Showing papers on "Convective available potential energy published in 2006"

The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection

Abstract: The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.

High-Resolution Simulation of Shallow-to-Deep Convection Transition over Land

Abstract: Results are presented from a high-resolution three-dimensional simulation of shallow-to-deep convection transition based on idealization of observations made during the Large-Scale Biosphere–Atmosphere (LBA) experiment in Amazonia, Brazil, during the Tropical Rainfall Measuring Mission (TRMM)-LBA mission on 23 February. The doubly periodic grid has 1536 1536 256 grid cells with horizontal grid spacing of 100 m, thus covering an area of 154 154 km 2 . The vertical resolution varies from 50 m in the boundary layer to 100 m in the free troposphere and gradually coarsens to 250 m near the domain top at 25.4 km. The length of the simulation is 6 h, starting from an early morning sounding corresponding to 0730 local time. Convection is forced by prescribed surface latent and sensible heat fluxes and prescribed horizontally uniform radiative heating Despite a considerable amount of convective available potential energy (CAPE) in the range of 1600– 2400 J kg 1 , and despite virtually no convective inhibition (CIN) in the mean sounding throughout the simulation, the cumulus convection starts as shallow, gradually developing into congestus, and becomes deep only toward the end of simulation. Analysis shows that the reason is that the shallow clouds generated by the boundary layer turbulence are too small to penetrate deep into the troposphere, as they are quickly diluted by mixing with the environment. Precipitation and the associated cold pools are needed to generate thermals big enough to support the growth of deep clouds. This positive feedback involving precipitation is supported by a sensitivity experiment in which the cold pools are effectively eliminated by artificially switching off the evaporation of precipitation; in the experiment, the convection remains shallow throughout the entire simulation, with a few congestus but no deep clouds. The probability distribution function (PDF) of cloud size during the shallow, congestus, and deep phases is analyzed using a new method. During each of the three phases, the shallow clouds dominate the mode of the PDFs at about 1-km diameter. During the deep phase, the PDFs show cloud bases as wide as 4 km. Analysis of the joint PDFs of cloud size and in-cloud variables demonstrates that, as expected, the bigger clouds are far less diluted above their bases than their smaller counterparts. Also, thermodynamic properties at cloud bases are found to be nearly identical for all cloud sizes, with the moist static energy exceeding the mean value by as much as 4 kJ kg 1 . The width of the moist static energy distribution in the boundary layer is mostly due to variability of water vapor; therefore, clouds appear to grow from the air with the highest water vapor content available. No undiluted cloudy parcels are found near the level of neutral buoyancy. It appears that a simple entraining-plume model explains the entrainment rates rather well. The least diluted plumes in the simulation correspond to an entrainment parameter of about 0.1 km 1 .

A Mass-Flux Scheme View of a High-Resolution Simulation of a Transition from Shallow to Deep Cumulus Convection

Abstract: In this paper, an idealized, high-resolution simulation of a gradually forced transition from shallow, nonprecipitating to deep, precipitating cumulus convection is described; how the cloud and transport statistics evolve as the convection deepens is explored; and the collected statistics are used to evaluate assumptions in current cumulus schemes. The statistical analysis methodologies that are used do not require tracing the history of individual clouds or air parcels; instead they rely on probing the ensemble characteristics of cumulus convection in the large model dataset. They appear to be an attractive way for analyzing outputs from cloud-resolving numerical experiments. Throughout the simulation, it is found that 1) the initial thermodynamic properties of the updrafts at the cloud base have rather tight distributions; 2) contrary to the assumption made in many cumulus schemes, nearly undiluted air parcels are too infrequent to be relevant to any stage of the simulated convection; and 3) a simple model with a spectrum of entraining plumes appears to reproduce most features of the cloudy updrafts, but significantly overpredicts the mass flux as the updrafts approach their levels of zero buoyancy. A buoyancy-sorting model was suggested as a potential remedy. The organized circulations of cold pools seem to create clouds with larger-sized bases and may correspondingly contribute to their smaller lateral entrainment rates. Our results do not support a mass-flux closure based solely on convective available potential energy (CAPE), and are in general agreement with a convective inhibition (CIN)-based closure. The general similarity in the ensemble characteristics of shallow and deep convection and the continuous evolution of the thermodynamic structure during the transition provide justification for developing a single unified cumulus parameterization that encompasses both shallow and deep convection.

