Convective Index Research Articles

Quantifying the Accuracy and Uncertainty of Diurnal Thermodynamic Profiles and Convection Indices Derived from the Atmospheric Emitted Radiance Interferometer

AbstractWhile radiosondes have provided atmospheric scientists an accurate high-vertical-resolution profile of the troposphere for decades, they are unable to provide high-temporal-resolution observations without significant recurring expenses. Remote sensing technology, however, has the ability to monitor the evolution of the atmosphere in unprecedented detail. One particularly promising tool is the Atmospheric Emitted Radiance Interferometer (AERI), a passive ground-based infrared radiometer. Through a physical retrieval, the AERI can retrieve the vertical profile of temperature and humidity at a temporal resolution on the order of minutes. The synthesis of these two instruments may provide an improved diagnosis of the processes occurring in the atmosphere. This study provides a better understanding of the capabilities of the AERI in environments supportive of deep, moist convection. Using 3-hourly radiosonde launches and thermodynamic profiles retrieved from collocated AERIs, this study evaluates the accuracy of AERI-derived profiles over the diurnal cycle by analyzing AERI profiles in both the convective and stable boundary layers. Monte Carlo sampling is used to calculate the distribution of convection indices and compare the impact of measurement errors from each instrument platform on indices. This study indicates that the nonintegrated indices (e.g., lifted index) derived from AERI retrievals are more accurate than integrated indices (e.g., CAPE). While the AERI retrieval’s vertical resolution can inhibit precise diagnoses of capping inversions, the high-temporal-resolution nature of the AERI profiles overall helps in detecting rapid temporal changes in stability.

Diurnal Variation of Summer Precipitation across the Central Tian Shan Mountains

AbstractThe climatic features of the diurnally varying summer precipitation over and around the central Tian Shan Mountains are investigated. Both the hourly rainfall data observed at eight stations along a transect across the mountains and the convective index derived from the satellite data show that there are three distinct regimes: the early morning peak at stations to the south of the mountains, the late afternoon peak at stations on the mountains, and the night peak at stations to the north of the mountains. The relation between regimes of diurnal variation is analyzed. By defining the regional rainfall event (RRE), the initial stations of each RRE are recorded. The early morning rainfall in the southern periphery of the mountains is triggered locally in the southern basin. Both the late afternoon peak over the mountains and the night peak in the northern periphery are influenced by mountain-originated rainfall events. These rainfall events appear over the mountains in the afternoon, and some of them move northward and lead to the nocturnal rainfall in the northern basin. The triggering of convection in the afternoon over the mountains and that in the early morning in the southern basin is related to the diurnally varying wind and thermodynamic conditions over and around the mountains. Low-level convergence with thermodynamic instability appears at noon (night) over the mountains (in the southern basin) just before the start of the convection.

Unstable convection flow as a controlling process of increasing salinity in the saline lake coastal aquifers: case of Maharlu Lake coastal aquifer

Deterioration of freshwater aquifers adjacent to saline lakes is a serious environmental and water resources management problem. Generally, different phenomena including saltwater intrusion, upconing, free and forced convection flow can lead to the salinization of fresh aquifers in the vicinity of saline lakes. The severity of this problem depends on hydrogeological and hydrogeochemical conditions of coastal saline lake aquifers. Accordingly, Maharlu Lake in the southeast of Shiraz plain is one of these saline lakes and has an important impact on its adjacent fresh aquifers so that the salinity of these aquifers has increased in recent years. Since the lake has special conditions such as very flat and low permeable floor and variable water level fluctuation during a year, with the onset of the rainy season after a long-term well pumping in the adjacent aquifer, the lake water covers a wide extent of the lake area in a short time. Then, because of the higher permeability of the surface of the lake in its southeast margin, conditions for convection flow are provided there. In this study, salinity distribution in one of the aquifers (Bokat aquifer) in the southeast of the lake is investigated using resistivity tomography along several profiles. Interpretation of inverse models of these profiles showed that convection flow is an important process in increasing the salinity of fresh aquifer. Comparison of the results of geo-tomography sections with convection index map of the Bokat aquifer showed that getting closer to the lake, free convection flow becomes the dominant process in the salinity distribution. In addition, in farther distances relative to the lake, due to increasing the pumping rate of the extraction wells, forced convection flow is an important process.

