Showing papers on "Convective available potential energy published in 2015"

The Deep Convective Clouds and Chemistry (DC3) Field Campaign

Abstract: The Deep Convective Clouds and Chemistry (DC3) field experiment produced an exceptional dataset on thunderstorms, including their dynamical, physical, and electrical structures and their impact on the chemical composition of the troposphere. The field experiment gathered detailed information on the chemical composition of the inflow and outflow regions of midlatitude thunderstorms in northeast Colorado, west Texas to central Oklahoma, and northern Alabama. A unique aspect of the DC3 strategy was to locate and sample the convective outflow a day after active convection in order to measure the chemical transformations within the upper-tropospheric convective plume. These data are being analyzed to investigate transport and dynamics of the storms, scavenging of soluble trace gases and aerosols, production of nitrogen oxides by lightning, relationships between lightning flash rates and storm parameters, chemistry in the upper troposphere that is affected by the convection, and related source character...

Climate and Hazardous Convective Weather

Abstract: Substantial progress has been made recently relating the large-scale climate system and hazardous convective weather (HCW; tornadoes, hail, and damaging wind), particularly over the USA where there are large societal impacts and a long observational record. Despite observational data limitations, HCW has shown to be influenced by the climate system and the tropical atmosphere via the Madden-Julian Oscillation and El Nino-Southern Oscillation. Analysis of the atmospheric environments favorable to HCW (e.g., convective available potential energy and vertical wind shear) avoids observational and model limitations. While few robust trends are seen over recent decades, future climate projections indicate increased frequency of such environments over the USA, Europe, and Australia, suggesting increased future HCW activity. A recent increase in the year-to-year variability of US tornado occurrence is striking, but not yet understood. Dynamical downscaling to convection-permitting resolutions promises improved understanding of the relationships between large-scale climate and HCW occurrence.

Temperature and CAPE dependence of rainfall extremes in the eastern United States

Abstract: We analyze how extreme rainfall intensities in the Eastern United States depend on temperature T, dew point temperature Td, and convective available potential energy CAPE, in addition to geographic sub-region, season, and averaging duration. When using data for the entire year, rainfall intensity has a quasi Clausius-Clapeyron (CC) dependence on T, with super-CC slope in a limited temperature range and a maximum around 25°C. While general, these features vary with averaging duration, season, the quantile of rainfall intensity, and to some extent geographic sub-region. By using Td and CAPE as regressors, we separate the effects of temperature on rainfall extremes via increased atmospheric water content and via enhanced atmospheric convection. The two contributions have comparable magnitudes, pointing at the need to consider both Td and atmospheric stability parameters when assessing the impact of climate change on rainfall extremes.

Environmental controls on storm intensity and charge structure in multiple regions of the continental United States

Abstract: A database consisting of approximately 4000 storm observations has been objectively analyzed to determine environmental characteristics that produce high radar reflectivities above the freezing level, large total lightning flash rates on the order of 10 flashes per minute, and anomalous vertical charge structures (most notably, dominant midlevel positive charge). The storm database is drawn from four regions of the United States featuring distinct environments, each with coinciding Lightning Mapping Array (LMA) network data. LMAs are able to infer total lightning flash rates using flash clustering algorithms, such as the one implemented in this study. Results show that anomalous charge structures inferred from LMA data, significant lightning flash rates, and increased radar reflectivities above the freezing level tend to be associated with environments that have high cloud base heights (approximately 3 km above ground level) and large atmospheric instability, quantified by normalized convective available potential energy (NCAPE) near 0.2 m s−2. Additionally, we infer that aerosols may affect storm intensity. Maximum flash rates were observed in storms with attributed aerosol concentrations near 1000 cm−3, while total flash rates decrease when aerosol concentrations exceed 1500 cm−3, consistent with previous studies. However, this effect is more pronounced in regions where the NCAPE and cloud base height are low. The dearth of storms with estimated aerosol concentrations less than 700 cm−3 (approximately 1% of total sample) does not provide a complete depiction of aerosol invigoration.