Mesoscale simulations of organized convection: Importance of convective equilibrium

Abstract: The validity of convective parametrization breaks down at the resolution of mesoscale models, and the success of parametrized versus explicit treatments of convection is likely to depend on the large-scale environment. In this paper we examine the hypothesis that a key feature determining the sensitivity to the environment is whether the forcing of convection is sufficiently homogeneous and slowly varying that the convection can be considered to be in equilibrium. Two case studies of mesoscale convective systems over the UK, one where equilibrium conditions are expected and one where equilibrium is unlikely, are simulated using a mesoscale forecasting model. The time evolution of area-average convective available potential energy and the time evolution and magnitude of the timescale of convective adjustment are consistent with the hypothesis of equilibrium for case 1 and non-equilibrium for case 2. For each case, three experiments are performed with different partitionings between parametrized and explicit convection: fully parametrized convection, fully explicit convection and a simulation with significant amounts of both. In the equilibrium case, bulk properties of the convection such as area-integrated rain rates are insensitive to the treatment of convection. However, the detailed structure of the precipitation field changes; the simulation with parametrized convection behaves well and produces a smooth field that follows the forcing region, and the simulation with explicit convection has a small number of localized intense regions of precipitation that track with the mid-levelflow. For the non-equilibrium case, bulk properties of the convection such as area-integrated rain rates are sensitive to the treatment of convection. The simulation with explicit convection behaves similarly to the equilibrium case with a few localized precipitation regions. In contrast, the cumulus parametrization fails dramatically and develops intense propagating bows of precipitation that were not observed. The simulations with both parametrized and explicit convection follow the pattern seen in the other experiments, with a transition over the duration of the run from parametrized to explicit precipitation. The impact of convection on the large-scaleflow, as measured by upper-level wind and potential-vorticity perturbations, is very sensitive to the partitioning of convection for both cases. © Royal Meteorological Society, 2006. Contributions by P. A. Clark and M. E. B. Gray are Crown Copyright.

Intense convective systems in West Africa and their relationship to the African easterly jet

Abstract: For May–September 1998, convective systems in West Africa were identified from observations by the Tropical Rainfall Measuring Mission satellite Microwave Imager at 85 GHz. Using re-analysis data, the 10-day average position of the African easterly jet (AEJ) was diagnosed from the 700 hPa zonal winds. The distance from each convective system's centroid to the axis of the AEJ was calculated. Each convective system's minimum brightness temperatures were ranked so that intense convective systems were defined as those in the 10th percentile or lower. The weak (>10th percentile) and intense convective systems were represented statistically as two separate populations, the weak by the skewed Gumbel distribution and the intense by the normal distribution. From May to August, the peak in activity of weak convective systems remained south of 10°N but shifted east of 10°E. The peak in activity of the intense convective systems followed the seasonal migration of the AEJ northwards and became increasingly separate from the peak in activity of weak convective systems. The majority of both weak and intense convective systems occurred within 0.50° of high terrain. The high convective available potential energy, high-shear AEJ environment in the vicinity of high terrain appeared to have the greatest probability of generating intense convective systems in the study area. Copyright © 2006 Royal Meteorological Society

Electrification and Lightning in an Idealized Boundary-Crossing Supercell Simulation of 2 June 1995*

Abstract: A nonhydrostatic cloud model with electrification and lightning processes was utilized to investigate how simulated supercell thunderstorms respond when they move into environments favorable for storm intensification. One model simulation was initialized with an idealized horizontally varying environment, characteristic of that observed across an outflow boundary in the west Texas Panhandle on 2 June 1995 with larger convective available potential energy (CAPE) and wind shear on the boundary’s cool side. That simulation was compared with a control simulation initialized without the boundary. The simulated rightmoving supercell rapidly increased in updraft strength and volume, low-level rotation, radar reflectivity, and 40-dBZ echo-top height as it crossed the boundary, whereas the supercell that did not cross the boundary failed to intensify. For the same kinematic and microphysical evolution and the same inductive charging parameterization, four noninductive (NI) charging parameterizations were tested. In all four cases, there was a general tendency for the charge regions to be lofted higher within the updraft after crossing the boundary. Once the precipitation regions between the main storm and a secondary storm started merging farther on the cool side of the boundary, a gradual deepening and strengthening of the lowest charge regions occurred with relatively large increases in hail and graupel volume, charging rates, charge volume, charge density, and intracloud and cloud-to-ground (CG) flash rates. The negative charge present on graupel within the downdraft appeared to have a common origin via strong NI charging within the midlevel updraft in all four NI cases. Positive channels were more consistent in coming closer to the ground with time compared to negative channels within this graupel and hail-filled downdraft (four of four cases). Those NI schemes that also set up a positive dipole (three of four cases) or inverted tripole (two of four cases) above the downdraft had downward-propagating positive channels that reached ground as positive CG (CG) flashes. The best overall performance relative to the 2 June 1995 CG lightning observations occurred within one of the rime-accretion-rate-based schemes and the Gardiner scheme as parameterized by Ziegler.

