Showing papers on "Convective available potential energy published in 2020"

Changes in the convective population and thermodynamic environments in convection-permitting regional climate simulations over the United States

Abstract: Novel high-resolution convection-permitting regional climate simulations over the US employing the pseudo-global warming approach are used to investigate changes in the convective population and thermodynamic environments in a future climate. Two continuous 13-year simulations were conducted using (1) ERA-Interim reanalysis and (2) ERA-Interim reanalysis plus a climate perturbation for the RCP8.5 scenario. The simulations adequately reproduce the observed precipitation diurnal cycle, indicating that they capture organized and propagating convection that most climate models cannot adequately represent. This study shows that weak to moderate convection will decrease and strong convection will increase in frequency in a future climate. Analysis of the thermodynamic environments supporting convection shows that both convective available potential energy (CAPE) and convective inhibition (CIN) increase downstream of the Rockies in a future climate. Previous studies suggest that CAPE will increase in a warming climate, however a corresponding increase in CIN acts as a balancing force to shift the convective population by suppressing weak to moderate convection and provides an environment where CAPE can build to extreme levels that may result in more frequent severe convection. An idealized investigation of fundamental changes in the thermodynamic environment was conducted by shifting a standard atmospheric profile by ± 5 °C. When temperature is increased, both CAPE and CIN increase in magnitude, while the opposite is true for decreased temperatures. Thus, even in the absence of synoptic and mesoscale variations, a warmer climate will provide more CAPE and CIN that will shift the convective population, likely impacting water and energy budgets on Earth.

Changes in Convective Available Potential Energy and Convective Inhibition under Global Warming

Abstract: Atmospheric convective available potential energy (CAPE) is expected to increase under greenhouse gas–induced global warming, but a recent regional study also suggests enhanced convective i...

What Drives the Intensification of Mesoscale Convective Systems over the West African Sahel under Climate Change

Abstract: Extreme rainfall is expected to increase under climate change, carrying potential socioeconomic risks. However, the magnitude of increase is uncertain. Over recent decades, extreme storms over the West African Sahel have increased in frequency, with increased vertical wind shear shown to be a cause. Drier midlevels, stronger cold pools, and increased storm organization have also been observed. Global models do not capture the potential effects of lower- to midtropospheric wind shear or cold pools on storm organization since they parameterize convection. Here we use the first convection-permitting simulations of African climate change to understand how changes in thermodynamics and storm dynamics affect future extreme Sahelian rainfall. The model, which simulates warming associated with representative concentration pathway 8.5 (RCP8.5) until the end of the twenty-first century, projects a 28% increase of the extreme rain rate of MCSs. The Sahel moisture change on average follows Clausius–Clapeyron scaling, but has regional heterogeneity. Rain rates scale with the product of time-of-storm total column water (TCW) and in-storm vertical velocity. Additionally, prestorm wind shear and convective available potential energy both modulate in-storm vertical velocity. Although wind shear affects cloud-top temperatures within our model, it has no direct correlation with precipitation rates. In our model, projected future increase in TCW is the primary explanation for increased rain rates. Finally, although colder cold pools are modeled in the future climate, we see no significant change in near-surface winds, highlighting avenues for future research on convection-permitting modeling of storm dynamics.

Climatology of Severe Local Storm Environments and Synoptic-Scale Features over North America in ERA5 Reanalysis and CAM6 Simulation