Correlations of Multispectral Infrared Indicators and Applications in the Analysis of Developing Convective Clouds

AbstractThree infrared (IR) indicators were included in this study: the 10.8-μm brightness temperature (BT10.8), the BT difference between 12.0 and 10.8 μm (BTD12.0–10.8), and the BT difference between 6.7 and 10.8 μm (BTD6.7–10.8). Correlations among these IR indicators were investigated using MTSAT-1R images for summer 2007 over East Asia. Temporal, spatial, and numerical frequency distributions were used to represent the correlations. The results showed that large BTD12.0–10.8 values can be observed in the growth of cumulus congestus and associated with the boundary of different terrain where convection was more likely to generate and develop. The results also showed that numerical correlation between any two IR indicators could be expressed by two-dimensional histograms (HT2D). Because of differences in the tropopause heights and in the temperature and water vapor fields, the shapes of the HT2Ds varied with latitude and the type of underlying surface. After carefully analyzing the correlations among the IR indicators, a conceptual model of the convection life cycle was constructed according to these HT2Ds. A new cloud convection index (CCI) was defined with the combination of BTD12.0–10.8 and BTD6.7–10.8 on the basis of the conceptual model. The preliminary test results demonstrated that CCI could effectively identify convective clouds. CCI value and its time trend could reflect the growth or decline of convective clouds.

Meteorological and hydrological analysis of major floods in NE Iberian Peninsula

This paper analyses the meteorological conditions and the specific peak flows of 24 catastrophic floods that affected NE Iberian Peninsula in the period 1842–2000. We classify these floods according to the affected area, peak flow magnitude, and damages. Additionally, the NOAA 6 Hourly 20th Century V2 Reanalysis Data Composites have been used to analyze the synoptic conditions during each flood and to evaluate several stability indices, such as the convective available potential energy (CAPE), or the lift index. We found a good correlation between stability indices and the season when the flood occurred. For instance, if maximum CAPE is considered, larger values are found for summer floods, moderate for autumn, and low values during winter floods. We select 5 representative episodes occurred in different seasons and areas to describe in detail the synoptic conditions and to show the temporal evolution of the stability indices. In one the summer floods analyzed in detail, the largest instability, according to all the convective indices, is found. On the contrary, the winter case shows very low values of the convective indices, and autumn cases lay in between. During the other analyzed summer flood instability was low but snow thaw played an important role in producing the flood. Regarding hydrological variables, clear differences between floods occurred at the coast or at the Pyrenees are found. Coastal specific peak flows are larger than Pyrenean ones, especially for small catchment areas. We also combine meteorological (rainfall duration, CAPE), hydrological (specific peak flow) and geomorphological (catchment area) variables to show that for many of the analyzed floods these variables are related: the specific peak flow generally shows larger values when CAPE is also large. However some differences appear depending on the season and area. In those summer floods, where snow thaw played doesn’t played any role, either Pyrenean or coastal, specific peak flow seems to be correlated with CAPE. For autumn floods, depending on the area different correlations were found: Pyrenean floods seem to be a correlation between CAPE and specific peak flows, but not for coastal ones. For winter coastal floods we couldn’t find any correlation between CAPE and specific peak flows.

