Showing papers on "Convective available potential energy published in 2008"

The Impact of Convection on ENSO: From a Delayed Oscillator to a Series of Events

Abstract: The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Nino–Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing “short-circuited” delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a...

The dependence of aerosol effects on clouds and precipitation on cloud-system organization, shear and stability

Abstract: Precipitation suppression due to an increase of aerosol number concentration in stratiform cloud is well-known. It is not certain whether the suppression applies for deep convection. Recent studies have suggested increasing precipitation from deep convection with increasing aerosols under some, but not all, conditions. Increasing precipitation with increasing aerosols can result from strong interactions in deep convection between dynamics and microphysics. High cloud liquid, due to delayed autoconversion, provides more evaporation, leading to more active downdrafts, convergence fields, condensation, collection of cloud liquid by precipitable hydrometeors, and precipitation. Evaporation of cloud liquid is a primary determinant of the intensity of the interactions. It is partly controlled by wind shear modulating the entrainment of dry air into clouds and transport of cloud liquid into unsaturated areas. Downdraft-induced convergence, crucial to the interaction, is weak for shallow clouds, generally associated with low convective available potential energy ( CAPE). Aerosol effects on cloud and precipitation can vary with CAPE and wind shear. Pairs of idealized numerical experiments for high and low aerosol cases were run for five different environmental conditions to investigate the dependence of aerosol effect on stability and wind shear. In the environment of high CAPE and strong wind shear, cumulonimbus- and cumulus-type clouds were dominant. Transport of cloud liquid to unsaturated areas was larger at high aerosol, leading to stronger downdrafts. Because of the large vertical extent of those clouds, strong downdrafts and convergence developed for strong interactions between dynamics and microphysics. These led to larger precipitation at high aerosol. Detrainment of cloud liquid and associated evaporation were less with lower CAPE and wind shear, where dynamically weaker clouds dominated. Transport of cloud liquid to unsaturated areas was not as active as in the environment of high CAPE and strong shear. Also, evaporatively driven differences in downdrafts at their level of initial descent were not magnified in clouds with shallow depth as much as in deep convective clouds as they accelerated to the surface over shorter distances. Hence the interaction between dynamics and microphysics was reduced, leading to precipitation suppression at high aerosol. These results demonstrate that increasing aerosol can either decrease or increase precipitation for an imposed large-scale environment supporting cloud development. The implications for larger-scale aspects of the hydrological cycle will require further study with larger-domain models and cumulus parameterizations with advanced microphysics. (Less)

Uplift of Saharan dust south of the intertropical discontinuity

Abstract: In situ observations from a flight made during the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation (GERBILS) field campaign (June 2007) show significant dust uplift into the monsoon flow immediately south of the intertropical discontinuity in the western Sahara. Dust loadings were highest in the moist monsoon air and the observations are consistent with dust uplift by the nocturnal monsoon winds. There is some evidence that cold pools within the monsoon flow contributed to the dust uplift: regions of elevated dust, water vapor, and ozone within the monsoon air are consistent with precipitation cooling and moistening air from upper levels and the resultant dusty cold pools propagating northward. However, only southward propagating cold pool outflows could be observed in satellite imagery. Using European Centre for Medium-Range Weather Forecasts analyses and satellite data, it is shown that the asymmetry in the seasonal dust cycle is closely related to the downdraft convective available potential energy (DCAPE) from convective storms. There is both more dust and more DCAPE during monsoon onset than during retreat. The larger DCAPE values during monsoon onset, as well as the stronger nocturnal monsoon flow and the stronger heat trough circulation, are expected to contribute to the higher dust loadings at this time. Both the monsoon flow and cold pool outflows within it result in dust uplift in the western Sahara during the monsoon onset, which is when the maximum dust uplift occurs. For dust modeling, this shows the importance of accurately modeling not only the monsoon flow itself but also deep convection and cold pools.