Abstract: Severe local storm (SLS) activity is known to occur within specific thermodynamic and kinematic environments. These environments are commonly associated with key synoptic-scale features--including southerly Great Plains low-level jets, drylines, elevated mixed layers, and extratropical cyclones--that link the large-scale climate to SLS environments. This work analyzes spatiotemporal distributions of both the environmental parameters and synoptic-scale features in ERA5 reanalysis and in Community Atmosphere Model version 6 (CAM6) during 1980--2014 over North America. Compared to radiosondes, ERA5 successfully reproduces SLS environments, with strong spatiotemporal correlations and low biases, especially over the Great Plains. Both ERA5 and CAM6 reproduce the climatology of SLS environments over the central United States as well as its strong seasonal and diurnal cycles. ERA5 and CAM6 also reproduce the climatological occurrence of the synoptic-scale features, with the distribution pattern similar to that of SLS environments. Compared to ERA5, CAM6 exhibits a high bias in Convective Available Potential Energy over the eastern United States primarily due to a high bias in surface moisture, and to a lesser extent, storm-relative helicity due to enhanced low-level winds. Composite analysis indicates consistent synoptic anomaly patterns favorable for significant SLS environments over much of the eastern half of the United States in both ERA5 and CAM6, though the pattern differs for the southeastern United States. Overall, results indicate that both ERA5 and CAM6 are capable of reproducing SLS environments as well as the synoptic-scale features and transient events that generate them.

Climatology of severe local storm environments and synoptic-scale features over North America in ERA5 reanalysis and CAM6 simulation

Abstract: Severe local storm (SLS) activity is known to occur within specific thermodynamic and kinematic environments. These environments are commonly associated with key synoptic-scale features--including southerly Great Plains low-level jets, drylines, elevated mixed layers, and extratropical cyclones--that link the large-scale climate to SLS environments. This work analyzes spatiotemporal distributions of both the environmental parameters and synoptic-scale features in ERA5 reanalysis and in Community Atmosphere Model version 6 (CAM6) during 1980--2014 over North America. Compared to radiosondes, ERA5 successfully reproduces SLS environments, with strong spatiotemporal correlations and low biases, especially over the Great Plains. Both ERA5 and CAM6 reproduce the climatology of SLS environments over the central United States as well as its strong seasonal and diurnal cycles. ERA5 and CAM6 also reproduce the climatological occurrence of the synoptic-scale features, with the distribution pattern similar to that of SLS environments. Compared to ERA5, CAM6 exhibits a high bias in Convective Available Potential Energy over the eastern United States primarily due to a high bias in surface moisture, and to a lesser extent, storm-relative helicity due to enhanced low-level winds. Composite analysis indicates consistent synoptic anomaly patterns favorable for significant SLS environments over much of the eastern half of the United States in both ERA5 and CAM6, though the pattern differs for the southeastern United States. Overall, results indicate that both ERA5 and CAM6 are capable of reproducing SLS environments as well as the synoptic-scale features and transient events that generate them.

A Lagrangian analysis of upper-tropospheric anticyclones associated with heat waves in Europe

Abstract: . This study presents a Lagrangian analysis of upper-tropospheric anticyclones that are connected to surface heat waves in different European regions for the period 1979 to 2016. In order to elucidate the formation of these anticyclones and the role of diabatic processes, we trace air parcels backwards from the upper-tropospheric anticyclones and quantify the diabatic heating in these air parcels. Around 25 %–45 % of the air parcels are diabatically heated during the last 3 d prior to their arrival in the upper-tropospheric anticyclones, and this amount increases to 35 %–50 % for the last 7 d. The influence of diabatic heating is larger for heat-wave-related anticyclones in northern Europe and western Russia and smaller in southern Europe. Interestingly, the diabatic heating occurs in two geographically separated air streams; 3 d prior to arrival, one heating branch (remote branch) is located above the western North Atlantic, and the other heating branch (nearby branch) is located over northwestern Africa and Europe to the southwest of the target upper-tropospheric anticyclone. The diabatic heating in the remote branch is related to warm conveyor belts in North Atlantic cyclones upstream of the evolving upper-level ridge. In contrast, the nearby branch is diabatically heated by convection, as indicated by elevated mixed-layer convective available potential energy along the western side of the matured upper-level ridge. Most European regions are influenced by both branches, whereas western Russia is predominantly affected by the nearby branch. The remote branch predominantly affects the formation of the upper-tropospheric anticyclone, and therefore of the heat wave, whereas the nearby branch is more active during its maintenance. For long-lasting heat waves, the remote branch regenerates. The results from this study show that the dynamical processes leading to heat waves may be sensitive to small-scale microphysical and convective processes, whose accurate representation in models is thus supposed to be crucial for heat wave predictions on weather and climate timescales.

