Convective Inhibition Energy Research Articles

Nested high‐resolution modeling of the impact of urbanization on regional climate in three vast urban agglomerations in China

In this paper, the Weather Research and Forecasting Model, coupled to the Urban Canopy Model, is employed to simulate the impact of urbanization on the regional climate over three vast city agglomerations in China. Based on high‐resolution land use and land cover data, two scenarios are designed to represent the nonurban and current urban land use distributions. By comparing the results of two nested, high‐resolution numerical experiments, the spatial and temporal changes on surface air temperature, heat stress index, surface energy budget, and precipitation due to urbanization are analyzed and quantified. Urban expansion increases the surface air temperature in urban areas by about 1°C, and this climatic forcing of urbanization on temperature is more pronounced in summer and nighttime than other seasons and daytime. The heat stress intensity, which reflects the combined effects of temperature and humidity, is enhanced by about 0.5 units in urban areas. The regional incoming solar radiation increases after urban expansion, which may be caused by the reduction of cloud fraction. The increased temperature and roughness of the urban surface lead to enhanced convergence. Meanwhile, the planetary boundary layer is deepened, and water vapor is mixed more evenly in the lower atmosphere. The deficit of water vapor leads to less convective available potential energy and more convective inhibition energy. Finally, these combined effects may reduce the rainfall amount over urban areas, mainly in summer, and change the regional precipitation pattern to a certain extent.

Heavy rainfall occurrences in northeast India

AbstractIn operational meteorology, forecasting heavy rainfall (HRF) events has been a long‐standing challenge in India. This is especially true in certain regions where the physical geography lends itself to the creation of such HRF events. Northeast India (NEI) is one such region within the Asian monsoon zone, which receive very HRF during the pre‐monsoon and summer monsoon season and the summer–autumn transition month of October. These events cause flooding, damage crops and bring life to standstill. In the present work, the characteristics of HRF events in NEI are studied. The seasonal and spatial variations of HRF occurrences are analysed using 31 year (1971–2001) of daily rainfall data from 15 rainfall stations. Using the daily data obtained from the Indian Meteorological Department, the most favorable locations were found for the stations between 27.5°N and 28.1°N. The most favorable time of occurrence of these events are between 10 June and 5 August. July records the largest number of HRF events followed by June and August. The aggregate of extreme rain events over the region has a significant decreasing trend over the region. Before the monsoon sets in, there is considerable thunderstorm (TS) activity in this region in the month of April and May that are also the cause of HRF events. While many of these HRF events occur associated with the pre‐monsoon Nor'westers (tornadoes), some severe TSs may occur during the monsoon season. So, we present a climatology of severe TS days. Also we present the annual and seasonal variation of convective available potential energy (CAPE) and convective inhibition energy (CINE) at Guwahati as the index of the thermal instability. Between 1973 and 2001, CAPE shows a decreasing trend whereas CINE shows an increasing trend which seems reasonable due to the HRF events' decreasing trend. Copyright © 2012 Royal Meteorological Society

A composite stability index for dichotomous forecast of thunderstorms

Thunderstorms are the perennial feature of Kolkata (22° 32′ N, 88° 20′ E), India during the premonsoon season (April–May). Precise forecast of these thunderstorms is essential to mitigate the associated catastrophe due to lightning flashes, strong wind gusts, torrential rain, and occasional hail and tornadoes. The present research provides a composite stability index for forecasting thunderstorms. The forecast quality detection parameters are computed with the available indices during the period from 1997 to 2006 to select the most relevant indices with threshold ranges for the prevalence of such thunderstorms. The analyses reveal that the lifted index (LI) within the range of −5 to −12 °C, convective inhibition energy (CIN) within the range of 0–150 J/kg and convective available potential energy (CAPE) within the ranges of 2,000 to 7,000 J/kg are the most pertinent indices for the prevalence thunderstorms over Kolkata during the premonsoon season. A composite stability index, thunderstorm prediction index (TPI) is formulated with LI, CIN, and CAPE. The statistical skill score analyses show that the accuracy in forecasting such thunderstorms with TPI is 99.67 % with lead time less than 12 h during training the index whereas the accuracies are 89.64 % with LI, 60 % with CIN and 49.8 % with CAPE. The performance diagram supports that TPI has better forecast skill than its individual components. The forecast with TPI is validated with the observation of the India Meteorological Department during the period from 2007 to 2009. The real-time forecast of thunderstorms with TPI is provided for the year 2010.