An Analysis of the Soil Moisture Feedback on Convective and Stratiform Precipitation

Abstract: Land–atmosphere interactions in midlatitude continental regions are particularly active during the warm season. It is still unclear whether and under what circumstances these interactions may involve positive or negative feedbacks between soil moisture conditions and rainfall occurrence. Assessing such feedbacks is crucially important to a better understanding of the role of land surface conditions on the regional dynamics of the water cycle. This work investigates the relationship between soil moisture and subsequent precipitation at the daily time scale in a midlatitude continental region. Sounding data from 16 locations across the midwestern United States are used to calculate two indices of atmospheric instability—namely, the convective available potential energy (CAPE) and the convective inhibition (CIN). These indices are used to classify rainfall as convective or stratiform. Correlation analyses and uniformity tests are then carried out separately for these two rainfall categories, to asse...

Connections between Potential Vorticity Intrusions and Convection in the Eastern Tropical Pacific

Abstract: The connections between intrusions of stratospheric air into the upper troposphere and deep convection in the tropical eastern Pacific are examined using a combination of data analysis, potential vorticity (PV) inversion, and numerical simulations. Analysis of NCEP–NCAR reanalyses and satellite measurements of outgoing longwave radiation during intrusion events shows increased cloudiness, lower static stability, upward motion, and a buildup of convective available potential energy (CAPE) at the leading edge of the intruding tongue of high PV. Potential inversion inversion calculations show that the upper-level PV makes the dominant contribution to the changes in the quantities that characterize convection. This supports the hypothesis that upper-level PV anomalies initiate and support convection by destabilizing the lower troposphere and causing upward motion ahead on the PV tongue. The dominant role of the upper-level PV is confirmed by simulations using the fifth-generation Pennsylvania State U...

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

Abstract: This 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

Extreme Helicity and Intense Convective Towers in Hurricane Bonnie

Abstract: Helicity was calculated in Hurricane Bonnie (1998) using tropospheric-deep dropsonde soundings from the NASA Convection and Moisture Experiment. Large helicity existed downshear of the storm center with respect to the ambient vertical wind shear. It was associated with veering, semicircular hodographs created by strong, vortex-scale, radial-vertical flow induced by the shear. The most extreme values of helicity, among the largest ever reported in the literature, occurred in the vicinity of deep convective cells in the downshear-left quadrant. These cells reached as high as 17.5 km and displayed the temporal and spatial scales of supercells. Convective available potential energy (CAPE) averaged 861 J/kg downshear, but only about one-third as large upshear. The soundings nearest the deep cells were evaluated using two empirical supercell parameters that make use of CAPE, helicity, and/or shear. These parameters supported the possible existence of supercells as a consequence of the exceptional helicity combined with moderate but sufficient CAPE. Ambient vertical wind shear exceeded 12 m/s for 30 h, yet the hurricane maintained 50 m/s maximum winds. It is hypothesized that the long-lived convective cells enabled the storm to resist the negative impact of the shear. Supercells in large-helicity, curved-hodograph environments appear to provide a useful conceptual model for intense convection in the hurricane core. Helicity calculations might also give some insight into the behavior of vortical hot towers, which share some characteristics with supercells.

Observed vertical structure of tropical oceanic clouds sorted in large-scale regimes

Abstract: [1] The CloudSat cloud water content (CWC) profiles are sorted by a number of large-scale parameters obtained from reanalysis and satellite observations, including 500 hPa vertical velocity, sea surface temperature and its gradient, surface divergence, precipitation, water vapor path, convective available potential energy and lower tropospheric static stability. The sorting is physics-based and phenomenon-oriented. We find different degrees of clustering of cloud vertical structure in various large-scale regimes. The dominant modes are the deep and shallow clouds with peak CWC above 7 km and below 2 km, respectively, corresponding to distinctly different large-scale regimes. A middle-level peak of CWC around 5–7 km is discernible associated with the large-scale conditions similar to the shallow clouds. This study provides the first quantitative and comprehensive view of tropical CWC distributions in large-scale regimes. These results offer insights into cloud parameterizations and serve as new observational metrics for evaluation of cloud simulations in models.