An attempt to explain recent changes in European snowfall extremes

Abstract: . The goal of this work is to investigate and explain recent changes in total and maximum yearly snowfall from daily data in light of current global warming or the interdecadal variability of atmospheric circulation. We focus on the period 1979–2018 and compare two different datasets: the ERA5 reanalysis data and the E-OBSv20.0 data, where snowfall is identified from rainfall by applying a threshold to temperature. We compute changes as differences from quantities computed for the periods 1999–2018 and 1979–1998. On the one hand, we show that the decline in average snowfall observed in almost all European regions is coherent with previous findings and can be linked to global warming. On the other hand, we observe contrasting changes in maxima and sometimes disagreement in the sign of changes in the two datasets. Coherent positive trends are found for a few countries in the Balkans. These have been investigated in details by looking at modifications in the atmospheric weather patterns as well as local thermodynamic factors concurring to large snowfall events. We link these changes to the stronger prevalence of Atlantic Ridge or blocking patterns associated with deeper cyclonic structures over the Adriatic and Tyrrhenian seas. These cyclones find warmer surfaces and large availability of humidity and convective available potential energy (CAPE), thus producing large snowfall amounts, enhanced by the Stau effect on the Balkan topography.

A kernel density estimation approach of North Indian Ocean tropical cyclone formation and the association with convective available potential energy and equivalent potential temperature

Abstract: Tropical cyclone (TC) is the one of the most devastating weather systems which causes enormous loss of life and property in the coastal regions of North Indian Ocean (NIO) rim countries. TC modelling can help decision-makers and inhabitants in shoreline zones to take necessary planning and actions in advance. To model TC activity, it is essential to know the factors that affect TC activities. The formation of tropical cyclones in the NIO basin is significantly modulated by Convective Available Potential Energy (CAPE) and Equivalent Potential Temperature (EPT). In this paper, a kernel density estimation approach (KDE) has been developed and evaluated to determine the extent of this modulation for the period 1979-2016. The distribution of genesis was defined by the KDE approach and validated by both classical and standard plug-in estimators. Results suggest a strong correlation of TC genesis densities with CAPE in the month of October-November (post-monsoon season) followed by the month of April-May (pre-monsoon season). Findings indicate the potential for predicting TC activities in the NIO well before the TC season.

A Statistical Investigation of Mesoscale Precursors of Significant Tornadoes: The Italian Case Study

Abstract: In this study, mesoscale environments associated with 57 significant tornadoes occurring over Italy in the period 2000–2018 are analyzed. The role of the vertical Wind Shear in the lower and middle troposphere, in terms of low-level shear (LLS) and deep-level shear (DLS), and of the convective available potential energy (CAPE) as possible precursors of significant tornadoes is statistically investigated. Wind shear and CAPE data are extracted from the ERA-5 and ERA-Interim reanalyses. Overall, the study indicates that: (a) values of these variables in the two uppermost quartiles of their statistical distribution significantly increases the probability of tornado occurrences; (b) the probability increases for increasing values of LLS and DLS, and (c) is maximum when either wind shear or CAPE are large. These conclusions hold for both the reanalysis datasets and do not depend upon the season and/or the considered area. With the possible exception of weak tornadoes, which are not included in our study, our results show that large wind shear, in the presence of medium-to-high values of CAPE, are reliable precursors of tornadoes.

Assessing atmospheric moisture effects on heavy precipitation during HyMeX IOP16 using GPS nudging and dynamical downscaling

