This study evaluates the performances of four different cloud microphysical parameterization (CMP... more This study evaluates the performances of four different cloud microphysical parameterization (CMP) schemes of the Weather Research and Forecasting (WRF) model at 3 km horizontal resolution (lead time up to 96 h) for the Heavy Rainfall Event (HRE) over Kerala in August 2018. The goal is to identify the major drivers for rain making mechanism and evaluate the ability of CMPs to accurately simulate the event with special emphasis on rainfall. It is found that the choice of CMP has a considerable impact on the rainfall forecast characteristics and associated convection. Results are validated against the India Meteorological Department (IMD) station data and Global Precipitation Measurement (GPM) observations and found that, among the four CMP schemes, viz., Milbrandt (MIL), Thompson Aerosol Aware (TAA), WRF double-moment 6-class scheme (WDM6) and WRF single-moment 6-class scheme (WSM6); WDM6 is the best performing scheme in terms of rainfall. It is noted that mixed phase processes are dominant in this scenario and the inability (ability) of MIL and TAA (WDM6 and WSM6) to predict the frozen hydrometeors, and thus simulating the cold rain processes realistically led to large (small) errors in the rainfall forecast. The moisture convergence was prominent in the foothills of the Western Ghats and highly influential in facilitating orography driven lifting of moisture. The moisture budget results suggest that horizontal moisture flux convergence (MFC) was the major driver of convection with WDM6 predicting the peaks of MFC most consistently with the observed Tropical Rainfall Measuring Mission (TRMM) rainfall product. Additionally, from Contiguous Rain Area analysis it is also found that the WDM6 has the least volumetric error. This is to highlight that hydrometeor distributions are strongly modulated by MFC, which further impacts the latent heat generation and rainfall over the region. Overall results infer the substantial influence of CMPs on the forecast of the Heavy Rainfall Event. The findings of this study will be highly useful for operational forecasting agencies and disaster management authorities for mitigation of damages caused by this kind of severe HREs in the future.
This study characterizes different rainfall types using surface-based instruments (i.e. micro rai... more This study characterizes different rainfall types using surface-based instruments (i.e. micro rain radar and laser precipitation monitor) installed at the Indian Institute of Technology Bhubaneswar Jatani, Odisha, India. A total of twelve rainfall cases including four from each season, i.e. pre-monsoon, monsoon and post-monsoon, are considered. The segregation of rainfall is carried out using radar reflectivity and rainfall intensity. In general, initial rainfall is dominantly convective and followed by a stratiform type. Two distinct maxima of radar reflectivity are noted at 3 and 5 km, suggesting the presence of high liquid water content and a melting band. The presence of liquid water content suggests occurrence of a warm rain process with shallow, intense convective cores. Results indicate a higher drop number density below 2 km with smaller size drops for convective rainfall and vice versa for the stratiform rainfall. Furthermore, Z-R relationships are computed for all the cases using a linear regression method, and the results suggest that the stratiform rainfall shows a higher slope parameter and lower intercept parameter as compared to convective rainfall. The distribution of drop number density shows a monomodal and bimodal pattern for convective and stratiform rainfall, respectively.
In this study, a comprehensive investigation is carried out to examine the sensitivity of troposp... more In this study, a comprehensive investigation is carried out to examine the sensitivity of tropospheric relative humidity (RH) on monsoon depressions (MDs) under a changing climate regime through surrogate climate change approach over the Indian region. Composite analysis of four MDs show a persistent warming (RH2+) and cooling (RH2−) throughout the troposphere in the sensitivity experiments. In-depth analysis of a MD over the Arabian Sea (AS) exhibits sustained warming for RH2+, which is accredited to 2.6% increase in stratiform clouds accounting for 13% increment in heating, whereas 5% increment in convective clouds hardly contribute to total heating. Frozen hydrometeors (graupel and snow) are speculated to be the major contributors to this heating. Stratiform clouds showed greater sensitivity to RH perturbations in the lower troposphere (1000-750 hPa), albeit very less sensitivity for convective clouds, both in the lower and mid-troposphere (700-500 hPa). Precipitation is enhanced in a moist situation (RH2+) owing to positive feedbacks induced by moisture influx and precipitation efficiency, while negative feedbacks suppressed precipitation in a dry troposphere (RH2−). In a nutshell, it is inferred that under moist (dry) situations, it is highly likely that intense (weak) MDs will occur in the near future over the Indian region. In an ever-changing climate with a consistently increasing trend in the global mean temperature, it is apparent that the water holding capacity of the atmosphere will increase at a rate governed by the Clausius-Clapeyron (CC, ~7% °C −1) relationship 1,2. Global land and ocean temperatures in 2016 set a record by overshooting the 1981-2010 average by 0.45° and 0.56 °C respectively, and as a consequence specific humidity (SH) peaked, reaching a record high well above the long-term average 3. Further, RH is projected to remain nearly constant with an increase in SH. The differential heating of land and ocean has been attributed for a small decrease in the near-surface RH over most land areas with exceptions over parts of Africa and the Indian subcontinent 4. Dai 5 documented similar trends from in situ observations (1975-2005) with exceptions over the central and eastern United States, India, and western China with RH increase ranging from 0.5-2% decade −1. It is inferred that this change is a result of increased RH coupled with moderate warming and enhanced low-level clouds during the analysis period. The earth's radiation budget is significantly affected by the presence of water vapor, owing to the absorption of radiation that contributes to changes in the water vapor feedback 6-8. A 10% increase in RH in the upper troposphere led to ~1.4 Wm −2 of radiative forcings 9. It is found that, if RH distribution is specified instead of absolute humidity, water vapor feedback to climate sensitivity doubled and the atmosphere took twice the time to reach radiative convective equilibrium 10. Further, studies demonstrated that in Deep Convective Systems (DCS), the convective cores bear the heavy precipitation with widespread rain in the stratiform region; the non-precipitating anvil canopy is dominant in the atmospheric radiation budget due to their sheer spatial coverage 11. DCS that last more than 6 hours have 50% more mid-tropospheric RH compared to short-lived systems, whereas, a dry mid-tropospheric profile can lead to suppressed deep convection in favor of a shallow convective regime 12,13. It was also found that an improved RH at the initial time in the model can bring better skills of MDs rainfall predictability (up to day 2) over the Indian region 14. Over the Indian subcontinent, the summer monsoon accounts for ~80% of annual precipitation which is crucial for an agrarian society like India 15. On an average, out of ~14 low-pressure systems that develop during the monsoon season, about 50% develop into depressions 16. Some concerns have been cited in recent literature regarding a decreasing trend in the number of monsoon depressions due to a decline in the mid-tropospheric RH and moisture flux convergence, weakening the low-level jet 17-20. Recent studies have also cautioned the use
In this study, the impact of different land initial conditions on the simulation of thunderstorms... more In this study, the impact of different land initial conditions on the simulation of thunderstorms and monsoon depressions is investigated using the Weather Research and Forecasting (WRF) model. A control run (CNTL) and a simulation with an improved land state (soil moisture and temperature) using the High Resolution Land Data Assimilation System (HRLDAS, experiment name: EHRLDAS) are compared for three different rainfall cases in order to examine the robustness of the assimilation system. The study comprises two thunderstorm cases (one in the pre-monsoon and one during the monsoon) and one monsoon depression case that occurred during the Interaction of Convective Organisation, Atmosphere, Surface and Sea (INCOMPASS) field campaign of the 2016 Indian monsoon. EHRLDAS is shown to yield improvements in the representation of location-specific rainfall, particularly over land. Further, it is found that surface fluxes as well as convective indices are better captured for the premonsoon thunderstorm case in EHRLDAS. By analysing components of the vorticity tendency equation, it is found that the vertical advection term is the major contributor towards the positive vorticity tendency in EHRLDAS compared to CNTL, hence improving localised convection and consequently facilitating rainfall. Significant improvements in the simulation of the pre-monsoon thunderstorm are noted, as seen using Automatic Weather Station (AWS) validation, whereas improvements in the monsoon depression are minimal. Further, it is found that vertical advection (moisture flux convergence) is the major driver modulating the convective circulation in localised thunderstorm (monsoon depression) cases and these dynamics are better represented by EHRLDAS compared to CNTL. These findings underline the importance of accurate and high resolution land-state conditions in model initial conditions for forecasting severe weather systems, particularly the simulation of localised thunderstorms over India.
Interaction of multiple oscillations of different time scales may result in severe weather events... more Interaction of multiple oscillations of different time scales may result in severe weather events. The presence of orography can modulate the intensity of these events even further. Kerala witnessed one such heavy rainfall event in August, 2018, claiming 483 lives and damages worth INR 200 billion. This study focuses on the peak rainfall duration (13–17 August) when the departure from normal was 42%. Segregating moisture transport into its mean and perturbation terms show that an anomalous moisture channel over the Arabian Sea supplied continuous moisture to the Western Ghats (WG), whereas anomalous wind due to a monsoon depression advected moisture towards the southern peninsula. It is evident in the form of Moisture Flux Convergence (MFC) towers traversing along the Eastern Ghats before merging with the semi-permanent MFC feature over the WG. The presence of positive quasi bi-weekly oscillations and of Intra Seasonal Oscillations (ISO) aggravated the event as they complemented the...