Simulation of shallow cumuli and their transition to deep convective clouds by cloud‐resolving models with different third‐order turbulence closures

Abstract: The abilities of cloud-resolving models (CRMs) with the double-Gaussian based and the single-Gaussian based third-order closures (TOCs) to simulate the shallow cumuli and their transition to deep convective clouds are compared in this study. The single-Gaussian based TOC is fully prognostic (FP), while the double-Gaussian based TOC is partially prognostic (PP). The latter only predicts three important third-order moments while the former predicts all the thirdorder moments. A shallow cumulus case is simulated by single-column versions of the FP and PP TOC models. The PP TOC improves the simulation of shallow cumulus greatly over the FP TOC by producing more realistic cloud structures. Large differences between the FP and PP TOC simulations appear in the cloud layer of the second- and third-order moments, which are related mainly to the underestimate of the cloud height in the FP TOC simulation. Sensitivity experiments and analysis of probability density functions (PDFs) used in the TOCs show that both the turbulence-scale condensation and higher-order moments are important to realistic simulations of the boundary-layer shallow cumuli. A shallow to deep convective cloud transition case is also simulated by the 2-D versions of the FP and PP TOC models. Both CRMs can capture the transition from the shallow cumuli to deep convective clouds. The PP simulations produce more and deeper shallow cumuli than the FP simulations, but the FP simulations produce larger and wider convective clouds than the PP simulations. The temporal evolutions of cloud and precipitation are closely related to the turbulent transport, the cold pool and the cloud-scale circulation. The large amount of turbulent mixing associated with the shallow cumuli slows down the increase of the convective available potential energy and inhibits the early transition to deep convective clouds in the PP simulation. When the deep convective clouds fully develop and the precipitation is produced, the cold pools produced by the evaporation of the precipitation are not favorable to the formation of shallow cumuli.

Structure of the Band-Shaped Precipitation System Inducing the Heavy Rainfall Observed over Northern Kyushu, Japan on 29 June 1999

Abstract: A band-shaped precipitation system, associated with a cold front, caused heavy rainfall over northern Kyushu, Japan, on 29 June 1999. This precipitation system had a hierarchical structure with different horizontal scales; the precipitation system consisted of several mesoscale convective systems (MCSs), and each MCS consisted of a few convective cells that formed successively on its upstream side. Each of them had different spatial and time scales, as well as different traveling speeds and directions.Low-level humid air from the southwest and middle-level dry air from the west continuously flowed into the precipitation system. This dry air was not colder than the surrounding atmosphere. The inflow of low-level humid air initiated the MCSs, and that of middle-level dry air enhanced, and maintained, the convective instability over the cold front. The heavy rainfall was brought under the maintenance condition of convective instability.The cloud top heights of convective cells were found differently between the western and central parts of the precipitation system. The factors to determine them were examined using the successfu1 simulation results of a nonhydrostatic cloud-resolving model, with the horizontal resolution of 2 km. The intrusion of middle-level dry air into convective cells was considerably larger in the western part than in the central part. This caused the difference in the cloud top heights of the convective cells; the convective cells were less than 7 km high in the western part, whereas, in the central part, most were between 5 and 7 km high and at the height of the tropopause. In the western part, most of the convective cells completely lost their buoyancy because of the significant evaporative cooling of hydrometeors. These results show the two different effects of middle-level dry air; a huge amount inflow suppresses the development of convective cells, while it strongly enhances the convective instability.

Bow Echo Sensitivity to Ambient Moisture and Cold Pool Strength

Abstract: Bow echo development within quasi-linear convective systems is investigated using a storm-scale numerical model. A strong sensitivity to the ambient water vapor mixing ratio is demonstrated. Relatively dry conditions at low and midlevels favor intense cold-air production and strong cold pool development, leading to upshear-tilted, “slab-like” convection for various magnitudes of convective available potential energy (CAPE) and low-level shear. High relative humidity in the environment tends to reduce the rate of production of cold air, leading to weak cold pools and downshear-tilted convective systems, with primarily cell-scale three-dimensionality in the convective region. At intermediate moisture contents, long-lived, coherent bowing segments are generated within the convective line. In general, the scale of the coherent three-dimensional structures increases with increasing cold pool strength. The bowing lines are characterized in their developing and mature stages by segments of the convectiv...