An examination of convective moistening of the lower stratosphere using satellite data

AbstractIn this paper, we use satellite data to test the hypothesis that deep convection moistens the lower stratosphere. Water vapor measurements from Earth Observing System‐Microwave Limb Sounder and Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer over North America are binned according to the International Satellite Cloud Climatology Project deep convection indices. The results show that in the North American region (50–112°W, 10–50°N) the convection‐impacted samples are significantly moister than the nonimpact samples in the lowermost stratospheric layer right above the tropopause, and a drier tendency is also noticed right above this moistened layer. Trajectory modeling is used to aid the identification of deep convection‐impacted water vapor samples. However, we find that a substantial fraction of high‐concentration (>8 ppmv) samples at 100 hPa cannot be attributed to nearby deep convections.

Statistical Characteristics and Variations of Thunderstorm Frequency in South China from 1980 to 2010

By using the observed data from 20 weather stations in South China,the characteristics of diurnal,seasonal,multi-year variations and relative atmospheric physical parameters and circulation fields were demonstrated. The results show ed that the thunderstorm frequency peak from afternoon to the evening(9% at 12: 00(UTC,hereafter the same)),and least in the morning(2% at 03: 00). The higher thunderstorm frequency occurred in July and August( 35%),and the low er thunderstorm frequency in December and January( 1%).The annual and summer thunderstorm frequency decreased with a rate of-1% ·(10a)-1and-3. 5% ·(10a)-1from 1980 s to 21 thcentury,yet after 21 thcentury,it has a weak increasing trend. On the whole year,the ratio of thunderstorm precipitation and total precipitation was 48%,but in summer,the value was 64%. The variation of annual(summer) thunderstorm precipitation were relatively similar with annual(summer) thunderstorm frequency,their correlation coefficient was 0. 46(0. 71). In summer,accompany with the variation of thunderstorm frequency,the atmospheric circulation exhibits large-scale abnormally in east Asia. During higher thunderstorm frequency month,the western Pacific subtropical high was abnormally weaker. Meanw hile,there was abnormally ascending flow s in the upper-middle troposphere over the South China. In addition,from the view of thermal instability,the abnormally high total tolals index and abnormally high convective available potential energy index has significant relationship with thunderstorm frequency,their correlation coefficients were 0. 58 and 0. 76. The thunderstorm frequency has statistical relevance with the surface temperature,how ever,it requires substantial investigation to make sure that whether it is a response of thunderstorms thermal and dynamical factor to the global climate change.

Satellite-derived normalized difference convection index for typhoon observations

This study used the spectral features of the geostationary satellite infrared window channel and the water vapor channel data to calculate a new parameter, normalized difference convection index (NDCI), to help determine the overshooting areas in typhoon cloud systems and the centers and intensity of typhoons. The results showed that the two-dimensional NDCI analysis helped to identify typhoon convective cloud systems and the positions of overshooting areas. In addition, because the NDCI values near a typhoon eye were rather significant, if a typhoon eye was formed, the NDCI cross-section analysis could help to confirm its position. When the center of a typhoon was covered by the high anvils and cirrus layers, it could still be qualitatively found through two-dimensional analysis. As for determining the intensity of typhoons, this study also tried to perform correlation analyses with NDCI and maximum sustained wind speed. The result showed that in the ranges within circles of 200 to 250 km radii with a typhoon eye as the center, the correlation between the area with the NDCI values <0 and the maximum sustained wind speed is high with a coefficient 0.7. Thus, the NDCI value could be a referential index to determine the intensity of a typhoon.

Monitoring a convective winter episode of the Iberian Peninsula using a multichannel microwave radiometer