Abstract: . Gaining insight into the interaction between atmospheric moisture and convection is determinant for improving the model representation of heavy precipitation, a weather phenomenon that causes casualties and monetary losses in the western Mediterranean region every year. Given the large variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation of moist convective processes. In this study, we use a diagnostic approach to assess the sensitivity of convective precipitation and underlying mechanisms during a heavy precipitation event (Hydrological cycle in the Mediterranean eXperiment Intensive Observation Period; HyMeX IOP16) to variations of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenized data set of the Global Positioning System-derived zenith total delay (GPS-ZTD) with sub-hourly temporal resolution (10 min) in 7 and 2.8 km simulations with the COnsortium for Small-scale MOdeling in CLimate Mode (COSMO-CLM) model over the western Mediterranean region. The analysis shows that (a) large atmospheric moisture amounts (integrated water vapour; IWV ∼ 40 mm ) precede heavy precipitation in the affected areas. This occurs 12 h prior to initiation over southern France and 4 h over Sardinia, north-eastern Italy and Corsica, which is our main study area. (b) We found that the moisture is swept from the Atlantic by a westerly large-scale front associated with an upper level low on the one hand and evaporated from the Mediterranean Sea and north Africa on the other. The latter moisture transport occurs in the 1 to 4 km layer. (c) COSMO-CLM overestimated the atmospheric humidity over the study region (Corsica), and this was, to a good extent, corrected by the GPS-ZTD nudging. This reduced maximum precipitation ( − 49 % for 7 km and − 16 % for 2.8 km ) drastically, considerably improving the precipitation representation in the 7 km simulation. The convection-permitting simulation (2.8 km ) without the GPS-ZTD nudging already did a good job in representing the precipitation amount. (d) The two processes that exerted the largest control on precipitation reduction were the decrease of atmospheric instability over Corsica (convective available potential energy; CAPE − 35 %) and the drying of the lower free troposphere bringing additional dry air entrainment. In addition, the 7 km simulation showed a weakening of the represented low-pressure system and the associated cyclonic wind circulation. This ultimately reduced the intensity and number of convective updrafts represented over the island. These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of processes leading to it across scales.

Short-duration precipitation extremes over Canada in a warmer climate

Abstract: Short-duration precipitation extremes are widely used in the design of engineering infrastructure systems and they also lead to high impact flash flood events and landslides. Better understanding of these events in a changing climate is therefore critical. This study assesses characteristics of short-duration precipitation extremes of 1-, 3-, 6- and 12-h durations in terms of the precipitation-temperature (P–T) relationship in current and future climates for ten Canadian climatic regions using the limited area version of the global environment multiscale (GEM) model. The GEM simulations, driven by ERA-Interim reanalysis and two coupled global climate models (CanESM2 and MPI-ESM), reproduce the general observed regional P–T relationship characteristics in current climate (1981–2010), such as sub-CC (Clausius–Clapeyron) and CC scalings for the coastal and northern, and inland regions, respectively, albeit with some underestimation. Analysis of the transient climate change simulations suggests important shifts and/or extensions of the P–T curve to higher temperature bins in future climate (2071–2100) for RCP4.5 and 8.5 scenarios, particularly for 1-h duration. Analysis of the spatial patterns of dew point depression (temperature minus dew point temperature) and convective available potential energy (CAPE) corresponding to short-duration precipitation extremes for different temperature bins show their changing relative importance from low to high temperature bins. For the low-temperature bins, short-duration precipitation extremes are largely due to high relative humidity, while for high-temperature bins, strong convection due to atmospheric instability brought by surface warming is largely responsible. The analysis thus addresses some of the key knowledge gaps related to the behavior of P–T relationship and associated mechanisms for the Canadian regions.

Observed monsoon precipitation suppression caused by anomalous interhemispheric aerosol transport

Abstract: This study uses observations and atmospheric reanalysis products in order to understand the impacts of smoke aerosols advected from the Southern Hemisphere on the dynamics of the West African monsoon. Seasonal biomass burning and resulting aerosol emissions have been well documented to affect regional weather patterns, especially low-level convection. Out of all monsoon months, precipitation shows the most variability over land during August, in which anomalous smoke aerosol values can increase (decrease) by 33% (29%) in the Northern Gulf of Guinea and precipitation can decrease (increase) by up to ~ 2.5 mm day−1 (~ 3 mm day−1) along the West African monsoon region accounting for a 17% (18%) change in precipitation. Smoke aerosols produced by biomass burning occurring near Central Africa are advected towards the Gulf of Guinea at elevations around the 850 hPa level. Satellite observations show an increase (decrease) in cloud fraction and optical depth below (above) the 300-hPa level in the Gulf of Guinea and along the West African coastline along with concurrent decreases (increases) in cloud droplet radius during dirty (clean) aerosol episodes. Additional observations of shortwave radiation quantify changes in cloud coverage and monsoon dynamics. On average, reductions in surface shortwave radiation of ~ 10–15 W m−2 occur over the Gulf of Guinea during increased aerosol transport, with aerosols accounting for ~ 33–50% of that reduction. Reductions in shortwave radiation are associated with decreased convective available potential energy (CAPE). This demonstrates that increased transport of aerosols perturbs surface radiation, convection in the lower troposphere and eventually cloud coverage, potentially leading to the observed monsoon precipitation suppression. In a broader social context, this region houses 200 million people and thus understanding these climate patterns may carry great importance.