Why does tropical convective available potential energy (CAPE) increase with warming

Abstract: Author(s): Seeley, JT; Romps, DM | Abstract: Recent work has produced a theory for tropical convective available potential energy (CAPE) that highlights the Clausius-Clapeyron (CC) scaling of the atmosphere's saturation deficit as a driver of increases in CAPE with warming. Here we test this so-called "zero-buoyancy" theory for CAPE by modulating the saturation deficit of cloud-resolving simulations of radiative-convective equilibrium in two ways: changing the sea surface temperature (SST) and changing the environmental relative humidity (RH). For earthlike and warmer SSTs, undilute parcel buoyancy in the lower troposphere is insensitive to increasing SST because of a countervailing CC scaling that balances the increase in the saturation deficit; however, buoyancy increases dramatically with SST in the upper troposphere. Conversely, in the RH experiment, undilute buoyancy throughout the troposphere increases monotonically with decreasing RH. We show that the zero-buoyancy theory successfully predicts these contrasting behaviors, building confidence that it describes the fundamental physics of CAPE and its response to warming.

Downscaled estimates of late 21st century severe weather from CCSM3

Abstract: High-resolution dynamical downscaling is used to explore 2080–2090 peak-season hazardous convective weather as simulated from the Community Climate System Model version 3. Downscaling to 4 km grid spacing is performed using the Weather Research and Forecasting model. Tornadoes, damaging wind gusts, and large hail are simulated using a model proxy at hourly intervals for locations east of the U.S. Continental Divide. Future period results are placed into context using 1980–1990 output. While a limited sample size exists, a statistically significant increase in synthetic severe weather activity is noted in March, whereas event frequency is shown to slightly increase in April, and stay the same in May. These increases are primarily found in the Mississippi, Tennessee, and Ohio River valleys. Diurnally, most of the increase in hazardous convective weather activity is shown to be in the hours surrounding local sunset. Peak-season severe weather is also shown to be more variable in the future with a skewed potential toward larger counts. Finally, modeled proxy events are compared to environmental parameters known to generate hazardous convective weather activity. These environmental conditions explain over 80 % of the variance associated with modeled reports during March–May and show an increasing future tendency. Finally, challenges associated with dynamical downscaling for purposes of resolving severe local storms are discussed.

Balanced dynamics and convection in the tropical troposphere

Abstract: This paper presents a conceptual picture of balanced tropical tropospheric dynamics inspired by recent observations. The most important factor differentiating the tropics from middle and higher latitudes is the absence of baroclinic instability; upward motion occurs primarily via deep convective processes. Thus, convection forms an integral part of large-scale tropical motions. Since convection itself is small-scale and chaotic in detail, predictability lies in uncovering the hidden hands that guide the average behavior of convection. Two appear, balanced dynamics and thermodynamic constraints. Contrary to conventional expectations, balanced dynamics plays a crucial role in the tropical atmosphere. However, due to the smallness of the Coriolis parameter there, nonlinear balance is more important in the tropics than at higher latitudes. Three thermodynamic constraints appear to play an important role in governing the average behavior of convection outside of the cores of tropical storms. First, convection is subject to control via a lower tropospheric buoyancy quasi-equilibrium process, wherein destabilization of the lower troposphere by nonconvective processes is balanced by convective stabilization. Second, the production of precipitation is extraordinarily sensitive to the saturation fraction of the troposphere. Third, “moisture quasi-equilibrium” governs the saturation fraction, with moister atmospheres being associated with smaller moist convective instability. The moist convective instability is governed by the balanced thermodynamic response to the pattern of potential vorticity, which in turn is slowly modified by convective and radiative heating. The intricate dance between these dynamic and thermodynamic processes leads to complex behavior of the tropical atmosphere in ways that we are just beginning to understand.

Increases in moist-convective updraught velocities with warming in radiative-convective equilibrium

Abstract: The scaling of updraught velocities over a wide range of surface temperatures is investigated in simulations of radiative-convective equilibrium with a cloud-system resolving model. The updraught velocities increase with warming, with the largest fractional increases occurring in the upper troposphere and for the highest percentile updraughts. A plume model approximately reproduces the increases in updraught velocities if the plume environment is prescribed as the mean profile in each simulation while holding the entrainment and microphysical assumptions fixed. Convective available potential energy (CAPE) also increases with warming in the simulations but at a much faster fractional rate when compared with the square of the updraught velocities. This discrepancy is investigated with a two-plume model in which a weakly entraining plume represents the most intense updraughts, and the environment is assumed to adjust so that a more strongly entraining plume has negligible buoyancy. The two-plume model suggests that updraught velocities increase with warming at a lower fractional rate than implied by the CAPE because of the influence of entrainment on both the mean stratification and the updraughts themselves.