The present study is aimed to investigate sub-surface ocean processes and their contribution to t... more The present study is aimed to investigate sub-surface ocean processes and their contribution to the intensification of a tropical cyclone (TC) from a coupled-modeling perspective. The Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model was employed to simulate TC Phailin, which originated over the Bay of Bengal and made landfall on the eastern coast of India in October 2013. Three sub-surface ocean condition datasets—viz., (a) the European Centre for Medium-Range Weather Forecast (ECMWF) Ocean Reanalysis, (b) the Climate Forecast System Version 2 (CFSV2) Operational Analysis, and (c) the Hybrid Coordinate Ocean Model (HYCOM) Reanalysis datasets—were used for the initial and boundary conditions for the oceanic component of the coupled model in three different simulations of TC Phailin. All the simulations showed a delay in intensification compared to the observation, and ECMWF simulated the most intensified TC. CFSV2 simulated a deeper mixed layer (ML) and higher mixing, ...
In this study, a comprehensive investigation is carried out to examine the sensitivity of troposp... more In this study, a comprehensive investigation is carried out to examine the sensitivity of tropospheric relative humidity (RH) on monsoon depressions (MDs) under a changing climate regime through surrogate climate change approach over the Indian region. Composite analysis of four MDs show a persistent warming (RH2+) and cooling (RH2−) throughout the troposphere in the sensitivity experiments. In-depth analysis of a MD over the Arabian Sea (AS) exhibits sustained warming for RH2+, which is accredited to 2.6% increase in stratiform clouds accounting for 13% increment in heating, whereas 5% increment in convective clouds hardly contribute to total heating. Frozen hydrometeors (graupel and snow) are speculated to be the major contributors to this heating. Stratiform clouds showed greater sensitivity to RH perturbations in the lower troposphere (1000–750 hPa), albeit very less sensitivity for convective clouds, both in the lower and mid-troposphere (700–500 hPa). Precipitation is enhanced...
A correction to this article has been published and is linked from the HTML and PDF versions of t... more A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.
In this study, the impact of different land initial conditions on the simulation of thunderstorms... more In this study, the impact of different land initial conditions on the simulation of thunderstorms and monsoon depressions is investigated using the Weather Research and Forecasting (WRF) model. A control run (CNTL) and a simulation with an improved land state (soil moisture and temperature) using the High Resolution Land Data Assimilation System (HRLDAS, experiment name: EHRLDAS) are compared for three different rainfall cases in order to examine the robustness of the assimilation system. The study comprises two thunderstorm cases (one in the pre-monsoon and one during the monsoon) and one monsoon depression case that occurred during the Interaction of Convective Organisation, Atmosphere, Surface and Sea (INCOMPASS) field campaign of the 2016 Indian monsoon. EHRLDAS is shown to yield improvements in the representation of location-specific rainfall, particularly over land. Further, it is found that surface fluxes as well as convective indices are better captured for the pre-monsoon t...
The state of Odisha is situated on the eastern coast of India and is highly vulnerable to massive... more The state of Odisha is situated on the eastern coast of India and is highly vulnerable to massive convective activity in the pre-monsoon season (PM), i.e., from March to May; however, there is a scarcity of studies in this context using long-term datasets. Therefore, an in-depth investigation of the variability in convective events and associated rainfall during PM over the state of Odisha has been carried out for the period 2009–2018 using the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis (ERA5) datasets. The convective events (severe and moderate) identified using two sets of threshold values of three different convective indices, i.e., convective available potential energy (CAPE), the K Index, and the Total Totals Index, show an increasing trend in recent years, with South Coastal Odisha (SCO) and North Coastal Odisha (NCO) showing the highest increase. Subsequently, the spatial distribution of rainfall suggests that the maximum convective precipitation (CP) is experienced over NCO and adjacent eastern districts of North Interior Odisha (NIO). The spatial distribution of the 2 m temperature suggests that there exists a strong temperature gradient between the western and eastern portions of the state. However, the gradient weakens for the years associated with the anomalous distribution of CP. The distinct tropospheric temperature difference between the lower levels (LL) and upper levels (UL) clearly suggests that the warming (cooling) of LL is associated with high (low) CP over the region. This is further established by the coherent signature of specific humidity. The frozen hydrometeors (cloud ice and snow) are the major facilitators for the occurrence of CP over the study region. The moisture transport (MT) is associated primarily with the anomalous distribution of spatial rainfall. The years with suppressed convective activity have a distinct signature of a negative MT anomaly along with anomalous north-easterly winds (as against the typical south-westerly flow). It is also demonstrated that the anomalous MT scenario is highly modulated by the land–sea temperature contrast over the region.
