Investigating the raindrop size distributions (DSDs) for different cloud types is essential for the rain characterization and understanding different microphysical processes within the cloud system. ...In this study, the simultaneous measurements from the Micro-Rain Radar (MRR) and Joss-Waldvogel Disdrometer (JWD) are used to investigate the DSDs of different precipitation categories during the Indian Summer Monsoon (ISM) season (June–September) for the period 2012–2015. Both the instruments deployed at Mahabaleshwar (17.92°N, 73.6°E, ~1.4km AMSL), which is located in the complex mountain terrain of the Western Ghats (WGs), India. From the MRR reflectivity factor and fall-velocity profiles, the observed precipitation systems are classified into four categories: shallow-convective, convective, stratiform and mixed convective-stratiform. In terms of rain occurrence frequency, it is found that the rain over Mahabaleshwar is mostly contributed by the shallow-convective (~89%) system while the stratiform system contribution is about 9% and the convective and the mixed convective-stratiform systems contributes <2%. For different precipitation categories, the rain microphysical parameters such as median volume diameter, rain liquid water content and normalized intercept parameter are estimated using the moment method (second, third and fourth moment) of the observed DSDs. The reflectivity–rainfall (Z–R) relation of the form Z=aRb is estimated for each precipitation categories. Studies of systematic and comprehensive classification of precipitation types in terms of their rain microphysical parameters and Z–R relationships over a region is important as it would improve the understanding on rain microphysics and rainfall estimation from active and passive remote sensing devices.
•Characterization of raindrop size distributions (DSDs) over the WGs•Precipitation systems are classified into shallow-convective, convective, stratiform and mixed convective–stratiform.•Most rain contribution is from the shallow-convective system.•DSD parameters and Z–R relationship vary for different precipitation systems.
Abstract
Diurnal variation of convective storms (CSs) during monsoon season and associated physical mechanisms are significantly important for accurate forecast of short-time and extreme ...precipitation. The diurnal cycle of CSs is investigated using ground-based X-band radar, Tropical Rainfall Measuring Mission Precipitation Radar, and reanalysis data during the summer monsoon (June–September of 2014) over complex mountain terrain of Western Ghats, India. Diurnally, CSs show a bimodal distribution in the coastal areas, but this bimodality became weak along the upslope regions and on the mountain top. The first occurrence mode of CSs is in the afternoon–evening hours, while the second peak is in the early-morning hours. The diurnal cycle’s intensity varies with location, such that it reaches maximum in the afternoon–evening hours and early morning on the mountain top and coastal areas, respectively. Two possible mechanisms are proposed for the observed diurnal variation in CSs (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between land-sea breeze, local topography and large-scale monsoon winds. It is also observed that the CSs developed on the mountain top during afternoon–evening hours are deeper than those along the coast. The higher moisture in the lower- and mid-troposphere, higher instability and strong upward motion facilitate deeper CSs during afternoon–evening hours.
The Indian summer monsoon trough (MT) is a dynamically active region of the quasi-stationary feature extending from north-western India towards the Bay of Bengal (BoB). The eastern rim of the MT ...(EMT) is modulated by the embedded synoptic-scale monsoon low-pressure systems (LPSs) in north BoB. We examine the spatiotemporal variability of the convective storm (CS) in the EMT region, which lies along the pathway of LPSs. A Lagrangian-based objective-tracking method, Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN), was injected into Kolkata S-band radar observations of nine wet seasons (June–September, 2009–2017). CSs frequently occur near the onshore areas and is linked to the formation and propagation of LPSs in the BoB. By examining the relationship of CS top heights with reflectivity lapse rate and fractional volume of 40 dBZ or more, three vertical categories of CS have been identified as shallow (SCS) below 5 km, medium depth (MDCS), and deep CS (DCS). The land–ocean contrast in the spatial distribution of MDCS and DCS and the contoured frequency by altitude diagram shows continental convective vigor. Although short-lived smaller CSs account for most storms in EMT, the bulk of the precipitation (75
%
) contributed from the infrequent largest storms. A weak (strong) linkage between the precipitation flux and CS top (area-time integral) gives a clue on the importance of CS voluminous organization rather than the linkage of heaviest rainfall to tallest CS. While most CSs propagate inland following the mass-weighted mean winds, the largest CS propagate offshore steered by the mid-level wind at 700 hPa.