Simultaneous influences of thermodynamics and aerosols on deep convection and lightning in the tropics

Abstract: Convective features (CFs) observed by the Tropical Rainfall Measuring Mission satellite between 2004 and 2011 are analyzed to determine the relative roles of thermodynamics and aerosols as they modulate radar reflectivity and lightning. We studied the simultaneous impacts of normalized convective available potential energy (NCAPE) and warm cloud depth (WCD) as well as cloud condensation nuclei concentrations (D ≥ 40 nm; N40) on total lightning density (TLD), average height of 30 dBZ echoes (AVGHT30), and vertical profiles of radar reflectivity (VPRR) within individual CFs. The results show that TLD increases by up to 600% and AVGHT30 increases by up to 2–3 km with increasing NCAPE and N40 for fixed WCD. The partial sensitivities of TLD/AVGHT30 to NCAPE and N40 separately were comparable in magnitude but account for a fraction of the total range of variability (i.e., when the influences of NCAPE and N40 are considered simultaneously). Both TLD and AVGHT30 vary inversely with WCD such that maxima of TLD and AVGHT30 are found for the combination of high NCAPE, high N40, and shallower WCD. The relationship between lightning and radar reflectivity was shown to vary as a function of N40 for a fixed thermodynamic environment. Analysis of VPRRs shows that reflectivity in the mixed phase region is up to 5.0–5.6 dB greater for CFs in polluted environments compared to CFs in pristine environments (holding thermodynamics fixed). This analysis favors a merged hypothesis for the simultaneous roles of thermodynamics and aerosols as they influence deep convective clouds in the Tropics.

Land and atmospheric controls on initiation and intensity of moist convection: CAPE dynamics and LCL crossings

Abstract: The local role that land-atmosphere interactions play in the rainfall process has been often explored by investigating the initiation of moist convection as the top of the atmospheric boundary layer (ABL) crosses the lifting condensation level (LCL). However, this LCL crossing alone is not a sufficient indicator of the probability and intensity of subsequent convective precipitation, which is instead better characterized by the added consideration of the so-called convective available potential energy (CAPE). In this study, both the LCL crossing and CAPE are jointly considered as the primary indicators of the occurrence and intensity of moist convection in order to analyze the land-atmosphere interactions through a simple soil-plant system and a zero-dimensional mixed-layer model. The approach is explored using the free atmospheric conditions observed at the Central Facility in the Southern Great Plains, where the ABL analysis shows both dry and wet soil can be conducive to early moist convection depending on atmospheric conditions but CAPE always tends to be larger under wetter soil conditions. The combination of the two indicators, LCL crossing and CAPE, further allows us to classify free atmosphere and soil moisture regimes into positive and negative feedback regimes for moist convection.

Soil moisture–precipitation coupling: observations from the Oklahoma Mesonet and underlying physical mechanisms

Abstract: . Interactions between soil moisture and the atmosphere are driven by the partitioning of sensible and latent heating, through which soil moisture has been connected to atmospheric modifications that could potentially lead to the initiation of convective precipitation. The majority of previous studies linking the land surface to subsequent precipitation have used atmospheric reanalysis or model data sets. In this study, we link in situ observations of soil moisture from more than 100 stations in Oklahoma to subsequent unorganized afternoon convective precipitation. We use hourly next generation (NEXRAD) radar-derived precipitation to identify convective events, and then compare the location of precipitation initiation to underlying soil moisture anomalies in the morning. Overall we find a statistically significant preference for convective precipitation initiation over drier than normal soils, with over 70 % of events initiating over soil moisture below the long-term median. The significant preference for precipitation initiation over drier than normal soils is in contrast with previous studies using satellite-based precipitation to identify the region of maximum precipitation accumulation. We evaluated 19 convective events occurring near Lamont, Oklahoma, where soundings of the atmospheric profile at 06:00 and 12:00 LST are also available. For these events, soil moisture has strong negative correlations with the level of free convection (LFC), planetary boundary layer (PBL) height, and surface temperature changes between 06:00 and 12:00 LST. We also find strong positive correlations between morning soil moisture and morning-to-afternoon changes in convective available potential energy and convective inhibition. In general, the results of this study demonstrate that both positive and negative soil moisture feedbacks are important in this region of the USA.