Severe Convective Wind Environments

Abstract: Nontornadic thunderstorm winds from long-lived, widespread convective windstorms can have a tremendous impact on human lives and property. To examine environments that support damaging wind producing convection, sounding parameters from Rapid Update Cycle model analyses (at 3-hourly intervals) from 2003 were compared with 7055 reports of damaging winds and 377 081 occurrences of lightning. Groundrelative wind velocity was the most effective at discriminating between damaging and nondamaging wind convective environments. Steep surface-based lapse rates (a traditional damaging wind parameter) generally did not discriminate between damaging and nondamaging wind convective environments. Other parameters, such as convective available potential energy, humidity aloft, and lapse rates aloft were moderately discriminating. This paper presents a composite damaging wind algorithm in which the two most discriminatory parameters were combined, yielding more skill than any individual parameter. Damaging wind environments are then examined further through a selection of cases that highlight common severe wind ingredients and failure modes. A primary result is that, even in seemingly favorable environments, when the winds at the top of the inflow layer were either parallel to the convective line or blowing from warm to cold over a front, damaging winds were less likely. In the former case, it appears that the downdraft winds and the cold pool’s gust-front-normal flow are not additive. In the latter case, it appears that convection becomes elevated and does not produce downdrafts that reach the surface. Combining the most discriminatory severe wind parameters with knowledge of these severe wind failure modes may help to improve the situational awareness of forecasters.

Recent Trends in Four Common Stability Indices Derived from U.S. Radiosonde Observations

Abstract: Daily sounding-derived atmospheric stability indices are typically employed for short-term severe weather forecasts. Over longer time periods, these indicators may convey changes in the potential for severe storm development over the United States. Daily (0000 UTC) observations from 48 radiosonde stations in the contiguous United States are extracted to assemble a ∼50 yr record of four common stability indices: the Lifted Index, the K-Index, convective available potential energy (CAPE), and the Air Force Severe Weather Threat Index. Because of radiosonde data inhomogeneities, the 1973–97 period is the focus of the analysis. Trends in the mean and extreme values of daily index observations are calculated for spring and summer seasons. In addition, climatological mean indices, as well as the mean frequency of index extremes, are determined for all U.S. regions. At stations free of obvious data discontinuities, the early part of the record (1948–65) is compared with more recent periods. In spring, f...

A Diagnostic Study on Interactions Between Atmospheric Thermodynamic Structure and Cumulus Convection over the Tropical Western Pacific Ocean and over the Indochina Peninsula

Abstract: Interactions between the convective activity and the atmospheric thermodynamic structures are analyzed utilizing upper-air rawinsonde observations obtained by R/V Mirai, R/V Kaiyo, R/V Natsushima, of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) over the western tropical Pacific Ocean, and those over three GEWEX Asian Monsoon Experiment (GAME) stations: Chiang Mai, Non Khai, and Ubon Ratchathani. Special emphases are placed on understanding the correlation between convection and the atmospheric thermodynamic structures in relation to the recent findings of trimodal cloud levels over the warm ocean (e.g., Johnson et al. 1999) and to the cloud diagnostics proposed by Raymond and Blyth (1992). We first examine the relationships between a convection index and thermodynamic structure indices. A large correlation is found between the convective activity and lower-tropospheric (600-800 hPa) humidity, while there is no significant correlation between the convective activity and Convective Available Potential Energy (CAPE) or Convective Inhibition (CIN). Next, we apply a cloud diagnostic model introduced by Raymond and Blyth (1992) (referred to as RB92) to the observed profiles. As a result, it is shown that there are fundamentally 3 peaks of detrain-ment levels, which are lower-troposphere (near 900 hPa), mid-troposphere (near 450 hPa), and upper-troposphere (near 150 hPa), over ocean as well as over land. In the soundings over ocean, when the lower-troposphere (600-800 hPa) is dry, there is a tendency for simultaneous existence of stable layers both in the lower-troposphere and in the mid-troposphere. Such atmospheric thermodynamic structure is diagnosed as favorable for strengthened detrainments in the low- and mid-troposphere and weakened in the upper-troposphere. Finally, meridional winds are composited to the north and to the south of the maximum convective activity in the Inter Tropical Convergence Zone (ITCZ) region, respectively, over the tropical western Pacific Ocean. It is confirmed with upper-air soundings that there is a significant meridional divergence near the melting layer level in the mid-troposhere around 500-600 hPa and a significant meridional convergece near 350-400 hPa, in addition to the lower-tropospheric convergence and the upper-tropospheric divergence of the local Hadley Circulation. These additional circulations in the mid-troposphere are consistent with detrainment profiles diagnosed for observed atmospheric profiles utilizing RB92 cloud model. After all, it is strongly suggested that the cloud microphysics, such as melting and freezing, play significant roles in determining the large-scale circulation.