This study characterizes different rainfall types using surface-based instruments (i.e. micro rai... more This study characterizes different rainfall types using surface-based instruments (i.e. micro rain radar and laser precipitation monitor) installed at the Indian Institute of Technology Bhubaneswar Jatani, Odisha, India. A total of twelve rainfall cases including four from each season, i.e. pre-monsoon, monsoon and post-monsoon, are considered. The segregation of rainfall is carried out using radar reflectivity and rainfall intensity. In general, initial rainfall is dominantly convective and followed by a stratiform type. Two distinct maxima of radar reflectivity are noted at 3 and 5 km, suggesting the presence of high liquid water content and a melting band. The presence of liquid water content suggests occurrence of a warm rain process with shallow, intense convective cores. Results indicate a higher drop number density below 2 km with smaller size drops for convective rainfall and vice versa for the stratiform rainfall. Furthermore, Z – R relationships are computed for all the cases using a linear regression method, and the results suggest that the stratiform rainfall shows a higher slope parameter and lower intercept parameter as compared to convective rainfall. The distribution of drop number density shows a mono-modal and bimodal pattern for convective and stratiform rainfall, respectively.
Journal of Advances in Modeling Earth Systems, Mar 1, 2017
A series of numerical experiments are carried out to investigate the sensitivity of a landfalling... more A series of numerical experiments are carried out to investigate the sensitivity of a landfalling monsoon depression to land surface conditions using the Weather Research and Forecasting (WRF) model. Results suggest that precipitation is largely modulated by moisture influx and precipitation efficiency. Three cloud microphysical schemes (WSM6, WDM6, and Morrison) are examined, and Morrison is chosen for assessing the land surface-precipitation feedback analysis, owing to better precipitation forecast skills. It is found that increased soil moisture facilitates Moisture Flux Convergence (MFC) with reduced moisture influx, whereas a reduced soil moisture condition facilitates moisture influx but not MFC. A higher Moist Static Energy (MSE) is noted due to increased evapotranspiration in an elevated moisture scenario which enhances moist convection. As opposed to moist surface, sensible heat dominates in a reduced moisture scenario, ensued by an overall reduction in MSE throughout the Planetary Boundary Layer (PBL). Stability analysis shows that Convective Available Potential Energy (CAPE) is comparable in magnitude for both increased and decreased moisture scenarios, whereas Convective Inhibition (CIN) shows increased values for the reduced moisture scenario as a consequence of drier atmosphere leading to suppression of convection. Simulations carried out with various fixed soil moisture levels indicate that the overall precipitation features of the storm are characterized by initial soil moisture condition, but precipitation intensity at any instant is modulated by soil moisture availability. Overall results based on this case study suggest that antecedent soil moisture plays a crucial role in modulating precipitation distribution and intensity of a monsoon depression.
Abstract Monsoon depressions (MDs) constitute a large fraction of the total rainfall during the I... more Abstract Monsoon depressions (MDs) constitute a large fraction of the total rainfall during the Indian summer monsoon season. In this study, the impact of high-resolution land state is addressed by assessing the evolution of inland moving depressions formed over the Bay of Bengal using a mesoscale modeling system. Improved land state is generated using High Resolution Land Data Assimilation System employing Noah-MP land-surface model. Verification of soil moisture using Soil Moisture and Ocean Salinity (SMOS) and soil temperature using tower observations demonstrate promising results. Incorporating high-resolution land state yielded least root mean squared errors with higher correlation coefficient in the surface and mid tropospheric parameters. Rainfall forecasts reveal that simulations are spatially and quantitatively in accordance with observations and provide better skill scores. The improved land surface characteristics have brought about the realistic evolution of surface, mid-tropospheric parameters, vorticity and moist static energy that facilitates the accurate MDs dynamics in the model. Composite moisture budget analysis reveals that the surface evaporation is negligible compared to moisture flux convergence of water vapor, which supplies moisture into the MDs over land. The temporal relationship between rainfall and moisture convergence show high correlation, suggesting a realistic representation of land state help restructure the moisture inflow into the system through rainfall-moisture convergence feedback.
Interaction of multiple oscillations of different time scales may result in severe weather events... more Interaction of multiple oscillations of different time scales may result in severe weather events. 8 The presence of orography can modulate the intensity of these events even further. Kerala 9 witnessed one such heavy rainfall event in August, 2018, claiming 483 lives and damages worth 10 INR 200 billion. This study focuses on the peak rainfall duration (13-17 August) when the 11 departure from normal was 42%. Segregating moisture transport into its mean and perturbation 12 terms show that an anomalous moisture channel over the Arabian Sea supplied continuous 13 moisture to the Western Ghats (WG), whereas anomalous wind due to a monsoon depression 14 advected moisture towards the southern peninsula. It is evident in the form of Moisture Flux 15 Convergence (MFC) towers traversing along the Eastern Ghats before merging with the semi-16 permanent MFC feature over the WG. The presence of positive quasi biweekly oscillations and 17 of Intra Seasonal Oscillations (ISO) aggravated the event as they complemented the anomalous 18 moisture transport, with ISO constantly providing winds of the order of 2-3 ms-1. In addition, 19 shedding of MFC towers by the depression is accredited to the synoptic scale oscillation.