Environmental Controls on Tropical Sea Breeze Convection and Resulting Aerosol Redistribution

Abstract: Sea breeze fronts propagate inland from the coastline, driving convective initiation and aerosol redistribution. Forecasting sea breezes is challenging due to uncertainties in the initial conditions, as well as the covariance and interaction of various meteorological and surface parameters. Using the Regional Atmospheric Modeling System coupled to an interactive land‐surface model, we conduct an ensemble of 130 idealized cloud‐resolving simulations by simultaneously perturbing six atmospheric and four surface parameters describing the initial conditions. To identify the key parameters impacting the inland characteristics and the intensity of the sea breeze convection in a tropical rainforest, we apply statistical emulation and variance‐based sensitivity analysis techniques. This study extends a previous study which explored the impacts of various parameters on sea breeze characteristics in arid environments devoid of moist convection. Wind speed is identified as the main contributor to the inland extent, similar to the arid environment study. However, the relative impacts of surface properties on the inland extent are less significant in the moist environment where land‐surface heating can be suppressed via moist convective processes and vegetation‐atmosphere interactions. Two sea breeze‐initiated convection regimes are also identified: shallow and deep. Over the shallow regime, where convective available potential energy is limited, the inversion layer strength is the primary control of the convective intensity. Over the deep regime, boundary layer temperature exerts a robust control over the convective available potential energy and hence the convective intensity. The potential vertical redistribution of aerosols is closely related to the convective intensity.

A Challenging Tornado Forecast in Slovakia

Abstract: An F1 tornado hit the village of Lekarovce in eastern Slovakia on the afternoon of 3 October 2018. The tornado, which occurred outside the main convective season in Slovakia, was not anticipated by the meteorologists of the Slovak Hydrometeorological Institute. The models available to the forecasters simulated an environment of marginal convective available potential energy (CAPE) and weakening vertical wind shear. This paper addresses forecasting challenges associated with events related to a tornado threat. To investigate conditions before tornado formation, observational datasets, including sounding, and vertical-azimuth display (VAD) data from a radar station and surface stations were used. Hodographs based on observational data and a higher-resolution run of the limited-area model showed stronger lower tropospheric shear than was formerly anticipated over the area of interest. The higher-resolution model was able to better represent the modification of the lower tropospheric flow by a mountain chain, which was crucial to maintaining the strong lower tropospheric shear in the early afternoon hours before the tornado’s occurrence. We discuss the importance of using both observational datasets and higher-resolution modeling in the simulation of lower tropospheric wind profiles, which affect the lower tropospheric storm relative helicity as one of the key ingredients in mesocyclonic tornadogenesis.

Wildfire Pyroconvection and CAPE: Buoyancy’s Drying and Atmospheric Intensification—Fort McMurray

