-19 pandemic in the absence of usual human activities. In the premonsoon months (March–May), the probability of$D_{m}$values exceeding 2 mm has increased in 2020, indicating the dominance of large raindrops, compared to the years 2017–2019. Increased number densities of larger drops have influenced the drop fall velocity spectrum as measured by a laser precipitation monitor in terms of the percentage occurrence of high-velocity small drops (superterminal) and low-velocity large drops (subterminal) for both convective and stratiform precipitations. As measured from a Ka-band microrain Doppler radar, the mean melting layer altitude during stratiform rain has decreased by ~800 m during the premonsoon of 2020 compared to 2017–2019. According to the ERA-5 reanalysis data, changing rain microphysical characteristics are related to decreasing zero-degree isotherm height and reduced wind shear.

The present study deals with the interplay of multiple atmospheric processes in changing the microphysical properties of precipitation during the pre-monsoon season (March–May) using the long-term experimental data of raindrop size distributions (DSDs) spanning over 15 years (2005–2019) obtained at an urban tropical location, Kolkata (22.57°N, 88.37°E), near the land–sea boundary. The changing pattern of air mass flows from the maritime and continental region, which contribute to the formation of precipitation processes, has been responsible for the varying characteristics of rain. Changes in raindrop sizes are related to aerosol properties, cloud features, temperature, and relative humidity that change mass-weighted mean drop diameter ($D_{m}$) differently in low and high rain rate regimes.$D_{m}$has shown an increasing trend over time for low rain rates ($D_{m}$at high rainfall rates.

Raindrop size distribution (DSD) parameters need to be estimated to determine rainfall from radar measurements of backscattered signals in the precipitating medium and to study microphysics of precipitation under varying atmospheric conditions. A technique is proposed to estimate the DSD parameters from Global Precipitation Measurement (GPM) dual-frequency products at Ku and Ka bands, which has been validated with ground-based disdrometer measurements obtained during the five-year period 2014-2018 over a tropical location, Kolkata (22°54’N, 88°35’E), India. The present method yields the three parameters of the DSD model in terms of a gamma function namely, intercept (N0), shape (μ) and slope (Λ). Based on a proposed optimization technique, the most appropriate μ-Λ relationship is determined during a GPM satellite pass to obtain the DSD parameters. The derived parameters have yielded mass-weighted mean diameter (Dm) data that show much better agreement with ground-based DSD measurements than that provided by GPM DPR product. This method has also been used to obtain the height profiles of DSD that broadly correlate with ground-based micro rain radar (MRR) observations, showing how DSD varies at different altitudes. The present results demonstrate that the proposed technique is effective in obtaining three-parameter DSDs over a tropical location, and offers a useful method to study the microstructure of rain using space-borne radar.

This article aims to classify precipitation into two categories, namely stratiform and convective. Multiple techniques, such as utilizing the micro rain radar (MRR), electric field monitor (EFM), radiometer, and disdrometer measurements, have been deployed for this purpose, at a tropical location Kolkata, India. A new rain classification technique, using logistic regression modeling of the sixth to third moment ratio (M6/M3), has been proposed. Classification of rain types based on the new technique shows high consistency with that based on radar reflectivity (Z) values obtained from disdrometer measurements. This article also distinguishes mixed rain from stratiform and convective rain. The observations on the bright band structure by MRR and on differential brightness temperature at 31.4 and 22.23 GHz by a radiometer are utilized to classify mixed rain types. Although the EFM measurements do not classify rain types directly, they give a distinct signature of the impending stratiform/convective rain events. A comparative analysis between the percentage of stratiform and convective rain durations shows significant dominance of stratiform rain over convective rain. At the present location, the convective phenomenon shows higher occurrences during the pre-monsoon period compared to the monsoon period.