This study evaluates the performances of four different cloud microphysical parameterization (CMP... more This study evaluates the performances of four different cloud microphysical parameterization (CMP) schemes of the Weather Research and Forecasting (WRF) model at 3 km horizontal resolution (lead time up to 96 h) for the Heavy Rainfall Event (HRE) over Kerala in August 2018. The goal is to identify the major drivers for rain making mechanism and evaluate the ability of CMPs to accurately simulate the event with special emphasis on rainfall. It is found that the choice of CMP has a considerable impact on the rainfall forecast characteristics and associated convection. Results are validated against the India Meteorological Department (IMD) station data and Global Precipitation Measurement (GPM) observations and found that, among the four CMP schemes, viz., Milbrandt (MIL), Thompson Aerosol Aware (TAA), WRF double-moment 6-class scheme (WDM6) and WRF single-moment 6-class scheme (WSM6); WDM6 is the best performing scheme in terms of rainfall. It is noted that mixed phase processes are dominant in this scenario and the inability (ability) of MIL and TAA (WDM6 and WSM6) to predict the frozen hydrometeors, and thus simulating the cold rain processes realistically led to large (small) errors in the rainfall forecast. The moisture convergence was prominent in the foothills of the Western Ghats and highly influential in facilitating orography driven lifting of moisture. The moisture budget results suggest that horizontal moisture flux convergence (MFC) was the major driver of convection with WDM6 predicting the peaks of MFC most consistently with the observed Tropical Rainfall Measuring Mission (TRMM) rainfall product. Additionally, from Contiguous Rain Area analysis it is also found that the WDM6 has the least volumetric error. This is to highlight that hydrometeor distributions are strongly modulated by MFC, which further impacts the latent heat generation and rainfall over the region. Overall results infer the substantial influence of CMPs on the forecast of the Heavy Rainfall Event. The findings of this study will be highly useful for operational forecasting agencies and disaster management authorities for mitigation of damages caused by this kind of severe HREs in the future.
This study characterizes different rainfall types using surface-based instruments (i.e. micro rai... more This study characterizes different rainfall types using surface-based instruments (i.e. micro rain radar and laser precipitation monitor) installed at the Indian Institute of Technology Bhubaneswar Jatani, Odisha, India. A total of twelve rainfall cases including four from each season, i.e. pre-monsoon, monsoon and post-monsoon, are considered. The segregation of rainfall is carried out using radar reflectivity and rainfall intensity. In general, initial rainfall is dominantly convective and followed by a stratiform type. Two distinct maxima of radar reflectivity are noted at 3 and 5 km, suggesting the presence of high liquid water content and a melting band. The presence of liquid water content suggests occurrence of a warm rain process with shallow, intense convective cores. Results indicate a higher drop number density below 2 km with smaller size drops for convective rainfall and vice versa for the stratiform rainfall. Furthermore, Z-R relationships are computed for all the cases using a linear regression method, and the results suggest that the stratiform rainfall shows a higher slope parameter and lower intercept parameter as compared to convective rainfall. The distribution of drop number density shows a monomodal and bimodal pattern for convective and stratiform rainfall, respectively.
In this study, a comprehensive investigation is carried out to examine the sensitivity of troposp... more In this study, a comprehensive investigation is carried out to examine the sensitivity of tropospheric relative humidity (RH) on monsoon depressions (MDs) under a changing climate regime through surrogate climate change approach over the Indian region. Composite analysis of four MDs show a persistent warming (RH2+) and cooling (RH2−) throughout the troposphere in the sensitivity experiments. In-depth analysis of a MD over the Arabian Sea (AS) exhibits sustained warming for RH2+, which is accredited to 2.6% increase in stratiform clouds accounting for 13% increment in heating, whereas 5% increment in convective clouds hardly contribute to total heating. Frozen hydrometeors (graupel and snow) are speculated to be the major contributors to this heating. Stratiform clouds showed greater sensitivity to RH perturbations in the lower troposphere (1000-750 hPa), albeit very less sensitivity for convective clouds, both in the lower and mid-troposphere (700-500 hPa). Precipitation is enhanced in a moist situation (RH2+) owing to positive feedbacks induced by moisture influx and precipitation efficiency, while negative feedbacks suppressed precipitation in a dry troposphere (RH2−). In a nutshell, it is inferred that under moist (dry) situations, it is highly likely that intense (weak) MDs will occur in the near future over the Indian region. In an ever-changing climate with a consistently increasing trend in the global mean temperature, it is apparent that the water holding capacity of the atmosphere will increase at a rate governed by the Clausius-Clapeyron (CC, ~7% °C −1) relationship 1,2. Global land and ocean temperatures in 2016 set a record by overshooting the 1981-2010 average by 0.45° and 0.56 °C respectively, and as a consequence specific humidity (SH) peaked, reaching a record high well above the long-term average 3. Further, RH is projected to remain nearly constant with an increase in SH. The differential heating of land and ocean has been attributed for a small decrease in the near-surface RH over most land areas with exceptions over parts of Africa and the Indian subcontinent 4. Dai 5 documented similar trends from in situ observations (1975-2005) with exceptions over the central and eastern United States, India, and western China with RH increase ranging from 0.5-2% decade −1. It is inferred that this change is a result of increased RH coupled with moderate warming and enhanced low-level clouds during the analysis period. The earth's radiation budget is significantly affected by the presence of water vapor, owing to the absorption of radiation that contributes to changes in the water vapor feedback 6-8. A 10% increase in RH in the upper troposphere led to ~1.4 Wm −2 of radiative forcings 9. It is found that, if RH distribution is specified instead of absolute humidity, water vapor feedback to climate sensitivity doubled and the atmosphere took twice the time to reach radiative convective equilibrium 10. Further, studies demonstrated that in Deep Convective Systems (DCS), the convective cores bear the heavy precipitation with widespread rain in the stratiform region; the non-precipitating anvil canopy is dominant in the atmospheric radiation budget due to their sheer spatial coverage 11. DCS that last more than 6 hours have 50% more mid-tropospheric RH compared to short-lived systems, whereas, a dry mid-tropospheric profile can lead to suppressed deep convection in favor of a shallow convective regime 12,13. It was also found that an improved RH at the initial time in the model can bring better skills of MDs rainfall predictability (up to day 2) over the Indian region 14. Over the Indian subcontinent, the summer monsoon accounts for ~80% of annual precipitation which is crucial for an agrarian society like India 15. On an average, out of ~14 low-pressure systems that develop during the monsoon season, about 50% develop into depressions 16. Some concerns have been cited in recent literature regarding a decreasing trend in the number of monsoon depressions due to a decline in the mid-tropospheric RH and moisture flux convergence, weakening the low-level jet 17-20. Recent studies have also cautioned the use
In this study, the impact of different land initial conditions on the simulation of thunderstorms... more In this study, the impact of different land initial conditions on the simulation of thunderstorms and monsoon depressions is investigated using the Weather Research and Forecasting (WRF) model. A control run (CNTL) and a simulation with an improved land state (soil moisture and temperature) using the High Resolution Land Data Assimilation System (HRLDAS, experiment name: EHRLDAS) are compared for three different rainfall cases in order to examine the robustness of the assimilation system. The study comprises two thunderstorm cases (one in the pre-monsoon and one during the monsoon) and one monsoon depression case that occurred during the Interaction of Convective Organisation, Atmosphere, Surface and Sea (INCOMPASS) field campaign of the 2016 Indian monsoon. EHRLDAS is shown to yield improvements in the representation of location-specific rainfall, particularly over land. Further, it is found that surface fluxes as well as convective indices are better captured for the premonsoon thunderstorm case in EHRLDAS. By analysing components of the vorticity tendency equation, it is found that the vertical advection term is the major contributor towards the positive vorticity tendency in EHRLDAS compared to CNTL, hence improving localised convection and consequently facilitating rainfall. Significant improvements in the simulation of the pre-monsoon thunderstorm are noted, as seen using Automatic Weather Station (AWS) validation, whereas improvements in the monsoon depression are minimal. Further, it is found that vertical advection (moisture flux convergence) is the major driver modulating the convective circulation in localised thunderstorm (monsoon depression) cases and these dynamics are better represented by EHRLDAS compared to CNTL. These findings underline the importance of accurate and high resolution land-state conditions in model initial conditions for forecasting severe weather systems, particularly the simulation of localised thunderstorms over India.
Interaction of multiple oscillations of different time scales may result in severe weather events... more Interaction of multiple oscillations of different time scales may result in severe weather events. The presence of orography can modulate the intensity of these events even further. Kerala witnessed one such heavy rainfall event in August, 2018, claiming 483 lives and damages worth INR 200 billion. This study focuses on the peak rainfall duration (13–17 August) when the departure from normal was 42%. Segregating moisture transport into its mean and perturbation terms show that an anomalous moisture channel over the Arabian Sea supplied continuous moisture to the Western Ghats (WG), whereas anomalous wind due to a monsoon depression advected moisture towards the southern peninsula. It is evident in the form of Moisture Flux Convergence (MFC) towers traversing along the Eastern Ghats before merging with the semi-permanent MFC feature over the WG. The presence of positive quasi bi-weekly oscillations and of Intra Seasonal Oscillations (ISO) aggravated the event as they complemented the...