Abstract: The accurate prediction of wildfire behavior and spread is possible only when fire and atmosphere simulations are coupled. In this work, we present a mechanism that causes a small fire to intensify by altering the atmosphere. These alterations are caused by fire-related fluxes at the surface. The fire plume and fluxes increase the convective available potential energy (CAPE) and the chance of the development of a strong pyroconvection system. To study this possible mechanism, we used WRF-Fire to capture fire line propagation as the result of interactions between heat and moisture fluxes, pressure perturbations, wind shear development and dry air downdraft. The wind patterns and dynamics of the pyroconvection system are simulated for the Horse River wildfire at Fort McMurray, Canada. The results revealed that the updraft speed reached up to 12 m/s. The entrainment mixed the mid and upper-level dry air and lowered the atmospheric moisture. The mid-level and upper-level dew point temperature changed by 5–10 ∘ C in a short period of time. The buoyant air strengthened the ascent as soon as the nocturnal inversion was eliminated by daytime heating. The 887 J/kg total increase of CAPE in less than 5 h and the high bulk Richardson number (BRN) of 93 were indicators of the growing pyro-cumulus cell. The presented simulation has not improved the original model or supported leading-edge numerical weather prediction (NWP) achievements, except for adapting WRF-Fire for Canadian biomass fuel. However, we were able to present a great deal of improvements in wildfire nowcasting and short-term forecasting to save lives and costs associated with wildfires. The simulation is sufficiently fast and efficient to be considered for a real-time operational model. While the project was designed and succeeded as an NWP application, we are still searching for a solution for the intractable problems associated with political borders and the current liable authorities for the further development of a new generation of national atmosphere–wildfire forecasting systems.

Quasi-stationary extreme rain produced by mesoscale convective system on the Mei-Yu front

Abstract: A mesoscale convective system (MCS) occurred on the Mei-Yu front in the Jiang-Huai River Basin of China on 7–8 July 2007, which caused extreme rainfall. The MCS formed in an environment of moderate convective available potential energy, high precipitable water, and an almost unidirectional southwesterly low-level jet. The favorable environment for MCS initiation and development featured a low-level convergence between northeasterly wind north of the Mei-Yu front and warm-moist southwesterly airflow. The evaporative cooling generated cold outflow which continuously promoted new convection at the leading edge of the MCS. WRF model simulations reproduced the observed back-building initiation and upscale organization. The flow-parallel MCS was affected by the low-level jet, vertical wind shear, and near-surface cold outflow in a stable nocturnal planetary boundary layer. Stratiform precipitation was reinforced by the downstream propagation of cumulonimbus and advection of ice-phase hydrometeors by the mid-upper level wind. The quasi-stationarity was a product of subtle dynamical balance between the near-surface cold outflow and the background low-level southerly flow. Sensitivity experiments addressed the role of near-surface outflow and diurnal forcing in MCS organization.

A rare winter thunderstorm event near Hong Kong

Abstract: A rare winter thunderstorm event near Hong Kong is documented in this paper. The thunderstorm occurrence was confirmed by the observation of one of the authors and lightning location information systems (both regional and global). It developed over the frontal cloud band associated with the winter monsoon. Observing the upper air measurements, however, the thermodynamic conditions did not seem favorable to the occurrence of the thunderstorm. The convective available potential energy could not be determined in the cool and dry lower troposphere. There was only a moister layer in the middle troposphere. The weather forecaster did not anticipate the occurrence of thunder and lightning. However, some features could have changed dynamically, which suggested the possibility that a thunderstorm would take place. For instance, waves could be identified in the middle troposphere and a broad troughing flow could be analyzed in the lower troposphere. The integrated wave vapor was also of a rather high level. It is hoped that this paper can serve as a useful reference for forecasters in assessing the possibility of thunderstorms occurring during winter over subtropical latitudes in association with frontal cloud bands.

Statistical Characteristics and Environmental Conditions of the Warm-Season Severe Convective Events over North China