Transition between suppressed and active phases of intraseasonal oscillations in the Indo-Pacific warm pool

Abstract: Intraseasonal oscillations (ISOs) are important large-amplitude and large-scale elements of the tropical Indo-Pacific climate with time scales in the 20-60-day period range, during which time they modulate higher-frequency tropical weather. Despite their importance, the ISO is poorly simulated and predicted by numerical models. A joint diagnostic and modeling study of the ISO is conducted, concentrating on the period between the suppressed and active (referred to as the "transition") period that is hypothesized to be the defining stage for the development of the intraseasonal mode and the component that is most poorly simulated. The diagnostic study uses data from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). It is found that during the transition period, the ocean and the atmosphere undergo gradual but large-scale and high-amplitude changes, especially the moistening of the lower troposphere caused jointly by the anomalously warm sea surface temperature arising from minimal cloud and low winds during the suppressed phase and the large-scale subsidence that inhibits the formation of locally deep convection. Using a cloud classification scheme based on microwave and infrared satellite data, it is observed that midtop (cloud with a top in the middle troposphere) nonprecipitating clouds are a direct response of the low-level moisture buildup. To investigate the sensitivity of ISO simulations to the transitional phase, the European Centre for Medium-Range Weather Forecasts (ECMWF) coupled ocean-atmosphere climate model is used. The ECMWF was run serially in predictive ensemble mode (five members) for 30-day periods starting from 1 December 1992 to 30 January 1993, encompassing the ISO occurring in late December. Predictability of the active convective period of the ISO is poor when initialized before the transitional phases of the ISO. However, when initialized with the correct lower-tropospheric moisture field, predictability increases substantially, although the model convective parameterization appears to trigger convection too quickly without allowing an adequate buildup of convective available potential energy during the transition period.

Proximity soundings of thundersnow in the central United States

Abstract: [1] Proximity balloon soundings for snow events with lightning and thunder during the period 1961 through 1990 reveal a less statically stable environment than similar nonthundering snow events. When thundersnow is present, a less stable environment (and in some cases subsequent upright convection) is found aloft in all of the thundering cases examined here; all of the events feature their most unstable parcel originating above a frontal inversion. In fact, only events in the cold air north of an extratropical cyclone are included in this study. Events with a lake effect or orographic enhancement are eliminated from the sample. The basic composite derived by averaging temperatures at an established interval reveals a nearly saturated lower atmosphere, below 0°C throughout its depth, with the frontal inversion present and its most unstable parcel occurring just above the top of the inversion. The feature-preserving composite approach of R. A. Brown (1993) better defines the frontal inversion bottom and top as well as the level and temperature of the most unstable parcel; these are the features in need of preservation, and a less statically stable environment emerges by doing so. Other salient features include the most unstable parcel originating some 30–50 mbar above the top of the frontal inversion and significant drying ∼100 mbar above the level of the most unstable parcel. The bulk sounding characteristics also favor the existence of lightning. The composite temperature at the level of the most unstable parcel is −8.7°C, which allows for enhanced amounts of supercooled water to enter any updraft that may form. The temperature of the most unstable parcel at its origin is also warmer than the charge reversal temperature; therefore convection of any appreciable depth will span that level. Moreover, the height of the composited −10°C level is 2959 m above ground level, which previous investigators have shown is sufficiently high to favor lightning production. Yet no convective available potential energy (CAPE) appears with either composite approach, which concurs with previous studies. While several of the composite members feature CAPE for elevated layers, the majority do not, suggesting that other processes (e.g., the release of symmetric instability), which are difficult to assess from a single sounding, tend to be at work.

A Study of Circumstances of Meso-β-Scale Systems of Strong Heavy Rainfall in Warm Sector Ahead of Fronts in South China

Abstract: The circulation characteristics and the influencing systems of the heavy rainfall causing flood in the Pearl River(the Xijiang River) valley in June 2005 are investigated.The heavy rainfall had the outstanding characteristics of multi-scale systems and it occured in warm sector ahead of the front,relating to monsoon activity,the influence of the planetary boundary layer and the dynamic lifting of the topography.The conclusions are as follows:(1) The late onset of the South China Sea monsoon and its continuous impact on the South China,as well as the further south position of the upper-level jet in the westerlies and the subtropical high,which maintained in 18°N,provided the very favorable large-scale circulations for sustaining the heavy rainfalls in South China.(2) There were the heavy rainfalls happening in warm sector ahead of the front in South China during the pre-rainy season.There always existed an approximately east-to-west oriented quasi-stationary front in South China.The calculated results of the frontogenesis function indicate that the weaker cold air played an important role in occurrence of heavy rainfall.The analysis of the vertical cross section of horizontal wind and vertical velocity indicates that there was ascending motion obviously ahead of the front,which was very favorable for the formation and development of the meso-scale convection.The weaker cold air from the north interacted with the warm and wet air coming along the south-west edge of the subtropical high,there is a kind of interaction between middle- and low-latitude systems.(3) The wind speed convergence of south-west wind at low level was the most obvious at 925 hPa.It indicates that the planetary boundary level played an very important role in the formation of the heavy rainfall,the current not only transported sufficient water vapor to the heavy rainfall area from the Sino-Indian Peninsula and the South China Sea,but also contributed to the initiation of the convection.(4) The trumpet-shaped terrain in the Nanling Mountains enhanced the ascending motion of the warm and wet air from the south,which benefited the occurrence of the heavy rainfall.The special terrain in South China,especially in the Xijiang River valley,can be one of the important conditions for the appearance of the continuous heavy rainfalls.(5) The meso-scale convective rainy clusters were the direct influencing systems of the heavy rainfall.There were at least four meso-β-scale rainy clusters existing over the Xijiang River valley(A,B,C,D).Fortunately,A and D rainy clusters were located near Guilin and Wuzhou observation stations,respectively.So the environment of A and D rainy clusters can be carefully studied.The rainfall areas closely related to the convergence center of the low-level wind speed,and the convective available potential energy is cumulated,released and rebuilt again and again.Under such circumstances,the meso-scale convective system and heavy rainy were very active.Furthermore,the convective rainy clusters followed the flood peak,namely moved towards the lower reaches of the river,resulting in the "coupling" between the heavy rainfall and the flood peak,then brought the very serious disasters.(6) Based on the above-mentioned research,a schematic diagram of the influencing factors for the heavy rainfall in warm sector ahead of the front in South China during the period of pre-rainy season is given.