Structure and Formation Mechanism on the 24 May 2000 Supercell-Like Storm Developing in a Moist Environment over the Kanto Plain, Japan

Abstract: The structure and formation mechanism of a supercell-like storm in a moist environment below a melting layer were investigated using dual-Doppler radar analysis and a cloud-resolving storm simulator (CReSS). The supercell-like storm developed over the Kanto Plain, Japan, on 24 May 2000. The environment of the supercell-like storm possessed large convective available potential energy (1000 J kg−1), strong vertical wind shear (4.2 × 10−3 s−1 between the surface and 5 km above sea level), and a moist layer (the relative humidity was 60%–90% below a melting layer at 3 km in height). The dual-Doppler radar analysis with a variational method revealed that the supercell-like storm had similar structures to those of a typical supercell in a dry environment below a melting layer, such as that in the Great Plains in the United States. The structures included a hook echo, an overhanging echo structure, and a strong updraft with strong vertical vorticity. However, some of the characteristics of the supercell...

Atmosphere-ocean conditions jointly guide convection of the Boreal Summer Intraseasonal Oscillation: Satellite observations

Abstract: The water-vapor and air-temperature profiles from the Atmospheric Infrared Sounder (AIRS), in combination with surface wind from Quick Scatterometer (QuikSCAT) and rainfall and sea surface temperature (SST) from Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI), are used to document surface conditions and vertical moist thermodynamic structures of the 2003-2006 Boreal Summer Intraseasonal Oscillation (BSISO) over the Indo-Pacific warm pool. The composite based on Wheeler and Hendon's intraseasonal oscillation index reveals that Convective Available Potential Energy (CAPE), SST, and surface convergence lead convection in both northward and eastward directions. The preconditioning of CAPE is much earlier than that of SST, implying that the atmosphere internal processes precondition CAPE. On the other hand, the ocean positively feeds back to the atmosphere from bottom up, forming a smooth transition from boundary layer moistening, shallow convection at lower or middle level, to the deep convection all through the troposphere. The preconditioning of the boundary layer moist (dry) anomalies to the subsequent positive (negative) rainfall maximum is as far as 60-90 degrees in longitude (15 degrees in latitude) and quarter-to-half cycle in time. In contrast, this boundary layer preconditioning is virtually undetected from conventional NCEP reanalysis. Finally, the implications of these new findings on the frictional "convective interaction with dynamics'' (CID) theory of intraseasonal oscillation are also discussed.

Vertical attributes of precipitation systems in West Africa and adjacent Atlantic Ocean

Abstract: This study reports the findings of TRMM (Tropical Rainfall Measuring Mission) satellite data analyses undertaken to investigate differences in intensity and depth of precipitating systems in the transition region from continental to maritime environments in West Africa during the rainy season of June to September in 1998–2004. The results of this study are interpreted in the context of regional thermodynamic variables such as equivalent potential temperature and equivalent convective available potential energy to discern the processes governing storm development. Over continental West Africa, convective-type precipitating storms exhibit a substantially larger vertical extent compared to the ones over the eastern Atlantic Ocean. In contrast, the stratiform precipitating systems show similar vertical reflectivity patterns, depth and intensity over both land and adjacent ocean in West Africa. The differences in the attributes of storms, as they move from the continent to the ocean, can be partly explained in terms of the surface-atmosphere interactions that provide the necessary transports of energy and water vapor from the surface to the cloud layer.

Convective inhibition beneath an upper-level PV anomaly

Abstract: Upper-level potential-vorticity (PV) anomalies reduce the convective stability of the troposphere through their impact on the vertical potential-temperature profile, thus reducing convective inhibition (CIN) and increasing convective available potential energy. Here, by contrast, we show the impact of a layer of stable air that was intrinsically linked with an upper-level PV anomaly and that increased CIN. This layer descended and tracked beneath the small upper-level PV anomaly, which in this case was a shallow upper-level trough. This low-humidity, relatively high-PV layer originated from the tropopause fold, generated by a breaking Rossby wave, which also produced the upper-level PV anomaly two days later. Despite conditions favourable for deep convection (as demonstrated by the development of a single storm), the CIN produced by this dry layer or lid was largely responsible for capping convection over much of southern England at around 2.5 km during the case presented here, which comes from the Convective Storm Initiation Project. Copyright (c) 2008 Royal Meteorological Society.