Abstract: Based on severe weather reports, surface precipitation observations, surface routine observations, and the European Center for Medium-Range Weather Forecasts ERA5 reanalysis dataset during the warm seasons (May–September) of 2011–2018 over North China, this paper analyzes the statistical characteristics and environmental conditions of three types of severe convective events. Results are compared between events with different altitudes (i.e., mountains and plains), severities (i.e., ordinary and significant), and months. Hail and thunderstorm high winds (THWs) are more common over the mountains whereas short-duration heavy rainfall (SDHR) is more frequent over the plains. The occurrence frequency of severe convective events exhibits distinct monthly and diurnal variations. Analyses of the environmental parameters provide reference for the potential forecasting of severe convective events over this region. Specifically, the 850–500 hPa temperature lapse rate (LR85), pseudo-equivalent potential temperature at 500 hPa (thetase500), and precipitable water (PW) are skillful in distinguishing hail and THW environments from SDHR environments, and thetase500 is useful in discriminating between hail and THW environments. The convective environments over the plains are characterized by significantly higher (lower) PW (LR85) compared with mountains. The skill of these parameters in forecasting the severity of the convective hazards is limited. Probability distributions in the two parameters space indicate that the occurrence of significant hail requires both higher most unstable convective available potential energy (MUCAPE) and stronger 0–6 km bulk wind shear (SHR6) compared with ordinary hail. Compared with ordinary THWs, the significant THWs over the mountains depend more on the SHR6 whereas those over the plains rely more on the MUCAPE. The significant SDHR events over the plains tend to occur under a variety of instability conditions. The thermodynamic parameters (i.e., MUCAPE, thetase500, and downdraft convective available potential energy), and PW are significantly higher in July–August, whereas the LR85 and vertical wind shear are apparently higher in May, June and September.

Investigation of WRF’s ability to simulate the monsoon-related seasonal variability in the thermodynamics and precipitation over southern peninsular India

Abstract: The goal of this study is to evaluate the ability of the state-of-the-art, higher-resolution, convection-permitting, weather research forecasting (WRF) model in predicting the changes in precipitation regimes which come in response to the seasonal changes in the large-scale environmental forcing. The simulation days are selected in the year 2009 and according to four environmental regimes defined by the daily flow direction (Ragi et al. (IEEE Trans Geosci Remote Sens 55:3466–3474, 2017)) using QuikSCAT scatterometer and the comparison of the same with National Center for Environmental Prediction (NCEP) final analysis (FNL) data. The observations used for analysis are from Indian Meteorological Department, Wyoming, TRMM satellite data, and NCEP-NCAR reanalysis data. This study finds that WRF is capable of reproducing the season-specific differences in the precipitating patterns that reflect the different phases of the monsoon. Extensive comparisons to observations point out that the model simulates reasonably well the temperature and the humidity fields, including their diurnal variability and vertical structure. However, the model-produced precipitation and winds do not compare so well, especially the winds. The simulated large-scale monsoon circulation and rainfall patterns indicate a wet bias in the model rainfall simulations than the TRMM rainfall observations over the selected region. In particular, WRF overestimates the rain. The base variables such as outgoing longwave radiation (OLR), latent and sensible heat fluxes, and convective available potential energy (CAPE) and convective inhibition energy (CIN) are nearly in agreement with the observations. In effect, WRF is skilled to represent the variability in different seasons and its spatial distribution, an important characteristic of the precipitation, especially concerning prediction of the monsoon onset. The disagreements between the observed and the model precipitation and winds can be due to the WRF model physics which generates different dynamics and different precipitating systems and initial conditions.

Probing for overshooting as extreme event of thunderstorms

Abstract: Thunderstorm overshooting is rare but not an unusual phenomenon in a metropolitan of India, Kolkata (22.57° N; 88.36° E) during the pre-monsoon months (April–May). An attempt is made in this study to identify the important parameters differentiating the thunderstorms in overshooting and non-overshooting categories through data analytics from 2000 to 2015. The present investigation on parametric classification would facilitate in estimating the predictability of thunderstorms with overshooting which subsequently might assist in operational forecast of thunderstorm severity over Kolkata. The altitudes of lifting condensation level (LCL), wind shear, bulk Richardson number (BRN), gust speed, boundary layer characteristics and their correlation with thunderstorm cloud top height (CTH) and also their variation and distribution during overshooting (OTS) and non-overshooting (TS) thunderstorms are analyzed in this study. The result depicts that over Kolkata the intensity of storms during OTS is higher than during TS though the frequency of OTS is less than that of TS. The results further show that the potential temperature (θ), equivalent potential temperature (θe), mixing ratio (es) in the boundary layer, convective available potential energy, convective inhibition energy, BRN and gust speed play significant roles in regulating the CTH during OTS and TS thunderstorms over Kolkata.