The present study is aimed to investigate sub-surface ocean processes and their contribution to t... more The present study is aimed to investigate sub-surface ocean processes and their contribution to the intensification of a tropical cyclone (TC) from a coupled-modeling perspective. The Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model was employed to simulate TC Phailin, which originated over the Bay of Bengal and made landfall on the eastern coast of India in October 2013. Three sub-surface ocean condition datasets—viz., (a) the European Centre for Medium-Range Weather Forecast (ECMWF) Ocean Reanalysis, (b) the Climate Forecast System Version 2 (CFSV2) Operational Analysis, and (c) the Hybrid Coordinate Ocean Model (HYCOM) Reanalysis datasets—were used for the initial and boundary conditions for the oceanic component of the coupled model in three different simulations of TC Phailin. All the simulations showed a delay in intensification compared to the observation, and ECMWF simulated the most intensified TC. CFSV2 simulated a deeper mixed layer (ML) and higher mixing, ...
In this study, a comprehensive investigation is carried out to examine the sensitivity of troposp... more In this study, a comprehensive investigation is carried out to examine the sensitivity of tropospheric relative humidity (RH) on monsoon depressions (MDs) under a changing climate regime through surrogate climate change approach over the Indian region. Composite analysis of four MDs show a persistent warming (RH2+) and cooling (RH2−) throughout the troposphere in the sensitivity experiments. In-depth analysis of a MD over the Arabian Sea (AS) exhibits sustained warming for RH2+, which is accredited to 2.6% increase in stratiform clouds accounting for 13% increment in heating, whereas 5% increment in convective clouds hardly contribute to total heating. Frozen hydrometeors (graupel and snow) are speculated to be the major contributors to this heating. Stratiform clouds showed greater sensitivity to RH perturbations in the lower troposphere (1000–750 hPa), albeit very less sensitivity for convective clouds, both in the lower and mid-troposphere (700–500 hPa). Precipitation is enhanced...
A correction to this article has been published and is linked from the HTML and PDF versions of t... more A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.
In this study, the impact of different land initial conditions on the simulation of thunderstorms... more In this study, the impact of different land initial conditions on the simulation of thunderstorms and monsoon depressions is investigated using the Weather Research and Forecasting (WRF) model. A control run (CNTL) and a simulation with an improved land state (soil moisture and temperature) using the High Resolution Land Data Assimilation System (HRLDAS, experiment name: EHRLDAS) are compared for three different rainfall cases in order to examine the robustness of the assimilation system. The study comprises two thunderstorm cases (one in the pre-monsoon and one during the monsoon) and one monsoon depression case that occurred during the Interaction of Convective Organisation, Atmosphere, Surface and Sea (INCOMPASS) field campaign of the 2016 Indian monsoon. EHRLDAS is shown to yield improvements in the representation of location-specific rainfall, particularly over land. Further, it is found that surface fluxes as well as convective indices are better captured for the pre-monsoon t...
The state of Odisha is situated on the eastern coast of India and is highly vulnerable to massive... more The state of Odisha is situated on the eastern coast of India and is highly vulnerable to massive convective activity in the pre-monsoon season (PM), i.e., from March to May; however, there is a scarcity of studies in this context using long-term datasets. Therefore, an in-depth investigation of the variability in convective events and associated rainfall during PM over the state of Odisha has been carried out for the period 2009–2018 using the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis (ERA5) datasets. The convective events (severe and moderate) identified using two sets of threshold values of three different convective indices, i.e., convective available potential energy (CAPE), the K Index, and the Total Totals Index, show an increasing trend in recent years, with South Coastal Odisha (SCO) and North Coastal Odisha (NCO) showing the highest increase. Subsequently, the spatial distribution of rainfall suggests that the maximum convective precipitation (CP) is experienced over NCO and adjacent eastern districts of North Interior Odisha (NIO). The spatial distribution of the 2 m temperature suggests that there exists a strong temperature gradient between the western and eastern portions of the state. However, the gradient weakens for the years associated with the anomalous distribution of CP. The distinct tropospheric temperature difference between the lower levels (LL) and upper levels (UL) clearly suggests that the warming (cooling) of LL is associated with high (low) CP over the region. This is further established by the coherent signature of specific humidity. The frozen hydrometeors (cloud ice and snow) are the major facilitators for the occurrence of CP over the study region. The moisture transport (MT) is associated primarily with the anomalous distribution of spatial rainfall. The years with suppressed convective activity have a distinct signature of a negative MT anomaly along with anomalous north-easterly winds (as against the typical south-westerly flow). It is also demonstrated that the anomalous MT scenario is highly modulated by the land–sea temperature contrast over the region.