AbstractOn 4 March 2011, a heavy snowfall episode affected the central Iberian Peninsula. Under the TECOAGUA Project (aimed at the study of winter cloud masses that produce snow in the Guadarrama Mountains near Madrid), measurements using a ground‐based multichannel microwave radiometer (MMWR) with vertical range 10 km recorded this episode of winter convection embedded within stratiform precipitation. In contrast to radiosondes, data retrieval from the MMWR has a clear advantage for identifying hazardous weather phenomena of short duration, such as winter convective episodes. From these continuous measurements, we analyzed the behavior of variables such as temperature, surface pressure, relative humidity, liquid water content, liquid water path, water vapor content, and integrated water vapor throughout the day. The continuous measurements also permitted construction of skew‐T log‐P profiles every 15 min during the convective episode, indicating vertical evolution of an event with an appearance similar to a “zipper” in which temperature and dew point temperature profiles are “closed” from the surface to 400 hPa and “reopen” at the end of the event. Finally, we selected six indices of stability most suitable for the study of winter convection, namely, the Showalter index, low‐topped convection index, most unstable lifted index, most unstable convective available potential energy (MUCAPE), convective inhibition, and MUCAPE level of free convection. Each of these indices has been evaluated for their capacity to warn of meteorological conditions leading to a convective heavy snowfall event.

Dependence of IMF By penetration into the neutral sheet on IMF Bz and geomagnetic activity

AbstractThe penetration of interplanetary magnetic field (IMF) By into the magnetosphere plays an important role in determining magnetospheric configuration and its dynamics. In this paper, using 9 years of Cluster data at the center of neutral sheet, we statistically study the relations of the penetration of IMF By in the neutral sheet (NS) with IMF Bz and the Kp index. The correlation coefficient between NS By and IMF By is enhanced during the periods of southward IMF Bz and large Kp indexes. The penetration efficiency of IMF By, which is defined as the slope of the linear fit of the points in the By‐IMF By space, is larger during southward IMF Bz than during northward IMF Bz. The penetration efficiency of IMF By also increases with increasing Kp index. Since the Kp index can be considered as an index of magnetospheric convection, this means that the penetration of IMF By into the magnetosphere is enhanced during the periods of strong magnetospheric convection. These results indicate that the IMF Bz and magnetospheric convection can influence the neutral sheet By and even magnetotail dynamics by changing the penetration of IMF By.

Assessment of Convective Activity Using Stability Indices as Inferred from Radiosonde and MODIS Data

The combined use of both radiosonde data and three-dimensional satellite derived data over ocean and land is useful for a better understanding of atmospheric thermodynamics. Here, an attempt is made to study the thermodynamic structure of convective atmosphere during pre-monsoon season over southwest peninsular India utilizing satellite derived data and radiosonde data. The stability indices were computed for the selected stations over southwest peninsular India viz: Thiruvananthapuram and Cochin, using the radiosonde data for five premonsoon seasons. The stability indices studied for the region are Showalter Index (SI), K Index (KI), Lifted Index (LI), Total Totals Index (TTI), Humidity Index (HI), Deep Convective Index (DCI) and thermodynamic parameters such as Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE). The traditional Showalter Index has been modified to incorporate the thermodynamics over tropical region. MODIS data over South Peninsular India are also used for the study. When there is a convective system over south peninsular India, the value of LI over the region is less than ?4. On the other hand, the region where LI is more than 2 is comparatively stable without any convection. Similarly, when KI values are in the range 35 to 40, there is a possibility for convection. The threshold value for TTI is found to be between 50 and 55. Further, we found that prior to convection, dry bulb temperature at 1000, 850, 700 and 500 hPa is minimum and the dew point temperature is a maximum, which leads to increase in relative humidity. The total column water vapor is maximum in the convective region and minimum in the stable region. The threshold values for the different stability indices are found to agree with that reported in literature.