Impact of convection and stability parameters on lightning activity over Andhra Pradesh, India

Abstract: This paper brings out the interconnection of flash rate density (FRD) with convection and stability parameters over Andhra Pradesh (AP), India. The convection parameters include rainfall, relative humidity, specific humidity, surface air temperature (SAT) and air temperature (at 850 mb). The stability parameters include convective available potential energy (CAPE), lifted index, K-index, total totals index (TTI), humidity index and total precipitable water. Both convective and stability parameters indicate good correlation with lightning activity. SAT and AT 850 mb have shown good correlations with lightning, which is a clear indication of interaction between warm air and dry air. CAPE and TTI have shown strong positive correlation with lightning activity. The correlation coefficient between FRD and CAPE is 0.81. We have also studied the influence of convective and stability parameters during lightning and no lightning activity. Later, we also attempted the estimation of lightning activity by using artificial neural network model. By using convection and stability parameters, six learning algorithms were used for training the artificial neural network. Scaled conjugate gradient backpropagation training algorithm has given the better estimation, whereas resilient backpropagation training algorithm has shown the poor estimation of FRD.

Mesoscale characteristics of a rare severe hailstorm event

Abstract: This study analyzed the environmental field, physical quantify fields, convection parameters and radar echo characteristics of a severe hailstorm that occurred on June 18, 2013, due to a mesoscale convective system. The analysis was performed using ground meteorological data, sounding data and GFS-ANL (gfs 0.5° × 0.5°) reanalysis data in combination with satellite cloud imagery and Doppler radar observations. The results showed that this rare hailstorm event was generated under the influence of a weakening and eastwardly moving low vortex over Lake Balkhash; and a distinct shear line in the middle and lower troposphere over the western South Xinjiang. Strong vertical wind shears, deep conditionally unstable stratification, ample water vapor in the lower troposphere and suitable wet-bulb zero heights are important contributors to the formation of severe hailstorms. An increase in convective available potential energy (CAPE) and a decrease in the lifting index are strong indicators of a severe hailstorm. For the studied hailstorm event, the time corresponding to the maximum value of the normalized CAPE was close to the timing of the actual hailstorm. The severe hailstorm, consisting of multiple supercells, exhibited notable characteristics of hail clouds and supercells. The formation, disappearance and replacement of strong convection cells occurred while the supercells were sustained. The tracking and forecasting results obtained using the Thunderstorm Identification, Tracking, Analysis and Nowcasting system are found to be, to a certain extent, able to indicate of the direction of the supercell storms during this severe hailstorm event.

Diurnal variation of radar reflectivity factor during intense convective clouds over Indonesia

Abstract: Intense convective clouds is represented by very high of clouds tops that produce heavy rainfall/intense convective precipitation. The vertical structure of intense convective clouds over Indonesia was investigated using radar reflectivity factor (dBZ) data from Tropical Rainfall Measuring Mission (TRMM) satellite-Precipitation Radar (PR) 2A25 product during 1998-2014. The vertical distribution of dBZ was classified into two convective cells following the classification proposed by some previous studies. The first type, Comulonimbus Tower (CbT) that contains Z threshold of 20 dBZ in 12 km altitude with at least 9 km depth and Intense Convective Clouds (ICC) that contains Z threshold of 30 and 41 dBZ at 8 and 3 km, respectively. The distribution of intense convective clouds is more frequently observed over land such as Sumatra, Kalimantan, Java and Irian Jaya than over ocean areas. To observe the pattern of vertical structure of intense convective clouds, the vertical profile of dBZ for several locations that represent land, coastal and ocean areas were analyzed. The land and coastal areas show similar vertical structure of intense convective. On seasonal basis, intense convective clouds are more frequently found during March, April and May (MAM) which coincided with the maximum of convective available potential energy (CAPE) value. Furthermore, on diurnal basis, intense convective clouds are significantly observed during 16-18 LT (local time) and 04-09 LT over mainland and ocean areas, respectively