Surface Boundaries of the Southern Plains: Their Role in the Initiation of Convective Storms

Abstract: The nature of the different types of surface boundaries that appear in the southern plains of the United States during the convectively active season is reviewed. The following boundaries are discussed: fronts, the dryline, troughs, and outflow boundaries. The boundaries are related to their environment and to local topography. The role these boundaries might play in the initiation of convective storms is emphasized. The various types of boundary-related vertical circulations and their dynamics are discussed. In particular, quasigeostrophic and semigeostrophic dynamics, and the dynamics of solenoidal circulations, density currents, boundary layers, and gravity waves are considered.

An Unusual Hailstorm on 24 June 2006 in Boulder, Colorado. Part I: Mesoscale Setting and Radar Features

Abstract: An unusual, isolated hailstorm descended on Boulder, Colorado, on the evening of 24 June 2006. Starting with scattered large, flattened, disk-shaped hailstones and ending with a deluge of slushy hail that was over 4 cm deep on the ground, the storm lasted no more than 20 min and did surprisingly little damage except to vegetation. Part I of this two-part paper examines the meteorological conditions preceding the storm and the signatures it exhibited on Weather Surveillance Radar-1988 Doppler (WSR-88D) displays. There was no obvious upper-tropospheric forcing for this storm, vertical shear of the low-level wind was minimal, the boundary layer air feeding the storm was not very moist (maximum dewpoint 8.5°C), and convective available potential energy calculated from a modified air parcel was at most 1550 J kg−1. Despite these handicaps, the hail-producing storm had low-level reflectivity exceeding 70 dBZ, produced copious low-density hail, exhibited strong rotation, and generated three extensive bo...

Cloud modeling in the tropical deep convective regime

Abstract: Understanding cloud processes and the associated interactions with their environment is crucial for better predictions of tropical climate. The cloud-resolving model is demonstrated to be a powerful tool for process studies. In this paper, cloud modeling in the tropical deep convective regime is reviewed based on the author's research work. The review focuses on model setup, cloud-radiation interaction processes, convective-radiative processes associated with the diurnal variation of tropical oceanic convection, dominant cloud microphysical processes producing precipitation, precipitation efficiency, physical processes responsible for the phase relation between surface rain rate and convective available potential energy, and the effects of precipitation on the tropical upper ocean.

Impact of convective downdrafts on model simulations: results from aqua-planet integrations

Abstract: The role of convective scale downdrafts has been examined, using the NCAR-CAM30 aqua-planet configuration We find that, convective downdrafts make the atmosphere more unstable thus increasing the convective available potential energy (CAPE) of the atmosphere It is noticed that, although the rate of CAPE consumption increases with the incorporation of downdrafts, the generation of CAPE increases with a higher rate Also, it is noted that there is a reduction in the deep convective rainfall, with the inclusion of downdrafts, which is primarily due to the re-evaporation of precipitation within the downdrafts There is a large increase in the low cloud fraction and the shortwave cloud forcing with the inclusion of convective scale downdrafts in the cumulus scheme, which along with the evaporation within the downdraft causes cooling in the troposphere

The Influence of Soil and Vegetation Parameters on Atmospheric Variables Relevant for Convection in the Sahel

Abstract: A key issue in modeling the Sahelian climate is to correctly predict the energy fluxes between the land surface and the atmosphere. A problem faced by land surface models in the Sahel is the horizontal heterogeneity of soil and vegetation properties in the region, where measured data are scarce. Experiments have been designed to evaluate a land surface model both in offline mode and coupled to the Advanced Regional Prediction System (ARPS), a mesoscale atmospheric model. For the evaluation in offline mode, an observational dataset of 58 days from the Hydrological and Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) is gathered to interpret the results. For the evaluation in the coupled mode, boundary layer development is simulated for 4 individual days. The model is able to reproduce the observations close to measurement errors. Sensitivity experiments are conducted to identify the most important parameters that affect the simulation of the convective available potential energy (CAPE) and ...