Abstract
This study investigates the diurnal cycle, propagation, and progression of convective storms (CSs) on the eastern edge of India’s monsoon trough (MT) using 9 years of S-band radar ...measurements with satellite and reanalysis datasets. CSs initiate over ocean during midnight–early morning hours and propagate onshore in succeeding hours. CSs exhibit two semidiurnal peaks, one during afternoon hours over inland areas and another during midnight–early morning hours in oceanic/coastal locations. The deep and intense afternoon peak over inland regions is attributed to land surface heating and associated destabilization. The weak and shallower but organized midnight–morning peak and propagation of CSs toward the coast are attributed to the nocturnal land breeze and its interaction with prevailing onshore flow. The observed lead–lag of a few hours in the diurnal cycle of different cumulus modes correspond to the transition of congestus into deep and then, often, into overshooting modes. Moisture budget analysis showed atmospheric regulation of this transition through thermodynamic (congestus moistening) and dynamic processes (vertical advection). Theoretical time scales were invoked to estimate the relative role of vertical advective versus congestus moistening for promoting the afternoon transition from congestus to deeper modes. Comparing the time scales for congestus moistening (18–46 h) and dynamics (3 h) with the actual transition time scales (2–4 h) reveal that congestus moistening is too slow to explain the observed lead–lag in CS modes. Though both thermodynamic and dynamic processes moisten the midlevel prior to deep/overshooting convection, vertical advection is the dominant dynamic process for the observed congestus–deep–overshooting transition.
Significance Statement
Tropical rainfall is usually linked with convection in the morning and afternoon hours. We look at the basic physical processes that lead to those convective activities peaks. The afternoon peak is linked to maximum heating, resulting in an unstable environment, whereas the morning peak is linked to the interaction of large-scale monsoon flow with a land breeze. Furthermore, daily solar heating visually shows a shallow-to-deep progression of convection. The moist midlevel environment was shown to precede such convective development in a day. The large-scale monsoon flow is a dominant cause of this moistening. The monsoon dynamic flow takes roughly 2–3 h to sufficiently moist shallow storms into deep storms, whereas the local thermodynamic moistening process takes about 18–46 h.
Monsoon trough (MT) convection exhibits multiscale spatial variability ranging from cellular storms to large organized convective systems. Understanding the relationships between storm‐scale ...convection and the large‐scale environment is crucial for accurately representing the properties and effect of convection in climate models. We investigate these relationships using nine wet seasons (June–September of 2009–2017) of S‐band radar observations over a tropical location (Kolkata) in the Indian summer monsoon trough region. A Lagrangian‐based objective cell‐tracking method, Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN), is applied to the volumetric reflectivity data to identify and track the convective storms. ERA5 hourly reanalysis products from the European Centre for Medium‐Range Weather Forecasts (ECMWF) are utilized for the field variables representing the large‐scale environment. We focus on the relationships of radar‐derived storm properties with large‐scale dynamic, thermodynamic, and stability variables such as convergence, mid‐level humidity, convective available potential energy (CAPE), and convective inhibition (CIN). Storm‐scale convection shows a selective nature, characterized as embedded cells in their unique environment. Furthermore, storms are classified as storm cells (simple and intense) and multicellular (linear and nonlinear) based on their geometrical properties and intensity. MT convection is predominantly composed of storm cells and characterized by an environment of moist mid‐level, low‐to‐moderate CAPE, low CIN, and strong dynamic forcing, that is, vertical ascent. In dry and weak dynamic forcing (i.e., subsiding atmosphere) with high CAPE and CIN, the highest rain intensity is associated with a few linear and nonlinear multicellular storms. In the development of such major linear and nonlinear multicellular storms over the eastern flank of the MT, the mid‐troposphere acts as a dry capping inversion. It is observed that the storm‐scale convection varies with the large‐scale dynamic forcing. During weak (strong) dynamic forcing conditions, the CAPE (mid‐level humidity) relates well with the observed storm‐scale convection.