MSE Minus CAPE is the True Conserved Variable for an Adiabatically Lifted Parcel

Abstract: For an adiabatic parcel convecting up or down through the atmosphere, it is often assumed that its moist static energy (MSE) is conserved. Here, it is shown that the true conserved variable for this process is MSE minus convective available potential energy (CAPE) calculated as the integral of buoyancy from the parcel’s height to its level of neutral buoyancy and that this variable is conserved even when accounting for full moist thermodynamics and nonhydrostatic pressure forces. In the calculation of a dry convecting parcel, conservation of MSE minus CAPE gives the same answer as conservation of entropy and potential temperature, while the use of MSE alone can generate large errors. For a moist parcel, entropy and equivalent potential temperature give the same answer as MSE minus CAPE only if the parcel ascends in thermodynamic equilibrium. If the parcel ascends with a nonisothermal mixed-phase stage, these methods can give significantly different answers for the parcel buoyancy because MSE minus...

Aerosol‐cloud associations over Gangetic Basin during a typical monsoon depression event using WRF‐Chem simulation

Abstract: To study aerosol-cloud interactions over the Gangetic Basin of India, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) has been applied to a typical monsoon depression event prevalent between the 23 and 29 August 2009. This event was sampled during the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) aircraft campaign, providing measurements of aerosol and cloud microphysical properties from two sorties. Comparison of the simulated meteorological, thermodynamical, and aerosol fields against satellite and in situ aircraft measurements illustrated that the westward propagation of the monsoon depression and the cloud, aerosol, and rainfall spatial distribution was simulated reasonably well using anthropogenic emission rates from Monitoring Atmospheric Composition and Climate project along with cityZEN projects (MACCity)+Intercontinental Chemical Transport Experiment Phase B anthropogenic emission rates. However,the magnitude of aerosol optical depth was underestimated by up to 50%. A simulation with aerosol emissions increased by a factor of 6 over the CAIPEEX campaign domain increased the simulated aerosol concentrations to values close to the observations, mainly within boundary layer. Comparison of the low-aerosol simulation and high-aerosol simulation for the two sorties illustrated that more anthropogenic aerosols increased the cloud condensing nuclei (CCN) and cloud droplet mass concentrations. The number of simulated cloud droplets increased while the cloud droplet effective radii decreased, highlighting the importance of CCN-cloud feedbacks over this region. The increase in simulated anthropogenic aerosols (including absorbing aerosols) also increased the temperature of air parcels below clouds and thus the convective available potential energy (CAPE). The increase in CAPE intensified the updraft and invigorated the cloud, inducing formation of deeper clouds with more ice-phase hydrometeors for both cases. These case studies provide evidence of aerosol-induced cloud invigoration over the Gangetic Basin.

A baseline climatology of sounding-derived parameters associated with heavy rainfall over Gauteng, South Africa

Abstract: Irene weather office sounding data are considered a proximity sounding for the Gauteng province of South Africa. Sounding-derived parameters are analysed for 35 austral summers from 1977 to 2012. The goal of this study is to provide a climatology of sounding-derived parameters commonly used as ingredients to forecast heavy rainfall. The emphasis is placed on identifying those variables that distinguish between climatology and heavy rainfall events. Special attention is given to how the critical values associated with heavy rainfall change from early to late summer. During early summer (October to December), the atmospheric circulation over Gauteng is markedly extra-tropical in nature. Heavy rainfall occurs in a conditionally unstable atmosphere and is associated with conditions conducive to the development of severe storms, such as large wind shear and convective available potential energy (CAPE) values, strong upper tropospheric winds and large temperature lapse rates. In late summer (January to March), the atmosphere takes on distinct tropical characteristics and becomes increasingly convectively unstable. During this time of year there is abundant moisture in circulation and the storms that develop are highly efficient in producing precipitation. Forecasting heavy rainfall in early summer requires different techniques than in late summer. Sounding parameters, which provide information about the moisture content of the atmosphere, are capable of distinguishing between climatology and heavy rainfall during all summer months. The only other variables capable of doing this are the average meridionial wind direction in the 800 to 600 hPa layer, the mean layer equivalent potential temperature, the Showalter Index (SI), the K-index (KI) and the elevated K-index (EKI). However, critical values associated with heavy rainfall for all these parameters change month by month.