Structure of Mesoscale Heavy Precipitation Systems Originated from the Changma Front

Abstract: Analyses of observational data and numerical simulations were performed to understand the mechanism of MCSs (Mesoscale Convective Systems) occurred on 13-14 July 2004 over Jindo area of the Korean Peninsula. Observations indicated that synoptic environment was favorable for the occurrence of heavy rainfall. This heavy rainfall appeared to have been enhanced by convergence around the Changma front and synoptic scale lifting. From the analyses of storm environment using Haenam upper-air observation data, it was confirmed that strong convective instability was present around the Jindo area. Instability indices such as K-index, SSI-index showed favorable condition for strong convection. In addition, warm advection in the lower troposphere and cold advection in the middle troposphere were detected from wind profiler data. The size of storm, that produced heavy rainfall over Jindo area, was smaller than according to radar observation. The storm developed more than 10 km in height, but high reflectivity (rain rate 30 mm/hr) was limited under 6 km. It can be judged that convection cells, which form cloud clusters, occurred on the inflow area of the Changma front. In numerical simulation, high CAPE (Convective Available Potential Energy) was found in the southwest of the Korean Peninsula. However, heavy rainfall was restricted to the Jindo area with high CIN (Convective INhibition) and high CAPE. From the observations of vertical drop size distribution from MRR (Micro Rain Radar) and the analyses of numerically simulated hydrometeors such as graupel etc., it can be inferred that melted graupels enhanced collision and coalescence process of heavy precipitation systems.

Climatology of Stability Indices and Environmental Parameters Derived from Rawinsonde Data over South Korea

Abstract: The climatology of various stability indices (SIs) and environmental parameters (EPs) widely used to forecast severe convection over South Korea has been studied by using the five upper air observation data for a period of 10 years (1997-2006). Virtual temperature correction is applied when calculating SIs and EPs, such as the equilibrium level (EL), level of free convection (LFC), convective available potential energy (CAPE), and convection inhibition (CIN). The characteristics (spread ranges of SIs and EPs) of the atmospheric environment are significantly affected by geographic location and seasonal features. Most of the selected SIs and EPs show significant variation as a result of the seasonal march of Asian monsoons, but their magnitudes are depend on the geographic location, SI and EP. The seasonal variations of SIs and EPs show that summer is the most favorable season for deep convection. Osan and Gwangju show larger instability and more favorable conditions for deep convection than other regions under the influence of the ocean. Also, most of the selected SIs and EPs show significant diurnal variations with the largest (lowest) instability at 12 UTC (00 UTC). The diurnal variation is more significant in the inland area (Osan and Gwangju), than on the coast (Baengnyeongdo, Jeju, and Pohang). The diurnal variations of SIs and EPs are likely to be related to the diurnal cycle of solar heating and water vapor at the lower layer. The frequency distributions of SIs classified by the Korea Meteorological Administration criteria show that the threshold values should be re-inspected for efficient use of SIs over South Korea, because the frequency distributions in a similar instability class are quite different among SIs.

Analyses of Formation Mechanisms of a Rainstorm during the Declining Phase of a Northeast Cold Vortex

Abstract: The rainstorm usually occurs in the developing stage of northeast cold vortex,not in the declining phaseSo people are apt to miss the rainstorm that appears during the declining phase of cold vortexBased on the observed data and the numerical simulation results,the process of the rainstorm which occurs during the declining phase of one northeast cold vortex is analyzed in middle and eastern Jilin Province on 21 Jun 2005The results show that there is one instable layer at the center of the rainstorm,with a little convective available potential energy(CAPE)The coupling and fluctuation of low levels jet in different layers present the "relay shape" in time and space,which leads to the especial water-vapor configuration and the right instability condition,which further lead to the intense development of moist potential vorticity in the heavy rain areaUnder the condition of convective instability(CI) and conditional symmetric instability(CSI),the mesoscale convective system(MCS) forms and the convection develops in the stratiformis cloud,which directly bring the precipitationGravity wave is the possible trigger mechanism of rainstorm

Analysis of atmospheric stability using AIRS data

Abstract: An optimal cloud-clearing method using collocated MODIS clear pixels is applied to AIRS partly cloudy radiance. Temperature and humidity profiles with 100 layers are retrieved from cloud-free and cloud-cleared AIRS radiance. Using these retrieved profiles, atmospheric environment parameters (include Convective Available Potential Energy, CAPE; Convective Inhibition, CIN) and instability indices (include K index, Lifted Index, Showalter Index) are calculated and compared with that calculated from AIRS and ATOVS operational products and ECMWF analysis fields. These different datasets have different spatial resolution (both in vertical and in horizontal) and spectral resolution. Results show that spatial resolution have obviously influence to the calculations, the lower spatial resolution is likely to have some small energy region lost or reduce the calculation precision. And the profiles derived from partly cloudy area are more meaningful to weather events. The CAPE values of typhoon rain region show differences to that of trough region.