Storm‐ to large‐scale environment relationships are examined using long‐term observations. The study evidenced a “natural selection” effect of mid‐level humidity. A random to deterministic relationship is observed during weak and strong synoptic forcing. Scatter plots between VMFC and (a) CAPE, (b) mid‐level humidity (RH650), and (c) CIN are presented where dotted, solid, and dashed lines show 95 percentile, median, and 5 percentile values, respectively.
X‐band radar observations at Mandhardev (18.04°N, 73.85°E) are used to investigate statistics of convective clouds over the Western Ghats during monsoon season (June–September 2014). Convective ...storms (cells) are identified using an objective‐tracking method to examine their spatiotemporal variability, thus quantifying the time‐continuous aspects of convective cloud population over the region for the first time. An increased frequency of storm location and initiation along the windward mountains compared to coastal and lee side highlights orographic response to southwesterly flow, with superimposed diurnal cycle. An eastward progression of convective activity from upstream the barrier through windward slopes of mountains over to the lee side is observed. Storm area, height, and duration follow lognormal distributions; wherein, small‐sized storms contribute more to total population and unimodal distribution of 35 dBZ top heights (peaking at 5.5 km) depicts the dominance of shallow convection. Storms exhibit a pronounced diurnal cycle with a peak in afternoon hours, while the convective area maximum is delayed by several hours to that of precipitation flux. Cell lifetime and propagation show that cells move with slow speeds and have mean duration of 46 min. They align east‐west nearly parallel to mountain ridges, and their direction of movement is steered mostly by large‐scale winds at lower levels. Based on top heights, convective cells are further classified into cumulus, congestus, and deep clouds. In general, congestus (deep) cells are most abundant in the windward (leeward) side. A lead‐lag relationship between congestus and deep cells indicates midtroposphere moistening by congestus cells prior to deep convection.
Key Points
Spatial distribution of convective storms highlights effect of orography on convection initiation
Convective storm properties (area, height, and duration) follow lognormal frequency distribution
Overall, shallow convection dominates the region and persists for an average duration of 46 min
X‐band radar observations are integrated with lightning location network observations to investigate the relationship between convective storm properties and lightning over the Western Ghats during a ...monsoon season (June–September 2014). Convective storms (cells) were identified using an objective‐tracking method and instantaneous lightning strikes within the radar domain were then linked with observed storms. This spatio‐temporal sampling of individual convective cells and lightning has facilitated process‐based study of electrified convection over the Ghats for the first time. Storm and lightning occurrences are typically high during monsoon onset and withdrawal months of June and September, respectively. A spatial correspondence between deep‐intense storms, lightning, and intense convective cores indicated presence of large hydrometeors in the mixed‐phase region of storm supported by strong updrafts and is essential for lightning. The large‐scale instability that peaked during afternoon hours was a key factor in the formation of deep‐intense storms and lightning. Results show that majority of lightning‐producing storms are located on the leeward side as opposed to the windward side. These storms have deeper top‐heights, larger areas and vertically integrated liquid, and an enhanced hail probability than those devoid of lightning. Warm season convection in the study area is accompanied by the preponderance of negative Cloud to Ground (−CG) flashes over positive Cloud to Ground (+CG) lightning. Storms with +CG features exhibited much higher (>2 times) vertical airmass flux in the mid‐troposphere (6–9 km) than storms without +CG features. Furthermore, for majority of +CG storms, intracloud flash occurrences increased significantly above the freezing level.
Key Points
Spatial agreement between deep intense storms, lightning, and intense convective cores infers lofting of hydrometeors in mixed‐phase region
Electrified storms are frequent over the leeward side as compared to the windward side. Negative Cloud to Ground flashes outnumbered the positive ones
Storms with lightning have deeper top heights, larger areas, and larger vertically integrated liquid than storms without lightning
This study examined a typical case of deep convective storm that formed over southwest India on October 12, 2011, using ground-based X-band radar measurements and Weather Research and Forecasting ...(WRF) model simulations. The radar observation showed isolated pockets of convective storm, which merged later to form a convective cluster. The observed storms were tall, extending well into the mixed-phase region. Few storms even extended up to the tropopause height. Three different WRF cloud microphysics schemes (WRF Double-Moment 6-Class, Morrison Double-Moment, and Milbrandt–Yau Double-Moment) were used to simulate the observed deep convective storm to examine the vertical structure of hydrometeors. All the cloud microphysics schemes were able to reproduce the convective storm event with a lag time of almost two and a half hours. The WRF Double-Moment 6-Class scheme better simulates the vertical structure of storm compared to the other two microphysics schemes. The WRF model reasonably simulated the observed patterns of convective storm when the WRF cloud microphysics scheme better simulate the graupel and snow. The differences in simulated storm structure obtained by different microphysics schemes compared to observation highlight the deficiency involved in the simulations in capturing the microphysics that is guiding the intensity of convective storms. The present study thus underscores the importance of microphysics in different parameterization schemes of WRF simulation over southwest India, which has an implication in the forecasting of convective storms.