Simulation of Heavy Rainfall Events during Retreat Phase of Summer Monsoon Season over Parts of Andhra Pradesh

The main aim of this paper is to simulate monsoon heavy rainfall episodes that caused floods across some parts of Andhra Pradesh (AP) state, India during 29th September through 2nd October, 2009. A heavy rainfall quantity of 21 cm was observed near Amaravathi station (16.15°N; 80.5°E) in Guntur district due to a meso-α low pressure system extended from the Bay of Bengal and widespread rainfall episodes were also appeared to many adjoining places in other three districts namely Mahaboob Nagar, Kurnool and Krishna in AP state simultaneously on 29th September. The rainy situation continued till 2nd October and caused floods over above districts of AP state which lead to a death toll of 33 people and heavy crop loss. To quantify the above catastrophic monsoon heavy precipitation events a high resolution (9 km) Weather Research and Forecast (WRF-ARW) model is centered at Amaravathi station to simulate rainfall episodes over the study region. In the present case study the simulated sensitive experiment highlights the dynamical characteristics of the meso-α system interms of circulation changes at different levels. Secondly, the thermodynamical characteristics for the generation of convective activity of this meso-α event in terms of Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE) are also simulated. Dynamical and thermodynamical simulated results support heavy rainfall episodes due to a low pressure system around Amaravathi station. Thus circulation changes, high CAPE and low CINE magnitudes have well defined not only the strength of meso-α system, but also quantum of rain-fall to a tune of 19 cm near Amaravathi station on 29th September. The observed rainfall was 21 cm on 29th September and thus this model underestimates rainfall about 9.5% not only at Amaravathi station, but also at other stations as well. Similar results are noticed over the study region on other three days. In this numerical study heavy rainfall events are better represented by Kain-Fritsch (KF) scheme than Betts-Miller-Janjic (BMJ) and Grell-Deveneyi (GD) schemes. Finally simulated circulation features and rainfall quantities are validated with observed rainfall of the India Meteorological Department (IMD) and satellite derived datasets of KALPANA-1, while CAPE and CINE quantities are checked against available Wyoming University observations. The results are promising.

A Probe for Consistency in CAPE and CINE During the Prevalence of Severe Thunderstorms:Statistical – Fuzzy Coupled Approach

Thunderstorms of pre-monsoon season (April – May) over Kolkata (22° 32’N, 88° 20’E), India are invariably accompanied with lightning flashes, high wind gusts, torrential rainfall, occasional hail and tornadoes which significantly affect the life and property on the ground and aviation aloft. The societal and economic impact due to such storms made accurate prediction of the weather phenomenon a serious concern for the meteorologists of India. The initiation of such storms requires sufficient moisture in lower troposphere, high surface temperature, conditional instability and a source of lift to initiate the convection. Convective available potential energy (CAPE) is a measure of the energy realized when conditional instability is released. It plays an important role in meso-scale convective systems. Convective inhibition energy (CINE) on the other hand acts as a possible barrier to the release of convection even in the presence of high value of CAPE. The main idea of the present study is to see whether a consistent quantitative range of CAPE and CINE can be identified for the prevalence of such thunderstorms that may aid in operational forecast. A statistical – fuzzy coupled method is implemented for the purpose. The result reveals that a definite range of CINE within 0 – 150 Jkg-1 is reasonably pertinent whereas no such range of CAPE depicts any consistency for the occurrence of severe thunderstorms over Kolkata. The measure of CINE mainly depends upon the altitude of the level of free convection (LFC), surface temperature (T) and surface mixing ratio (q). The box-and-whisker plot of LFC, T and q are drawn to select the most dependable parameter for the consistency of CINE in the prevalence of such thunderstorms. The skills of the parameters are evaluated through skill score analyses. The percentage error during validation with the observation of 2010 is estimated to be 0% for the range of CINE and 3.9% for CAPE.