Measuring Atmospheric Stability with GPS

Abstract: Nowcasting of convective systems plays a crucial role in weather forecasting. The strength of convection depends on the (in)stability of the air column. The stability can be detected by radiosonde observations. However, these observations are not frequent (typically 2 times per day) and are expensive to deploy. In this article a method is presented to detect the stability of the atmosphere based on high-frequency global positioning system (GPS) path-delay observations. The convection parameter derived from these observations is the power of the nonisotropic GPS path-delay signal. Comparisons with the convective available potential energy obtained from radiosonde observations show a correlation with the convection parameter obtained from GPS. This result implies that, because of the continuous availability of GPS estimates and the good land coverage, this method of detecting atmospheric stability may be beneficial to forecasters.

Moist convective instability over the Arabian Sea during the Asian summer monsoon, 2002

Abstract: The moist convective instability over the Arabian Sea during the Arabian Sea Monsoon Experiment 2002 is studied using high resolution radiosonde data over the Arabian Sea off the west coast of India and inland at Goa (15°21′N,73°51′E). The vertical structure of the lower troposphere is discussed in terms of the profiles of conserved variables and the time series of convective available potential energy (CAPE), mixed-layer depth, lifting condensation level and level of free convection. Analysis shows that the Arabian Sea within 200 km of the west coast of India is characteristically a low-CAPE region irrespective of whether it is a good or bad monsoon season. Little or no variability in the mean (0–500 Pa) lapse rate during July and August indicates the quasi-equilibrium state of the tropical atmosphere during the summer monsoon with an approximate balance between the convective and large-scale forces. Large-scale forcing for the ascent of surface air seems to be of prime importance for triggering deep convection and rainfall over the East Arabian Sea. Case studies of precipitating convection over inland at Goa during the monsoon indicate a moistening and warming of the lower troposphere following the convection

The Synoptic and Climatic Characteristic Studies of Thunderstorm Winds in Beijing

Abstract: Synoptic and climatic characteristic of thunderstorm winds in Beijing are studied by using the observational data and sounding data during 1990 to 2004 The obviously diurnal characteristics was showed to thunderstorm winds occurrence The ambient circulation, sounding structure and convection parameters are studied to discover the most favorable condition for the occurrence of the thunderstorm wind in Beijing The instability index and energy parameters are studied as well It shows that warm and wet at low level, dry and cold at middle level, strong convective instability, and relative strong vertical wind sheer are favorable to thunderstorm wind occurrences More convective parameters associated to strong storms are also studied, such as the best convective available potential energy (BCAPE), downdraft convective available potential energy (DCAPE), Storm-Relative Environmental Helicity (SREH), Wind Index (WINDEX), Storm Severity Index (SSI), Deep Convective Index (DCI) and so on, which are important to the potential development of the strong convection in Beijing

An Observational Study of Mesoscale Convective Systems Producing Severe Heavy Rainfall in the Huaihe River Basin During 3-4 July 2003