The Medieval Climate Anomaly and the Little Ice Age in the eastern Ecuadorian Andes

Abstract. To better characterize the climate variability of the last millennium in the high Andes, we analyzed the pollen content of a 1150-yr-old sediment core collected in a bog located at 3800 m a.s.l. in the páramo in the eastern Cordillera in Ecuador. An upslope convective index based on the ratio between cloud transported pollen from the Andean forest to the bog (T) and Poaceae pollen frequencies, related to the edaphic moisture of the páramo (P), was defined. This index was used to distinguish changes in the atmospheric moisture from the soil moisture content of the páramo and their associated patterns of interdecadal El Niño–Southern Oscillation (ENSO) variability and South American summer monsoon (SASM) activity. Results show that between 850 and 1250 AD, the Medieval Climate Anomaly interval was warm and moist with a high transported pollen/Poaceae pollen (T/P) index linked to high ENSO variability and weak SASM activity. Between 1250 and 1550 AD, a dry climate prevailed, characterized by an abrupt decrease in the T/P index and therefore no upslope cloud convection, related to lower ENSO variability and with significant impact on the floristic composition of the páramo. During the Little Ice Age, two phases were observed: first, a wet phase between 1550 and 1750 AD linked to low ENSO variability in the Pacific and warm south equatorial Atlantic sea surface temperatures (SSTs) favored the return of a wet páramo, and then a cold and dry phase between 1750 and 1800 AD associated with low ENSO variability and weak SASM activity resulted in drying of the páramo. The current warm period marks the beginning of a climate characterized by high convective activity – the highest in the last millennium – and weaker SASM activity modifying the water storage of the páramo. Our results show that the páramo is progressively losing its capacity for water storage and that the interdecadal variability of both tropical Pacific and Atlantic SSTs matter for Andean climate patterns, although many teleconnection mechanisms are still poorly understood.

Seasonal and interannual variability of an index of deep atmospheric convection over western boundary currents

AbstractThe seasonal and interannual variability of an index measuring the potential for deep (surface‐to‐tropopause) convection over the extratropical oceans is studied using reanalysis data. It is found that most of the conditional instability is concentrated over the world's western boundary currents during winter, but shifts equatorward of the currents in summer. Conditional instability is only detected over the Gulf Stream and the East Australian Current in their respective summer season.The coupled ocean–atmosphere mechanisms controlling the variability of the convective index are then studied. It is found that the convective index displays a large interannual variability, which is primarily controlled by the erratic displacements of the storm tracks. Only weak negative feedback from the oceans is singled out on short (intraseasonal) time‐scales, reflecting the stabilization of the troposphere through the development of cold sea‐surface temperature anomalies. A larger role for warm oceanic advection in destabilizing the troposphere is, however, suggested on longer (interannual and decadal) time‐scales.

Diurnal variability of stability indices observed using radiosonde observations over a tropical station: Comparison with microwave radiometer measurements

Convection plays an important role in maintaining the thermodynamic structure of the atmosphere particularly in the tropical regions and it is often associated with clouds and precipitation. In the present study we report the diurnal variation of various stability indices observed using intensive radiosonde observations made during October 2010 to October 2011 over Indian tropical region, Gadanki (13.5°N, 79.2°E). Simultaneous co-located microwave radiometer (MWR) observations collected during April–October 2011 are used for comparison. Detailed comparison between these two independent techniques has been made which will be very much useful in assessing the data from MWR for Nowcasting. In general, MWR observations show warm (cold) bias in the temperature, except at 0.5km, when compared to radiosonde observations below (above) 3–4km, assuming latter as a standard technique. In case of water vapor, MWR observations show wet (dry) bias below (above) 2–3km depending on the time. Nevertheless, very good comparison in several convection indices is noticed between the two different techniques, particularly in the trends though some differences are noticed in the amplitudes. For about 25% of time MWR is unable to estimate the Convective Available Potential Energy (CAPE) as equilibrium level is above the altitude that MWR can detect. Strong diurnal variation in CAPE and other thermodynamic parameters is noticed with maximum in the afternoon and minimum in the early morning hours in all the seasons except in winter over this tropical station.