Multiscale interaction with topography and extreme rainfall events in the northeast Indian region

Flash floods associated with extreme rain events are a major hydrological disaster in the northeast Indian (NEI) region because of the unique topographic features of the region as well as increased frequency of occurrence of such events. Knowledge of the spatiotemporal distribution of these events in the region and an understanding of the factors responsible for them, therefore, would be immensely useful for appropriate disaster preparedness. Using daily rainfall data from 15 stations over the region for 32 years (1975–2006), it is shown that the frequency of occurrence of these events is largest not during the premonsoon thunderstorm season but during the peak monsoon months (June–July–August). This fact together with the fact that most of these events occur during long rainy spells indicate that the extreme events in the NEI region largely occur in association with the monsoon synoptic events rather than isolated thunderstorms. We also find that the aggregate of extreme rain events over the region has a significant decreasing trend in contrast to a recent finding of an increasing trend of such events in central India (Goswami et al., 2006). This decreasing trend of extreme events is consistent with observed decreasing trend in convective available potential energy and increasing convective inhibition energy over the region for the mentioned period. Examination of the structure of convection associated with the extreme rain events in the region indicates that they occur through a multiscale interaction of circulation with the local topography. It is found that at all the stations, the events are associated with a mesoscale structure of convection that is embedded in a much larger scale convective organization. We identify that this large‐scale organization is a manifestation of certain phases of the tropical convergence zone associated with the northward propagating active‐break phases of the summer monsoon intraseasonal oscillation. Further, it is shown that the mesoscale circulation interacting with the local topography generates southward propagating gravity waves with diurnal period. The strong updrafts associated with the gravity waves within the mesoscale organization leads to very deep convective events and the extreme rainfall. The insights provided by our study would be useful when designing models to improve the prediction of extreme events.

Convective energies in forecasting severe thunderstorms with one hidden layer neural net and variable learning rate back propagation algorithm

Thunderstorms are perennial features of India. However, the severe thunderstorms of pre — monsoon season (April–May) over Kolkata (22°32′N, 88°20′E) are of great concern for imparting devastating effect on life and property on the ground and aviation aloft. The study is thus, focused on developing one hidden layer neural network model with variable learning rate back propagation algorithm to forecast such thunderstorms. Convective available potential energy (CAPE) and convective inhibition energy (CIN) are selected as the input parameters of the model after the estimation of various skill scores like, Probability of Detection (POD), False Alarm Ratio (FAR), Heidke Skill Score (HSS) and Odds Ratio Skill Score (Yule’s Q) on different stability indices. During training the model, the squared error for forecasting severe thunderstorms is observed to be 0.0022 when the values of CIN within the range of 0 to 140 J kg−1 is taken as the input whereas the error is observed to be 0.0114 while the values of CAPE within the range of 2000 to 7000 J kg−1 is considered as the input. The values of CIN and CAPE at twelve to six hours prior to the occurrence of severe thunderstorms are considered in this study. During validation of the model, the percentage of prediction error with the values of CIN as input is observed to be 0.042% and that with CAPE as input is 0.162%. The values of CIN within the range of 0–140 J kg−1 are observed to be more persistent in forecasting severe thunderstorms over Kolkata than the values of CAPE within the range of 2000–7000 J kg−1.

Sensitivity of a modelled life cycle of a mesoscale convective system to soil conditions over West Africa

AbstractCloud‐resolving real‐case simulations initialized with European Centre for Medium‐range Weather Forecasts (ECMWF) analysis data were performed to investigate the sensitivity of a mesoscale convective system (MCS) to soil properties. Four scenarios were investigated: original ECMWF soil moisture (MOI); ECMWF soil moisture reduced by 35% (CTRL), which better fits with the satellite observations; homogeneous soil type and soil moisture (HOM); and homogeneous soil type with a north–south oriented drier band of two degrees width (BAND).Initiation of convection was observed in all experiments. The initiation area was characterised by very low convective inhibition (CIN) and high convective available potential energy (CAPE). The simulations showed some evidence that convection was initiated in the vicinity of orography and along soil moisture inhomogeneities. Except for the MOI run, MCSs developed in all experiments. In CTRL, a weakening of the system was observed when approaching an area with reduced total water content and enhanced CIN. In BAND, convection developed almost simultaneously in the original initiation area and inside the dry band, where convergence, generated by thermally‐induced circulation and supported by downward mixing of momentum from the African Easterly Jet, triggered convection. When the mature MCS approached the band from the east it weakened, caused by increasing CIN ahead of the band. Triggering of convection on this day was favoured by drier surfaces and/or soil moisture inhomogeneities, while a mature system weakened in the vicinity of a drier surface. Thus a positive feedback between soil moisture and convection existed for a mature system whereas a negative feedback was found for triggering of convection. Copyright © 2009 Royal Meteorological Society