Abstract: Three long life-cycle severe heavy rainfall induced the flooding of the Huaihe River basin from 29 June to 11 July 2003,the most severe heavy rainfall during 3-5 July,which moved from upper reaches to lower reaches of the Huaihe River basin,cause the flooding more dangerous.In this paper,firstly,synoptic condition of the heavy rainfall process is diagnosed. Secondly,the upper-air sounding,satellite and Doppler radar data are employed to analyze the mesoscale convective system producing the most severe heavy rainfall at Chuzhou and Nanjing.The synoptic weather pattern analysis shows that the eastward propagation of trough results in the rainfall area moving from upper to lower reaches of the Huaihe River basin.The invasion of dry air in the middle troposphere prior to the rainfall intensifies the convective instability in the lower and middle troposphere.The transportation of vapor increases the humidity and Convective Available Potential Energy(CAPE).The inversion near surface produced by downdraft during the heavy rainfall is not favorable for the maintenance of convective systems.The large area rainfall is produced by meso-α convective system,but the local severe rainfall at Chuzhou and Nanjing is caused by meso-β or meso-γ convective systems.The rainfall most frequently occurs while the convective system develops to the mature and dissipating periods.It is found that the rainfall at the mature stage is not always stronger than that at the dissipating stage.The analysis of infrared brightness temperature and radar reflectivity reveals that severe radar echo is not consistent with the area enclosed by-52℃.For present case,the strong radar echo is only located in the central and southern part of the area of-52℃ cloud brightness temperature,and there is the cloud anvil to the north of strong radar echo.The radar reflectivity from Hefei radar indicates that the top of radar echo is higher than 10 km during the heavy rainfall period and the reflectivity more than 45 dBZ occurs below 5 km.The two periods of Chuzhou rainfall happen during the mature and dissipating stages of the convective systems.The retrieval data of dual Doppler radar from Hefei and Maanshan reveal that the mesoscale convective line and its cells along the mesoscale convergence line produce directly the heavy rainfall at Chuzhou.The mesoscale convergence lines appear in the lower troposphere and tilt northward from low to middle level.The location of convection line is consistent with the convergence line at the height of about 3 km.

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

Abstract: 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.

Use of high-resolution WRF simulations to forecast lightning threat

Abstract: Recent observational studies have confirmed the existence of a robust statistical relationship between lightning flash rates and the amount of large precipitating ice hydrometeors in storms. This relationship is exploited, in conjunction with the capabilities of recent forecast models such as WRF, to forecast the threat of lightning from convective storms using the output fields from the model forecasts. The simulated vertical flux of graupel at -15C is used in this study as a proxy for charge separation processes and their associated lightning risk. Six-h simulations are conducted for a number of case studies for which three-dimensional lightning validation data from the North Alabama Lightning Mapping Array are available. Experiments indicate that initialization of the WRF model on a 2 km grid using Eta boundary conditions, Doppler radar radial velocity and reflectivity fields, and METAR and ACARS data yield the most realistic simulations. An array of subjective and objective statistical metrics are employed to document the utility of the WRF forecasts. The simulation results are also compared to other more traditional means of forecasting convective storms, such as those based on inspection of the convective available potential energy field.

Electrical behavior of downburst-producing convective storms over the High Plains

Abstract: A great body of research literature pertaining to microburst generation in convective storms has focused on thermodynamic factors of the pre-convective environment as well as storm morphology as observed by radar imagery. Derived products based on Geostationary Operational Environmental Satellite (GOES) sounder data have been found to be especially useful in the study of thermodynamic environments. However, addressed much less frequently is the relationship between convective storm electrification, lightning phenomenology and downburst occurrence. Previous research in lightning production by convective storms has identified that electrification, phenomenology (i.e. flash rate, density), and polarity are dependent upon the thermodynamic structure of the ambient atmosphere, especially vertical moisture stratification. Thus, relevant parameters to describe the thermodynamic setting would include convective available potential energy (CAPE), due to its influence on updraft strength, and moisture characteristics of the boundary layer, due to its relationship to precipitation physical processes. It has already been addressed that buoyant energy and moisture stratification are important factors in convective storm development and downburst generation. This research effort investigates and derives a qualitative relationship between lightning phenomenology in convective storms and downburst generation. Downburst-producing convective storms will be analyzed by comparing preconvective environments, as portrayed by GOES microburst products, storm morphology, as portrayed by radar imagery, and electrical behavior, as indicated by National Lightning Detection Network (NLDN) data. In addition, this paper will provide an update to validation efforts for the GOES Wet Microburst Severity Index (WMSI) and Hybrid Microburst Index (HMI) products to include data from both the 2005 and 2006 convective seasons. The case study presented will demonstrate the effectiveness of the coordinated use of the GOES WMSI and HMI products during convective storm events over the southern High Plains.