This study characterizes different rainfall types using surface-based instruments (i.e. micro rai... more This study characterizes different rainfall types using surface-based instruments (i.e. micro rain radar and laser precipitation monitor) installed at the Indian Institute of Technology Bhubaneswar Jatani, Odisha, India. A total of twelve rainfall cases including four from each season, i.e. pre-monsoon, monsoon and post-monsoon, are considered. The segregation of rainfall is carried out using radar reflectivity and rainfall intensity. In general, initial rainfall is dominantly convective and followed by a stratiform type. Two distinct maxima of radar reflectivity are noted at 3 and 5 km, suggesting the presence of high liquid water content and a melting band. The presence of liquid water content suggests occurrence of a warm rain process with shallow, intense convective cores. Results indicate a higher drop number density below 2 km with smaller size drops for convective rainfall and vice versa for the stratiform rainfall. Furthermore, Z – R relationships are computed for all the cases using a linear regression method, and the results suggest that the stratiform rainfall shows a higher slope parameter and lower intercept parameter as compared to convective rainfall. The distribution of drop number density shows a mono-modal and bimodal pattern for convective and stratiform rainfall, respectively.
Journal of Advances in Modeling Earth Systems, Mar 1, 2017
A series of numerical experiments are carried out to investigate the sensitivity of a landfalling... more A series of numerical experiments are carried out to investigate the sensitivity of a landfalling monsoon depression to land surface conditions using the Weather Research and Forecasting (WRF) model. Results suggest that precipitation is largely modulated by moisture influx and precipitation efficiency. Three cloud microphysical schemes (WSM6, WDM6, and Morrison) are examined, and Morrison is chosen for assessing the land surface-precipitation feedback analysis, owing to better precipitation forecast skills. It is found that increased soil moisture facilitates Moisture Flux Convergence (MFC) with reduced moisture influx, whereas a reduced soil moisture condition facilitates moisture influx but not MFC. A higher Moist Static Energy (MSE) is noted due to increased evapotranspiration in an elevated moisture scenario which enhances moist convection. As opposed to moist surface, sensible heat dominates in a reduced moisture scenario, ensued by an overall reduction in MSE throughout the Planetary Boundary Layer (PBL). Stability analysis shows that Convective Available Potential Energy (CAPE) is comparable in magnitude for both increased and decreased moisture scenarios, whereas Convective Inhibition (CIN) shows increased values for the reduced moisture scenario as a consequence of drier atmosphere leading to suppression of convection. Simulations carried out with various fixed soil moisture levels indicate that the overall precipitation features of the storm are characterized by initial soil moisture condition, but precipitation intensity at any instant is modulated by soil moisture availability. Overall results based on this case study suggest that antecedent soil moisture plays a crucial role in modulating precipitation distribution and intensity of a monsoon depression.
Abstract Monsoon depressions (MDs) constitute a large fraction of the total rainfall during the I... more Abstract Monsoon depressions (MDs) constitute a large fraction of the total rainfall during the Indian summer monsoon season. In this study, the impact of high-resolution land state is addressed by assessing the evolution of inland moving depressions formed over the Bay of Bengal using a mesoscale modeling system. Improved land state is generated using High Resolution Land Data Assimilation System employing Noah-MP land-surface model. Verification of soil moisture using Soil Moisture and Ocean Salinity (SMOS) and soil temperature using tower observations demonstrate promising results. Incorporating high-resolution land state yielded least root mean squared errors with higher correlation coefficient in the surface and mid tropospheric parameters. Rainfall forecasts reveal that simulations are spatially and quantitatively in accordance with observations and provide better skill scores. The improved land surface characteristics have brought about the realistic evolution of surface, mid-tropospheric parameters, vorticity and moist static energy that facilitates the accurate MDs dynamics in the model. Composite moisture budget analysis reveals that the surface evaporation is negligible compared to moisture flux convergence of water vapor, which supplies moisture into the MDs over land. The temporal relationship between rainfall and moisture convergence show high correlation, suggesting a realistic representation of land state help restructure the moisture inflow into the system through rainfall-moisture convergence feedback.
Interaction of multiple oscillations of different time scales may result in severe weather events... more Interaction of multiple oscillations of different time scales may result in severe weather events. 8 The presence of orography can modulate the intensity of these events even further. Kerala 9 witnessed one such heavy rainfall event in August, 2018, claiming 483 lives and damages worth 10 INR 200 billion. This study focuses on the peak rainfall duration (13-17 August) when the 11 departure from normal was 42%. Segregating moisture transport into its mean and perturbation 12 terms show that an anomalous moisture channel over the Arabian Sea supplied continuous 13 moisture to the Western Ghats (WG), whereas anomalous wind due to a monsoon depression 14 advected moisture towards the southern peninsula. It is evident in the form of Moisture Flux 15 Convergence (MFC) towers traversing along the Eastern Ghats before merging with the semi-16 permanent MFC feature over the WG. The presence of positive quasi biweekly oscillations and 17 of Intra Seasonal Oscillations (ISO) aggravated the event as they complemented the anomalous 18 moisture transport, with ISO constantly providing winds of the order of 2-3 ms-1. In addition, 19 shedding of MFC towers by the depression is accredited to the synoptic scale oscillation.
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