Sensitivity of Monthly Convective Precipitation to Environmental Conditions

Abstract Identifying dynamical and physical mechanisms controlling variability of convective precipitation is critical for predicting intraseasonal and longer-term changes in warm-season precipitation and convectively driven large-scale circulations. On a monthly basis, the relationship of convective instability with precipitation is examined to investigate the modulation of convective instability on precipitation using the Global Historical Climatology Network (GHCN) and NCEP–NCAR reanalysis for 1948–2003. Three convective parameters—convective inhibition (CIN), precipitable water (PW), and convective available potential energy (CAPE)—are examined. A lifted index and a difference between low-tropospheric temperature and surface dewpoint are used as proxies of CAPE and CIN, respectively. A simple correlation analysis between the convective parameters and the reanalysis precipitation revealed that the most significant convective parameter in the variability of monthly mean precipitation varies by regions and seasons. With respect to region, CIN is tightly coupled with precipitation over summer continents in the Northern Hemisphere and Australia, while PW or CAPE is tightly coupled with precipitation over tropical oceans. With respect to seasons, the identity of the most significant convective parameter tends to be consistent across seasons over the oceans, while it varies by season in Africa and South America. Results from GHCN precipitation data are broadly consistent with reanalysis data where GHCN data exist, except in some tropical areas where correlations are much stronger (and sometimes signed differently) with reanalysis precipitation than with GHCN precipitation.

Premonsoon estimates of convective available potential energy over the oceanic region surrounding the Indian subcontinent

Convective available potential energy (CAPE) and convective inhibition energy (CIN) are important parameters in determining the stability of the atmosphere for moist convection. This paper presents the estimates of CAPE and CIN during the premonsoon season over the oceanic region surrounding the Indian subcontinent. The high‐resolution radiosonde data used in this study were collected as a part of the Integrated Campaign for Aerosols gases and Radiation Budget (ICARB; March–May 2006), which covered the Bay of Bengal, Arabian Sea, and parts of North Indian Ocean. We discuss the spatiotemporal variability of CAPE and CIN during the premonsoon period and investigate the role of boundary layer, as well as free tropospheric parameters in controlling the CAPE and CIN values. During the convective event of 9 April the sensors on board the ship recorded 4 mm of rain and an overall reduction of CAPE by 620 J kg−1was seen. This corroborates with the concept that CAPE generated by the nonconvective processes is consumed by the convection for its intensification. However, the observed reduction in CAPE after this convective event is much less compared to the monsoon season reported elsewhere. CIN was found to be anticorrelated with the free convection depth (FCD), which is the distance through which the parcel ascends by its own buoyancy. Thus the variability in CAPE and CIN is found to be interlinked through the FCD. Apart from this, contribution to total CAPE from various levels are also estimated, which shows that the CAPE in the middle levels contributes most toward the total CAPE. Our investigations show that although the CAPE and CIN are related to the tropospheric parameters like temperature lapse rate, the variability in CAPE and CIN is essentially determined by the moisture in the atmospheric boundary layer. As the equivalent potential temperature (θE) in the ABL increases, CAPE increases, favoring the development of convection.

Impacts of urban expansion and future green planting on summer precipitation in the Beijing metropolitan area

In this study, an analysis of long‐term rainfall data reveals that the rapid urban expansion in Beijing since 1981 is statistically correlated to summer rainfall reduction in the northeast areas of Beijing from 1981 to 2005. This coincides with the period in which the shortage of water in the Beijing area has become a serious factor for sustainable economic development. Meanwhile, an analysis of the aerosol optical depth (AOD) from the Total Ozone Mapping Spectrometer spanning the years from 1980 to 2001 shows that there is no clear secular trend in summer AOD in Beijing. With the particular purpose of further understanding the effects of urban expansion on summer rainfall and the potential measures to mitigate such effects, a mesoscale weather/land‐surface/urban–coupled model along with different urban land‐use change scenarios are used to conduct numerical simulations for two selected heavy summer rainfall events with different, but representative, summer weather patterns in Beijing. Results show that urban expansion can produce less evaporation, higher surface temperatures, larger sensible heat fluxes, and a deeper boundary layer. This leads to less water vapor, more mixing of water vapor in the boundary layer, and hence less (more) convective available potential energy (convective inhibition energy). The combination of these factors induced by expanding urban surfaces is helpful in reducing precipitation for the Beijing area in general and, in particular, for the Miyun reservoir area (the major source for the local water supply). Increasing green vegetation coverage in the Beijing area would produce more rainfall, and model results show that planting grass seems more effective than planting trees. For the same vegetation, the rainfall difference from simulations using two green‐planting layouts (annular and cuneiform) is small.