Numerical simulation of the mid-night rainfall over Sichuan basin during August 2003

Abstract: Nine mid-night heavy rainfall events during August 2003 over the western part of Sichuan basin were simulated by using an Advanced Regional Eta-coordinate Model(AREM)The mechanisms responsible for the diurnal variation of the rainfall are addressed by analyzing the model outputsThe results show that mid-night rainfall over western Sichuan basin usually occurs under a specific large scale background circulation,both the orography forcing-induced convergence/ascending motion and the unstable layer resulted from the thermal forcing of ground surface contribute to the formation and development of the midnight rainfall over Sichuan basinDuring the daytime,the low-level temperature and humidity gradually increase with gradually intensified land-air heat flux exchanges,and reach their maximum values after the noonIn the mean time,the low-level warm/moist northeast flow associated with a cyclonic curvature over the western basin gradually strengthens,and the low-level convergence appears,when the warm/moist northeast flow encounters eastward-facing mountain slopes in the western basinThe low-level convergence transports the warm and moist air upward,which leads to the increasing of both the thickness and intensity of the unstable layer in the lower troposphereAfter sunset,the relative humidity in the lower atmosphere gradually increases due to radiation cooling,which makes the air easier to be saturated and the convective available potential energy(CAPE) to accumulate in the lower atmosphereIn the mean time,the topography-induced low-level convergence triggers the release of accumulated CAPE,which finally results in the occurrence of midnight convective rainfall

A Numerical Study of a Mesoscale Convective System Associated with the Heavy Rain Event over Guangdong Province in June 2005

Abstract: The Mesoscale Convective System(MCS) that produced the heavy rainfall during 20-21 Jun 2005 in Guangdong Province is examinedThe observations reveal that the MCS initiated and developed in front of a cold frontThe convergence of vapor flux in the ahead of the jet stream core at 925 hPa level appears to be a crucial factor for its developmentA 24-hour numerical simulation of the developing and mature as well as decline stages of the MCS is performed using the fifth generation Penn State/NCAR mesoscale model(MM5)The synoptic scale circulation evolutions and the mesoscale precipitation distributions associated with the MCS are reproduced successfully by the modelUsing the higher-resolution output of the model,the meso-β-scale structure and evolution of the MCS at the mature stage are examinedThe results show that:(1) with the development of the strong convective core,the associated positive relative vorticity core in the lower troposphere extends to the mid-upper troposphere,so as to form a deep and intense cyclonic air-column penetrating to the upper troposphere in the severe convective region of mature MCSIn the meantime,bands of positive and negative vertical vorticity,paralleled to the convective zone,are found in the middle troposphereThe maximum positive vorticity is near the severe convective zone,the negative vorticity near the rear of the convective zone and submaximum positive vorticity farther back(2) On the surface map,the MCS is composed of a mesolow and a mesohigh which are located in the front of severe convective zone and behind the zone,respectively,ie,the severe convective zone occurred at the transition zone between the mesolow and the mesohigh(3) The mesoscale circulation characteristics viewed in the coordinate system moving with the MCS include: a strong,nearly perpendicular convective scale updraft penetrates the troposphere of the severe convective zone;a convective scale downdraft within the lower troposphere is behind severe convective zone;two major branches of inflows which occurred respectively near 300 hPa and 900 hPa are ahead of the severe convective zone;the mesoscale updraft and downdraft are divided near 0℃ level in the stratiform precipitation region;the convergent inflows in the lower troposphere appear at 900 hPa and the divergent outflows in the upper troposphere appear at 200 hPa

AGCM experiment of the effect of cumulus suppression on convection center formation over the Bay of Bengal