Changes in the seasonality of tornado and favorable genesis conditions in the central United States

Abstract: We analyze changes in the seasonality of tornado reports and related environmental conditions over the central U.S. Tornado counts are normalized by annual totals to reduce effects of changing reporting practices. Wavelet analysis of the normalized tornado counts shows significant power at semiannual and annual frequencies. An environmental index based on convective available potential energy and storm relative helicity shows power at the same frequencies. Reconstruction of the normalized tornado counts and the environmental index at these frequencies provides a time-filtered description of their seasonal variation. Onset and peak dates of the tornado season defined from the reconstruction are significantly correlated with those of the environmental index over the period 1979–2013 and are found to have trends of −3.7 and −3.8 days/decade, respectively. The agreement between changes in tornado report seasonality and those of the environmental index is evidence that the trends have a physical origin.

Long-Lived Mesoscale Systems in a Low–Convective Inhibition Environment. Part I: Upshear Propagation

Abstract: Part II of this study of long-lived convective systems in a tropical environment focuses on forward-tilted, downshear-propagating systems that emerge spontaneously from idealized numerical simulations. These systems differ in important ways from the standard mesoscale convective system that is characterized by a rearward-tilted circulation with a trailing stratiform region, an overturning updraft, and a mesoscale downdraft. In contrast to this standard mesoscale system, the downshear-propagating system considered here does not feature a mesoscale downdraft and, although there is a cold pool it is of secondary importance to the propagation and maintenance of the system. The mesoscale downdraft is replaced by hydraulic-jump-like ascent beneath an elevated, forward-tilted overturning updraft with negligible convective available potential energy. Therefore, the mesoscale circulation is sustained almost entirely by the work done by the horizontal pressure gradient and the kinetic energy available from ...

The Relationship between Hurricane Potential Intensity and CAPE

Abstract: The theoretical minimum eyewall pressure of tropical cyclones can be computed from convective available potential energy (CAPE) if the buoyancy in the CAPE is allowed to feed back on the surface pressure via hydrostatic balance. The relationship between this so-called hurricane CAPE and the surface pressure is exploited by a widely used algorithm for hurricane potential intensity (PI). For the observed atmosphere, the algorithm is shown to yield significantly weaker pressure intensity (20%–25%) and velocity intensity (5%–10%) than the most familiar analytical formulas. This discrepancy is found to come mostly from thermodynamic approximations in the formulas.The CAPE–PI algorithm makes a significant adjustment to the hurricane CAPE by subtracting the environmental CAPE. Most of the environmental profile becomes irrelevant as a result. Other steady-state theories retain the influence of the full environmental column. The impact of this choice on the pressure and velocity PI is analyzed. Another imp...

The Simulated Structure and Evolution of a Quasi-Idealized Warm-Season Convective System with a Training Convective Line

Abstract: This study details the development and use of an idealized modeling framework to simulate a quasi-stationary heavy-rain-producing mesoscale convective system (MCS) A 36-h composite progression of atmospheric fields computed from 26 observed warm-season heavy-rain-producing training line/adjoining stratiform (TL/AS) MCSs was used as initial and lateral boundary conditions for a numerical simulation of this MCS archetypeA realistic TL/AS MCS initiated and evolved within a simulated mesoscale environment that featured a low-level jet terminus, maximized low-level warm-air advection, and an elevated maximum in convective available potential energy The first stage of MCS evolution featured an eastward-moving trailing-stratiform-type MCS that generated a surface cold pool The initial system was followed by rearward off-boundary development, where a new line of convective cells simultaneously redeveloped north of the surface cold pool boundary Backbuilding persisted on the western end of the new lin

Thermodynamic Constraints on the Morphology of Simulated Midlatitude Squall Lines