The Applicability of Bipartite Graph Model for Thunderstorms Forecast over Kolkata

Single Spectrum Bipartite Graph (SSBG) model is developed to forecast thunderstorms over Kolkata(22∘32′N,88∘20′E)during the premonsoon season (April-May). The statistical distribution of normal probability is observed for temperature, relative humidity, convective available potential energy (CAPE), and convective inhibition energy (CIN) to quantify the threshold values of the parameters for the prevalence of thunderstorms. Method of conditional probability is implemented to ascertain the possibilities of the occurrence of thunderstorms within the ranges of the threshold values. The single spectrum bipartite graph connectivity model developed in this study consists of two sets of vertices; one set includes two time vertices (00UTC, 12UTC) and the other includes four meteorological parameters: temperature, relative humidity, CAPE, and CIN. Three distinct ranges of maximal eigen values are obtained for the three categories of thunderstorms. Maximal eigenvalues for severe, ordinary, and no thunderstorm events are observed to be(2.6±0.12),(1.88±0.09), and(1.26±.03), respectively. The ranges of the threshold values obtained using ten year data (1997–2006) are considered as the reference range and the result is validated with the IMD (India Meteorological Department) observation, Doppler Weather Radar (DWR) Products, and satellite images of 2007. The result reveals that the model provides 12- to 6-hour forecast (nowcasting) of thunderstorms with 96% to 98% accuracy.

Aspects of the diurnal cycle in a regional climate model

We analyze the ability of the regional climate model CLM to simulate the European summer climate with particular consideration of mean diurnal cycles of precipitation, surface variables, convective indices and vertical temperature and humidity profiles. We consider three sets of simulations using different convective parametrizations (Tiedtke, Tiedtke-CAPE and Kain-Fritsch), each covering 6 summer seasons (April-September) driven by the ERA-40 reanalysis. Analysis shows that the afternoon peak of precipitation occurs 3-7 hours too early in the model. This may be linked to the underestimation of convective inhibition energy (CIN). We also observe an underestimation of the diurnal temperature range and a cold bias of about 1-3 K, resulting in a too shallow boundary layer. In addition, the boundary layer is found to be too well mixed and the night-time inversion to be strongly underestimated. Finally, the diurnal evolution of Iberian profiles is found to be influenced by the Iberian thermal low in both model and observations: Between 850 and 750 hPa, the temperatures are lower at noon than at midnight. As an explanation, we propose night-time subsidence heating in layers above 900 hPa.

Selected instability indices in Europe

A climatology of various parameters associated with severe weather and convective storms has been created for Europe that involves using radiosounding data collected at the University of Wyoming for the period from 1991 to 2005. The analysis is based on monthly means, frequency distributions of such parameters as convective available potential energy (CAPE), convective inhibition energy (CIN), KI - index, total totals index (TTI), and the severe weather threat index (SWEAT). Monthly average CAPE values exceeding 300 Jkg−1 are observed over the west Mediterranean Sea and the neighboring coastal countries. The similar seasonal cycle and spatial distributions exhibit CIN fields with summer monthly means above 100 Jkg−1 observed on the south part of the researched domain. The KI, TTI, and SWEAT indices, which assess both the lapse ratio between 850 and 500 hPa and low level humidity, show the privileged region (the Alpine area and the Carpathian Basin) with the highest instability conditions. Orography clearly plays an important role in this structure. Farther from this area, the monthly average decreases to the east, west, north, and south of the research domain. Ward’s procedure was applied to create objective regionalization according to instability conditions. This method tends to produce two regions with relatively different instability conditions and few subregions with similar conditions. The first region, covering the Alpine area, the west Mediterranean Sea, west Turkey and the southern Ukraine, is characterized by the highest instability. The rest of the investigated area is the second region with a more stable atmosphere.