Abstract: [1] We examined the effect of cumulus triggering on the distribution of precipitation during the Indian summer monsoon using the CCSR/NIES/FRCGC atmospheric general circulation model (AGCM) with different cumulus parameterizations. To investigate the effect, we carried out two kinds of simulation. The no-cumulus-suppression (NOCS) run used the original AGCM in which we implemented the prognostic Arakawa-Schubert (AS) scheme. In the other run, we introduced cumulus suppression (CS) as an additional condition of the AS scheme; the CS permitted cumulus convection only when the environmental relative humidity averaged in the modeled cumulus cloud region of the AS scheme exceeded 80%. Special attention was given to the formation of the convection center over the Bay of Bengal (CCBB). The NOCS run could not reproduce the CCBB because the original AS scheme was controlled solely by the convective available potential energy (CAPE). In the CS run, the CS suppressed the unrealistic precipitation simulated in the NOCS run east of the Indian subcontinent (around Sri Lanka) through large-scale topographical effects and well reproduced the westward propagating disturbances coming from the South China Sea and Indochina Peninsula. The westward propagating disturbances created heavy rain in the Bay of Bengal. As a result, the CS run reproduced the realistic distribution of precipitation during the Indian summer monsoon. The AGCM experiment showed that the triggering of cumulus convection played a key role in the distribution of precipitation in the Indian summer monsoon. For instance, the CS suppressed the unrealistic precipitation east of the Indian subcontinent. In addition, in the CS run, ascending motion orographically produced by the large-scale horizontal flow triggered cumulus convection around the northeast coast of the Bay of Bengal, and the propagating disturbances brought heavy rainfall to the Bay of Bengal.

Simulation Study About the Influence of Atmospheric Stratification on Lightning Activities

Abstract: A 2D model about charging and discharging processes in thundercloud is used to simulate three differential atmospheric stratifications resulting in discrepant thunderstorm processes in Beijing region. The dynamic and microphysical processes in thunderstorm and their influence on lightning activities are also discussed.The results indicate that ascending velocity and water vapor are the most important factors to influence lightning activities. At the same time, they affect each other and are together controlled by atmospheric stratification. The magnitude of the ascending velocity determines the intensity of storm and the time when the thunderstorm matured. The thunderstorm with strong updrafts can reach a large height in a short time. Strong persistent updrafts and suffcient water vapor which help to generate more ice phase hydrometeors that directly influence charging and discharging process will prolong the mature stage of the thunderstorm and thereby enhance lightning activities. Though the big density of ice phase hydrometeors can be formed, it is diffcult to sustain a long time in the condition of strong updrafts and scant water vapor. Under the condition of weak updrafts and suffcient water vapor in the whole levels, it is easy to form warm cloud process in which the ice phase process and lightning activities are weak. The favorable stratification conditions for strong lightning activities are the suffcient vapor in the lower atmosphere,moderate humidity in the mid troposphere, big instability energy and some suitable convective inhibition.Through calculating some atmospheric instability parameters, it is indicated that convective instability index smaller than -10 °C (negative means instable), convective available potential energy larger than 1000 J kg-1, convective inhibition larger than 40 J kg-1, the 700-hPa potential equivalent temperature larger than 340 K and the 35%-85% humidity in the mid troposphere (700-400 hPa) are the advantageous conditions for strong lightning activities.

An electric force facilitator in descending vortex tornadogenesis

Abstract: [1] We present a novel explanation of the physical processes behind one type of cloud and ground-level tornadogenesis within a supercell. We point out that the charge separation naturally found in these large thunderstorms can potentially serve to contract the preexisting angular momentum through the additional process of the electric force. On the basis of this, we present a plausible geometry that explains why many tornado vortices begin at storm midlevel and build downward into ground-level tornadoes. A simple model based on this geometry is used to demonstrate the strength of the electric force compared to the required centripetal acceleration to maintain cloud midlevel tornado vortices measurable as tornado vortex signatures (TVSs). Furthermore, a model based on this geometry is used to get a time estimate for tornado vortex formation. From this we are able to identify a plausible value for the threshold charge density that would lead to tornadogenesis and tornado maintenance on the timescale of a few minutes. We show that the proposed geometry can explain the observations that ground-level tornadoes thrive in regions with high shear and large convective available potential energy (CAPE) and are able to make some predictions of specific measurable quantities.