Accurate and real‐time precipitation estimation is a challenging task for current and future spaceborne measurements, which is essential to understand the global hydrological cycle. Recently, the ...Global Precipitation Measurement (GPM) satellites were launched as a next‐generation rainfall mission for observing the global precipitation characteristics. The purpose of the GPM is to enhance the spatiotemporal resolution of global precipitation. The main objective of the present study is to assess the rainfall products from the GPM, especially the Integrated Multi‐satellitE Retrievals for the GPM (IMERG) data by comparing with the ground‐based observations. The multitemporal scale evaluations of rainfall involving subdaily, diurnal, monthly, and seasonal scales were performed over the Indian subcontinent. The comparison shows that the IMERG performed better than the Tropical Rainfall Measuring Mission (TRMM)‐3B42, although both rainfall products underestimated the observed rainfall compared to the ground‐based measurements. The analyses also reveal that the TRMM‐3B42 and IMERG data sets are able to represent the large‐scale monsoon rainfall spatial features but are having region‐specific biases. The IMERG shows significant improvement in low rainfall estimates compared to the TRMM‐3B42 for selected regions. In the spatial distribution, the IMERG shows higher rain rates compared to the TRMM‐3B42, due to its enhanced spatial and temporal resolutions. Apart from this, the characteristics of raindrop size distribution (DSD) obtained from the GPM mission dual‐frequency precipitation radar is assessed over the complex mountain terrain site in the Western Ghats, India, using the DSD measured by a Joss‐Waldvogel disdrometer.
Key Points
For the first time, assessment of GPM‐IMERG rainfall as well as raindrop size distribution estimates was carried over Indian subcontinent
IMERG rainfall was assessed over different places in Western Ghats, India, where orography plays a major role in the monsoon precipitation
Assessment was performed on daily, diurnal, monthly, and seasonal basis over Indian region
Summer precipitation over the Western Ghats is an important facet of Indian monsoon. The vertical structure of mesoscale convection during dry and wet epochs of regional precipitation is studied ...using X‐band Doppler radar, for the first time. The observed characteristics are distinctly different in terms of small‐scale convective and large‐scale atmospheric features for the dry versus wet regimes. The depth and intensity of convection is analyzed using various echo‐top heights (ETHs). The frequency distribution of 0‐ and 15‐dBZ ETH exhibits bimodality during the dry period, which changes to unimodal in wet period. Top heights of precipitating convection (characterized by 30‐dBZ echoes) decreased with low‐level static stability, suggesting preponderance of shallow convection with little stratiform clouds. Suppressed heating in the dry period is a signature of reduced 0‐dBZ ETH occurrence. However, its enhanced occurrence during the wet period resulted in cooling below 2 km and increased heating aloft with maxima at 6 km. Near high precipitation events, midtropospheric humidity rapidly builds up over 2–3 days prior to increase in cloud‐top heights and areal coverage. Observations indicate large amplitude of ETH diurnal cycle during the dry period (especially over leeside) accompanied by weak upstream winds. However, during the wet period, smaller amplitude of ETH is evident (throughout the radar domain) under strong upstream wind conditions. Convective activity during the late afternoon hours produces higher lighting flashes in the dry period compared to the wet. The lightning occurrences are related with penetrations of 30‐dBZ ETH above the freezing level and convective area fractions.
Key Points
Dry and wet period convection characteristics are observed to be different in terms of their small‐scale convective and large‐scale features
Positive feedback between low‐level heating and static stability maintains the shallow nature of convection over the Western Ghats
Large (small) amplitude of convection diurnal cycle is observed during the dry (wet) period with weak (strong) cross‐shore upstream winds
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