Analysis of large‐scale conditions associated with convection over the Indian monsoon region

AbstractThis paper examines the thermodynamics of the atmosphere in relation to occurrence of convective rainfall over the Indian region. Various thermodynamical and kinematic parameters such as convective available potential energy (CAPE), convective inhibition energy (CINE), equivalent potential temperature and relative vorticity field are computed based on model analysis field of the limited area model of India Meteorological Department. The data period of this study is from 15 March 2001 to 28 February 2002. The study shows that the spatial distribution of CAPE in pre‐monsoon and monsoon season exhibits a belt of high CAPE along the east coast extending north up to Gangetic West Bengal and another zone of high CAPE along the southwest coast. During post‐monsoon and winter seasons, the belt of maximum CAPE shifts over the extreme southern Peninsula. The annual variation of CAPE over all stations is bi‐modal in nature with twin peaks each at the beginning and at the end of the monsoon season. Magnitude of CINE on the other hand is nearly zero during the monsoon season. The result shows that presence of strong thermodynamic environment is not sufficient for the occurrence of deep convection. Factors like proper dynamic conditions play a very important role in controlling the occurrence of deep convection. The surface warming during the pre‐monsoon season destabilizes the atmosphere, but large‐scale moist convection, generally, does not take place until CINE becomes close to zero (in the monsoon season). Preceding the monsoon season, the land sea contrast heating strengthens circulation in the lower atmosphere, which in turn leads to the moistening and destabilization of lapse rate. This process starts about a month prior to the actual start of the monsoon rainfall, allowing moisture to build up to lead to the widespread monsoon rainfall. During winter months, rainfall occurs over northwest India in association with high negative value of CINE and over southern Peninsula in association with high value of CAPE. Substantial changes are noticed in the equivalent potential temperature (θe) and vorticity profiles between days with and without rainfall over northwest India (Delhi). The results suggest that the increase in moisture in association with the occurrence of rainfall over Delhi is of advective origin. Where as, other regions (Kolkata, Mumbai and Thiruvananthapuram) display relatively smaller changes in the vertical profiles of θe, indicating an in situ presence of moisture. Copyright © 2007 Royal Meteorological Society

European climatology of severe convective storm environmental parameters: A test for significant tornado events

A climatology of various parameters associated with severe convective storms has been constructed for Europe. This involves using the reanalysis data base from ERA-40 for the period 1971–2000 and calculating monthly means, variability range and extremes occurrence of fields such as convective available potential energy, convective inhibition energy, mid-tropospheric lapse rate, low-tropospheric moisture content and storm relative helicity for different layers. This process is a first step towards development of a synthetic climatology of European severe weather, and is publicly available at the web site http://ecss.uib.es. Preliminary results derived from these products were presented during the ECSS 2004 conference. This paper is devoted to a more detailed presentation and discussion of the main results. It is hypothesized that preferred areas for severe thunderstorms occurrence in Europe would extend along a zonal belt over the south-central regions, where high helicity associated with the extratropical storm tracks and thermodynamically-favourable profiles established over the southern Atlantic and Mediterranean Sea would most likely be concatenated. Further, this effort has been complemented with a collection of existing reports of significant (at least F2) tornadoes in Europe during the period 1971–2003. We present this data set in this paper and it also can be found at the website http://ecss.uib.es. Thus, the tornado collection can be used to test the appropriateness of the parameters selected for the synthetic climatology. In particular, it is found that the convective available potential energy, low-tropospheric moisture content and environmental shear, when related to the monthly climatology, are reasonably good descriptors of the tornadic environments.

Towards a European climatology of meteorological parameters associated to the genesis of severe storms

In this paper, we analyse the relevance of some meteorological parameters to determine the presence of severe storms within the European domain. The exact values that these parameters take during a case of severe weather are not considered the most important matter, but rather the relation they have when being compared with local climatologies is taken into account. To do so, a list of events which occurred between 1970 and 2005 within the European domain is used, and they show how the CAPE (Convective Available Potential Energy), the contents of water vapour up to 850 hPa, the temperature at 850 hPa and the sea level pressure are very related to the severe weather phenomena; whereas others, such as the CAPEN (Convective Inhibition Energy), the temperature change between 700 and 500 hPa, the helicities relative to the storm and the geopotential height at 500 hPa, are less relevant.