Abstract: This study examines how environmental thermodynamics constrain the morphology of simulated idealized midlatitude squall lines (SLs). The thermodynamic soundings used for simulating various SLs are specified primarily by prescribed vertical profiles of the convective available potential energy (CAPE) and the level of free convection. This framework, which contemplates the latent instability properties of both low- and midtropospheric air, is considered to be convenient for investigating layer-lifting convective phenomena.Results show that frequently used CAPE indices are unsuitable for diagnosing SL characteristics, while integrated CAPE (ICAPE) discriminates the amplitude of the storm-induced heating for a given value of environmental shear. The skill of ICAPE follows from its relation to the buoyancy attained by low- and midtropospheric parcels as they ascend over the cold pool under layer-lifting convection. Environmental kinematics also affect the storm-induced heating, with stronger low-level ...

Environmental control of deep convective clouds on Titan: The combined effect of CAPE and wind shear on storm dynamics, morphology, and lifetime

Abstract: Titan has deep convective clouds driven by the release of latent from methane condensation. As on Earth, the presence of convective available potential energy (CAPE), which quantifies the amount of energy available through condensation, is required for storms to develop. While CAPE is a requirement for storms, the dynamics, morphology, and longevity of storms on Earth is controlled by both CAPE and wind shear, often expressed as a ratio in the form of the bulk Richardson Number. The impact of CAPE and wind shear on storms in a Titan-like environment are explored through numerical simulation. Model results indicate that Titan storms should respond to changes in the Richardson Number in a manner similar to storms on Earth. Very long-lived storms (>24 h) propagating for 1000 km or more might be possible on Titan when CAPE and wind shear are properly balanced. Some of the simulated storms exhibit dynamics similar to squall lines. Varying amounts of shear in the Titan environment might explain the variety of convective cloud expressions—varying from short-lived single cell storms to longer-lived linear features and large cloud bursts—identified in Cassini orbiter and ground-based observations. The varying amounts and spatial distribution of precipitation, as well as surface winds associated with storms, should have implications on the formation of fluvial and aeolian features and on the exchange of methane with the surface and lakes.

A Dynamically Computed Convective Time Scale for the Kain–Fritsch Convective Parameterization Scheme

Abstract: Many convective parameterization schemes define a convective adjustment time scale τ as the time allowed for dissipation of convective available potential energy (CAPE). The Kain–Fritsch scheme defines τ based on an estimate of the advective time period for deep convective clouds within a grid cell, with limits of 1800 and 3600 s, based on practical cloud-lifetime considerations. In simulations from the Weather Research and Forecasting (WRF) Model using 12-km grid spacing, the value of τ often defaults to the lower limit, resulting in relatively rapid thermodynamics adjustments and high precipitation rates. Herein, a new computation for τ in the Kain–Fritsch scheme is implemented based on the depth of the buoyant layer and the convective velocity scale. This new τ formulation is applied using 12- and 36-km model grid spacing in conjunction with a previous modification that takes into account the radiation effects of parameterized convective clouds. The dynamically computed convective adjustment ti...

Convective Rainfall in Amazonia and Adjacent Tropics

Abstract: Hourly rainfall estimates from integrated satellite data are used to build a dynamically based climatology of convectively generated rainfall across South America, including tropical, sub-tropical and oceanic regions. Herein, we focus on 0S to 15S, including greater Amazon and NE Brazil leeward of the South Atlantic Ocean. Emphasis is placed on rainfall resulting from organized convective regimes, which are known to produce the majority of seasonal rainfall in various parts of South America and other continents. The statistical characteristics of individual events are quantified and examined with respect to regional atmospheric conditions. Among the factors considered are steering winds and wind shear, convective available potential energy (CAPE), sea and land breezes, and the occurrence of transient disturbances such as Kelvin Waves and Easterly Waves. Forcing and convective triggering mechanisms are inferred from the diagnosis of systematic patterns as evidenced in the continental diurnal cycle and longer periods of natural variability. The episodes of organized convection are analyzed in terms of their duration, span, phase speed, starting and ending time, starting and ending longitude, month and year through frequency distribution analysis. Most episodes of organized convection tend to move westward across the Amazon Basin. Descriptive statistics indicate average phase speed of westward and eastward episodes of convection in the Amazon basin at -11.8 m.s-1 and 13.0 m.s-1, respectively. Eastward propagating systems are influenced by northeastward moving cold fronts in Southern South America and tend to trigger and to organize convection across the Amazon Basin. Hourly rainfall analyses indicate that convection over the Amazon region is often organized.