The profiles of rain microstructures have been investigated for different types of precipitation using microrain radar observations at a tropical location Kolkata (22.57° N, 88.37° E). A prominent dip in radar reflectivity ($Z$ ) profile has been observed near the boundary layer at high rain rates. This phenomenon is due to the break-up of raindrops into smaller sizes at around 2 km height during intense convective events. The boundary layer dynamics decides the dominance of one of the two processes, namely, coalescence and break-up of rain drops, and determines the drop size distribution profile near the boundary layer. A sharp gradient of vertical velocity at 1-2 km heights is responsible for enhanced drop break-up resulting in an increase in the small drop number density.

Boundary layer dynamics play an important role in determining precipitation structure during convective processes. The radar reflectivity profiles which depend on the sixth moment of rain drop size distribution (DSD) can be employed to sense the precipitation processes. The height profiles of DSD significantly depend on the vertical wind velocity. The observations with a micro rain radar (MRR), an FMCW radar, over Kolkata, have been utilized to study the drop size distribution and its relationship with radar reflectivity at Ka-band. A sharp decrease of radar reflectivity in the height range 1-2 km corresponds to an increase of downdraft velocity observed around the same height range. This indicates that the increased downdraft causes breakup of rain drops into smaller sizes. The split in the Doppler spectrum, known as aliasing effect, is most prominent near the boundary layer height and has been employed to estimate downward wind velocity.

Atmospheric gravity waves are generated by the interplay of earth's gravity and buoyancy effect on an air parcel. The zonal and meridional wind and temperature perturbations observed in the troposphere and stratosphere exhibit signature of gravity waves. These gravity waves can originate from different excitation mechanisms such as deep convection in the tropics, orographic effects and various meteorological disturbances. We have studied the gravity wave generated due to an intense tropical cyclone named, Phailin, using radio occultation data from COSMIC over the Indian region. The radio occultation data are used to obtain the vertical profiles of atmospheric refractivity that bear the manifestation of gravity waves at different heights. The dry temperature and water vapor pressure from the refractivity profile yield the potential energy and the water vapor intrusion due to gravity waves in the lower stratosphere (18-25 km) during the tropical cyclone.

A nowcasting technique has been proposed to estimate the impending rain accumulation using ground-based radiometric measurements at Kolkata (22.65°N, 88.45°E), a tropical location. It has been observed that the normalized variation of brightness temperature (BT) at 31 GHz along with the standard deviation of BT at 22 GHz and instability indices, namely, lifting index, have shown definite changes before rain events. A combination of these three parameters can be effective in predicting rain events both qualitatively and quantitatively. Accordingly, a prediction model is developed and tested on several intense rain events during the period 2014-2015. The model is found to perform reasonably well in predicting intense rain about 70-75 min in advance with an efficiency of 80%.

In this letter, the tropospheric scintillations occurring just before rain commencement have been investigated. This letter indicates the association of the prerain scintillations with both the cloud thickness and the prevailing convection. The convective available potential energy has been considered as a parameter indicating the strength of the associated convection.

The Van de Kamp (VDK) model, used to describe the probability distribution of the fade slope for temperate regions, has been modified for a tropical location, Kolkata, India. The fade-slope technique is adopted to predict the time series of rain attenuation of Ku-band signal during rain events for the tropical location. The matching between measurement and prediction has been improved by predicting the fade slope from the modified VDK model instead of using the direct slope measurement.

The phenomenon of scintillations in relation to rain attenuation of Ku-band satellite signals has been studied at a tropical location. The standard deviation (σ) of scintillations increases with attenuation up to a value in the range of 6-7 dB, beyond which σ decreases with attenuation. A technique is proposed to obtain some effective values of structure constant (Cn2) of refractive-index variation from the experimental observations of σ and rain rate (R). The value of Cn2also increases with attenuation up to values in the 6-7-dB range and decreases beyond that value. The eddies in turbulent raining medium grow with rain rate, and consequently with attenuation, causing an increase in the outer scale (LO) of turbulence and thus increasing σ until LOreaches the size of the first Fresnel zone. In a further development, the contribution of LO toward Cn2decreases, resulting in the decrease of fast fluctuations with rain attenuation.