Pre-convective environment of pre-monsoon thunderstorms around Chennai – A thermodynamical study

Pre-convective environment around Chennai during March – May 2003 has been studied using 0000 UTC upper air (RS/RW) and 0600 and 0900 UTC surface meteorological data. The study revealed the following results : (i) prevalence of positive (negative) dew point anomaly at 850 hPa, convective instability exceeding –6° K/km (convective stability, i.e., lapse rate less than –3° K/km) between 1000 and 700 hPa and positive (negative) relative humidity anomaly in the layer 850-500 hPa at 0000 UTC are associated with strong (no) convection (ii) SSEly to NNWly at 850 hPa is favourable for strong convection whereas ENEly to SSEly winds at 850 hPa are associated with no convection (iii) George’s K index, Deep Convective Index and Showalter’s stability index show better forecasting skill over the method of persistency (iv) Galway’s lifted index, SWEAT index, humidity index and Boyden index do not have forecasting skill over persistency and hence they are considered unsuitable for forecasting thunderstorm during pre-monsoon season (March – May) over Chennai and (v) forecast based on 0000 UTC convective available potential energy (CAPE) and convective inhibition energy (CINE) does not throw any light in improving our forecasting skill.

Evolution of the vertical structure of the atmosphere over North-West India during winter season

This paper examines the structural changes of the atmosphere in relation to the passage of Western Disturbances (WD) over Delhi, during winter season using data for the period December 2001 to February 2002. Various thermodynamic and dynamic parameters such as Convective Available Potential Energy (CAPE), Convective Inhibition Energy (CINE), precipitable water content and vorticity over Delhi have been computed based on model analysis fields of India Meteorological Department. Although, a clear increase in the value of CAPE is noticed on most of the days of occurrence of rainfall over the station, this is not a necessary condition for the occurrence of rainfall nor is the amount of rainfall very well correlated to the value of CAPE. The vertical profile of the daily specific humidity reveals that each spell of disturbed weather is preceded and accompanied by deep positive moisture anomalies over the station. The equivalent potential temperature profile shows that this sudden increase in moisture associated with the passage of WD is due to advection of moisture over the station. This again is validated by the synoptic charts, which demonstrate the moisture feed in the boundary layer from the Arabian Sea. The rainfall amount is not very well correlated to the thermodynamic parameters considered. However, rainfall amount is found to have a stronger dependence on positive vorticity field of the lower troposphere. This reflects the stronger dynamic forcing of the rainfall activity over the station. The northward tilt of the ridge line in the lower troposphere in association with rainfall demonstrates that the rainfall activity over the station is primarily caused by the induced surface low in association with the upper level troughs of a WD. Hence the trigger for the rainfall activity over the station and hence this region, during this season is primarily dynamic in nature and local convective structure of the atmosphere does not play a very determining role in the entire process.

The influence of the land surface on the transition from dry to wet season in Amazonia

Analysis of the fifteen years of European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis suggests that the transition from dry to wet season in Southern Amazonia is initially driven by increases of surface latent heat flux. These fluxes rapidly reduce Convective Inhibition Energy (CINE) and increase Convective Available Potential Energy (CAPE), consequently providing favourable conditions for increased rainfall even before the large-scale circulation has changed. The increase of rainfall presumably initiates the reversal of the cross-equatorial flow, leading to large-scale net moisture convergence over Southern Amazonia. An analysis of early and late wet season onsets on an interannual scale shows that a longer dry season with lower rainfall reduces surface latent heat flux in the dry and earlier transition periods compared to that of a normal wet season onset. These conditions result in a higher CINE and a lower CAPE, causing a delay in the increase of local rainfall in the initiating phase of the transition and consequently in the wet season onset. Conversely, a wetter dry season leads to a higher surface latent heat flux and weaker CINE, providing a necessary condition for an earlier increase of local rainfall and an earlier wet season onset. Our results imply that if land use change in Amazonia reduces rainfall during dry and transition seasons, it could significantly delay the wet season onset and prolong the dry season.