Recent trends and variabilities of convective parameters relevant for hail events in Germany and Europe

This study investigates whether any evidence can be found to support the occurrence of alterations in the hailstorm frequency in the last few decades (1978–2009) over Germany and Europe. Due to their local-scale extent and a lack of appropriate monitoring systems, hailstorms are not captured reliably and comprehensively over long periods by current observation systems. To overcome this constraint, we consider various convective indices and parameters (CPs) derived from soundings at 1200 UTC, and we evaluate which of them are appropriate for predicting hail damage days according to loss data from a building insurance company.Most of the CPs that rely on moisture at the near-surface layers exhibit a significant positive trend towards a higher convective potential. This finding applies to the 90% (10%) percentiles of the annual distribution and to the number of days that exceed (or fall below) a specific threshold, which is suitable for hailstorm prediction. Negative and, at most stations, insignificant trends of CPs that rely on moisture at higher levels or if the initial parameters of the lifting curve were mixed over the lowest layers can be attributed to inconsistency in the time series of the dew point. This in homogeneity is caused by changes in the instrumentation around the year 1990. The investigations show considerable spatial differences in the convective potential both in the mean and the trends, with a distinct north-to-south gradient and a less marked west-to-east gradient over Europe and Germany. The spatial distribution of the trends, however, is fairly consistent among the CPs that are based on the same principles.

Development of nowcasting technique and evaluation of convective indices for thunderstorm prediction in Gangetic West Bengal (India) using Doppler Weather Radar and upper air data

Thunderstorm and hailstorm are well known short term severe weather phenomena which sometimes turn in to natural hazard especially in Gangetic West Bengal region of India. Large vertical extent of the cumulonimbus cloud, very high reflectivity, squally wind speed sometimes exceeding 100 km/h and heavy rainfall are the main features of these thunderstorms during pre-monsoon period in this region. A study of 70 thunderstorms has been carried out during the pre-monsoon season (March-May) of the year 2005 around Kolkata (22.5° N, 88.5° E) using Doppler Weather Radar and Upper air data. Standard convective indices like CAPE, CINE, LI, BRN and VGP have been evaluated and analyzed statistically. As no definite thresholds of the convective indices are available for thunderstorm prediction in this region, an attempt has been made to find threshold of these indices for possible occurrences of thunderstorms in Gangetic West Bengal region after the analysis of the thunderstorms during year 2005. The validity of these convective indices has been checked with 34 occurrences of thunderstorms during 2006-2007 recorded by Doppler Weather Radar Kolkata. The study reveals that nowcasting of thunderstorms may be done at least 2-3 hrs in advance witha fair degree of accuracy using Doppler radar products only. However, the lead time of nowcasting may be further improved if the convective indices are also analyzed and used in addition to the DWR data. A simple technique has been suggested by the authors for better prediction of thunderstorms at least three to four hours in advance.

Soil moisture impacts on convective indices and precipitation over complex terrain

The impact of soil moisture on convective precipitation, convective indices, surface energy balance components, and near-surface meteorological variables is analysed for seven intensive observation periods of the Convective and Orographically induced Precipitation Study (COPS) conducted in summer 2007 using a non-hydrostatic limited-area atmospheric prediction model. The control runs are compared to sensitivity experiments under dry (-25 %) and wet (+25 %) initial soil moisture conditions. In the wet experiment, surface fluxes produce moister and cooler boundary layers with increased equivalent potential temperatures. Furthermore, the lifting condensation level and the level of free convection are lowered for all analysed regions, even under different synoptic controls. The comparison of boundary-layer and mid-tropospheric forcing regimes reveal that the impact of soil moisture on the atmosphere is not systematically higher for boundary-layer forcing. Whereas the Bowen ratio exhibits a clear dependence on soil moisture conditions, the impact on precipitation is complex and strongly depends on convective inhibition. A considerable, but non-systematic dependence of convective precipitation on soil moisture exists in the analysed complex orography. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields.