Some Aspects of Thunderstorm over India during Pre-Monsoon Season: A Preliminary Report-I

Abstract: Thunderstorm, resulting from vigorous convective activity, is one of the most magnificent weather phenomena in the earth’s atmosphere. The severe thunderstorms associated with thunder squall, hail storm, tornado, flash flood and lightning cause extensive damage and losses to lives and property. A common feature of the weather during the pre-monsoon season over the Indian region is the outburst of severe local convective storms. This paper presents on the aspects of the realized significant weather phenomena thunderstorm, which is supported through the analyses of thermodynamic instability indices based on the radiosonde and rawinsonde (RS/RW) ascent products from India Meteorological Department (IMD) for the pre-monsoon season for different identified cities of SAARC STORM project region of India. Doppler Weather Radar (DWR) images and Skew-T diagrams are also analyzed which support the thunderstorm activities in different locations of India. The convective available potential energy (CAPE) and convective inhibition (CIN) energy show the favorable conditions for the thunderstorm to occur in some of the identified stations; however, due to physiographic uniqueness of Indian subcontinent, the values of CAPE, CIN and other thermodynamic parameters show different values in different stations. Moreover, the variation in threshold values of CAPE in different regions makes thunderstorm forecasting difficult which may add uncertainty to loss estimation for risk assessment. A simple outline on thunderstorm risk assessment model development steps are also highlighted as a future work for the quantification of losses, so that the likely probability of occurrences of events with their frequency, location, severity and extent of losses can be modeled and accessed ahead of time for the betterment of the society.

The relative influence of environmental characteristics on tropical deep convective morphology as observed by CloudSat

Abstract: Utilizing a previously developed CloudSat cloud object database, the sensitivity of oceanic, mature, deep convective cloud morphology to cloud-scale environmental characteristics is examined. Convective available potential energy (CAPE), aerosol optical depth, midlevel vertical velocity, and tropospheric deep shear are all used to characterize the environment. The sensitivity of various aspects of convective morphology to each one of these environmental quantities is assessed individually. The results demonstrate that clouds tend to be invigorated by higher CAPE, aerosol, and upward midlevel vertical velocity. Stronger shear tends to make clouds wider but also shallower. The relative importance of each of these and some additional environmental measures to trends in cloud morphology are compared. It is found that aerosol, deep-layer shear, and sea surface temperature tend to be the most influential environmental factors to convective morphology. The results are shown to be insensitive to the manner in which the environmental characteristics are defined. The potentially surprising weak sensitivity of cloud morphology to CAPE is discussed in detail.

Sensitivity of the distribution of thunderstorms to sea surface temperatures in four Australian east coast lows

Abstract: The relationship between the sea surface temperature (SST) distribution and the locations of thunderstorms during four Australian east coast lows is investigated using both lightning observations and numerical simulation results. The focus is placed on investigating changes in convective instability caused by the introduction of complex, high-resolution ocean eddy, and frontal structures present in Bluelink SST datasets. Global Position and Tracking System lightning data are overlaid on maps of SST to investigate whether a thunderstorm–SST relationship is discernible. Weather Research and Forecast model simulations are used to establish what atmospheric changes contribute to the observed distributions of thunderstorms. Maximum convective available potential energy (MCAPE) analysis shows a distinct relationship to the SST distribution. In particular, areas of elevated MCAPE are related to regions of warmer SST with horizontal advection often displacing increased MCAPE downwind of the warmer SST. At short timescales of 3–6 h, the differences in MCAPE become larger and more localised and show a strong correlation with the observed lightning. This suggests that at times the thunderstorms are directly related to the complex structures in the detailed SST dataset. For the damaging Pasha Bulker case, the plume of thunderstorms associated with the coastal damage occurs downwind of the region of enhanced MCAPE on the southern flank of the warm eddy. Based on these results, it is concluded that the particular features of the warm eddy enhanced the thunderstorm potential over the coastal region during this event and helped in localising the area of greatest impact for thunderstorm-related intense rainfall.