Case study of a severe windstorm over Slovakia and Hungary on 25 June 2008

A system of thunderstorms approached the Slovakia and Hungary in the late evening hours of 25 June 2008, causing extensive damage and peak wind gusts up to 40 m/s. This study examines the macro- and mesosynoptic conditions for the windstorm using soundings, analyses, and forecasts of numerical models (ALADIN, ECMWF). A derecho-like character of the event is discussed. Meteosat Second Generation imagery and convective indices inferred from satellite and model data are used to assess the humidity distribution and the conditional instability of the thunderstorm environment. An intrusion of the environmental dry air into the convective system and intensification of downdrafts is considered to be one of the reasons for the damaging winds observed at some areas. This is supported by the radar imagery showing a sudden drop of radar reflectivity and creation of line echo wave patterns and bow echoes. A numerical simulation provided by the non-hydrostatic MM5 model indicated the development of meso-γ scale vortices embedded in the convective system. The genesis and a possible role of such vortices in creating rear-inflow jets and intensifying the low level winds are investigated with the help of the vorticity equation and several other diagnostic parameters. In addition, the effect of various physical parameterisations on the forecast of the windstorm is evaluated.

The dependence of convection‐related parameters on surface and boundary‐layer conditions over complex terrain

AbstractThe field campaign ‘Convective and Orographically‐induced Precipitation Study’ (COPS) was performed in south‐western Germany and eastern France in summer 2007. Within the COPS context this study focused on the process chain of soil moisture, surface fluxes, conditions of the convective boundary layer (CBL), and convection‐related parameters.The results were different for valley and mountain sites. Only in the Rhine valley did the ratios of sensible and latent heat to the net radiation at the surface, H0/Q0 and E0/Q0 respectively, reveal a weak dependence on soil moisture. H0/Q0 was lower and E0/Q0 was higher at higher soil moisture. The correlation of the diurnal increase of the equivalent potential temperature, Θe, with the energy supplied by H0 and E0 was found to be lower for higher surface inhomogeneity. Furthermore, only a weak dependence of the CBL depth on the sensible surface heat flux was found for valley sites and was non‐existent for the mountain crest.The convective indices in the whole COPS domain were found to depend on Θe in the CBL. The absolute values of conditional and potential instability are not necessarily the decisive parameters for convection to occur, because highest instability was observed in the Rhine valley while convection was preferably initiated over the mountains. Convective inhibition (CIN) was positively correlated with the capping strength and negatively with the CBL height: the higher the CBL, the lower the upper threshold of CIN. The frequency of low CIN was higher in the Black Forest mountains than in the Rhine valley, which facilitates convection initiation over the mountain sites. Copyright © 2011 Royal Meteorological Society

Study of thermodynamic indices in forecasting pre-monsoon thunderstorms over Kolkata during STORM pilot phase 2006–2008

The pre-monsoon convective atmosphere over Kolkata (22.52°N, 88.37°E) during STORM field phase 2006–2008 is investigated using 12 UTC radiosonde data and thermodynamic indices. In the present study, an attempt has been made to assess the skill of various indices and parameters and to propose suitable threshold values in forecasting the occurrence of thunderstorm activity at Kolkata. The thermodynamic indices and parameters used in the present study are lifted index (LI), K index (KI), severe weather threat index (SWEAT), total totals index (TTI), convective available potential energy (CAPE), deep convection index (DCI), humidity index (HI), Boyden index (BI), dew point temperature at 850 hpa (DEW), relative humidity at 700 hpa (RH), and bulk Richardson number (BRN). Validation of the suggested threshold values of indices was conducted on the days of thunderstorm activity. It was found that one index alone cannot predict the occurrence of thunderstorm over Kolkata region. The present study suggests that the indices with highest skill for thunderstorm prediction are KI, DCI, SWEAT, DEW, HI, RH, LI, TTI, while the prediction efficiency is poor for CAPE, BRN, and BI. Observed values of these indices also reveal that scattered, multi-cellular thunderstorms are possible over Kolkata during pre-monsoon months.