The Characteristics and Predictability of Convective System Based on GOES-9 Observations during the Summer of 2004 over East Asia

Abstract: Convective systems propagate eastward with a persistent pattern in the longitude-time space. The characteristic structure and fluctuation of convective system is helpful in determining its predictability. In this study, convective index (CI) was defined as a difference between GOES-9 window and water vapor channel brightness temperatures following Mosher (2001). Then the temporal-spatial scales and variational characteristics of the summer convective systems in the East Asia were analyzed. It is found that the average moving speed of the convective system is about 14 m/s which is much faster than the low pressure system in the summer. Their average duration is about 12 hours and the average length of the cloud streak is about 750km. These characteristics are consistent with results from other studies. Although the convective systems are forced by the synoptic system and are mostly developed in the eastern edge of the Tibetan Plateau, they have a persistent pattern, i.e., appearance of the maximum intensity of convective systems, as they approach the Korean Peninsula. The consistency of the convective systems, i.e., the eastward propagation, suggests that there exists an intrinsic predictability.

Electrical Behavior of Downburst-Producing Convective Storms over the Western United States

Abstract: A great body of research literature pertaining to microburst generation in convective storms has focused on thermodynamic factors of the pre-convective environment as well as storm morphology as observed by radar imagery. Derived products based on GOES sounder data have been found to be especially useful in the study of thermodynamic environments. However, addressed much less frequently is the relationship between convective storm electrification, lightning phenomenology and downburst generation. Previous research in lightning production by convective storms has identified that electrification, phenomenology (i.e. flash rate, density), and polarity are dependent upon the thermodynamic structure of the ambient atmosphere, especially vertical moisture stratification. Thus, relevant parameters to describe the thermodynamic setting would include convective available potential energy (CAPE), due to its influence on updraft strength, and cloud liquid water content, due to its relationship to precipitation physical processes. It has already been addressed that buoyant energy and moisture stratification are important factors in convective storm development and downburst generation. This research effort investigates and derives a qualitative relationship between lightning phenomenology in convective storms and downburst generation. Downburst-producing convective storms will be analyzed by comparing pre-convective environments, as portrayed by GOES microburst products, storm morphology, as portrayed by radar imagery, and electrical behavior, as indicated by NLDN data.

Research on Assessment of Model-Generated Sounding and Application in Forecasting Strong Convective Weather

Abstract: Because of the shortage of data density of conventional sounding data and the quality problems of some sounding data,it is so important to pay attention to the application of model-generated sounding.After analysed mode-generated sounding and observation data in detail during the high frequency convective weather season(April and May) in South China,the direct assessment of model-generated sounding data shows that the error amount of forecast element on surface and high levels is bigger than middle levels,for example,forecast element of temperature at 700-hPa level is the best,and the element forecast of surface temperature is very sensitive and difficult.Based on error analysis of multiple stations and real time forecast in long period,the possible reason is discussed.Assessment of instability indexes also shows that the performance of those instability indexes which is only considered the mid-level elements is relatively steady,but some indexes which is considered surface elements might be relatively sensitive,and their quality could be greatly improved by adding surface observation data.Case study shows that hourly model-generated sounding data are proved to be very useful in strong convective weather forecast,such as CAPE(Convective Available Potential Energy) index,it could be made good use of guiding to convective weather forecast.In the case study,hourly variation of four sounding stations data in South China is mainly analysed.The analysis shows that model-generated sounding data of the four stations(Lianping,Qingyuan,Yangjiang and Hong Kong) are very close to the corresponding observation value after added surface observation,and those model-generated sounding data of hourly forecast really enriches a lot of variation details which observation could not gave us,which details is consistent very well with the occurring of strong convective storm.Case study also shows that the fine features of afternoon storm and morning storm could be obtained by using model-generated sounding data,and these features are very important for forecasting severe storm in a few hours later.Field distribution analysis shows that the change of CAPE index could help us forecasting convective weather that takes place two or three hours later and some indexes only considered mid-level element such as K index and SI(Showalter Index) are very useful for very short range(212 hours) forecasting.There are a lot of cases that mesoscale convective systems derived from terrain-effect,which could be successfully simulated by a mesoscale numerical model, but for strong convective storms derived from large-scale weather systems,it would become very complicated,one reason might be that the initial data for mesoscale numerical model is not completely proper,another reason might be due to mesoscale numerical model,for example subgrid scale parameterization scheme is not perfect,so at present time,it might be the right way to forecast strong convective storm by combining model-generated sounding with conventional surface observation,radar echo and satellite image data.

Mechanism of strong rainstorm and structure of mesoscale system

Abstract: Based on the synoptic analysis of the strong rainstorm of Hunan on June 23,2004, the nonhydrostatic version of mesoscale numerical model MM5-V3.6 was used to simulate this case.The results indicate that high resolution model MM5 can preferably simulate the occurrence and development of the low mesovortex with shear line,and that there are many disturbances in front of the low vortex and the strong mesoscale convective system inspired with the development of the low vortex.Over the rainstorm area,the vertical double-circulation causes the mesoscale convective system to become much more systematic.The development of the rainstorm and MCS were investigated with moist potential vorticity principle and slantwise vorticity development theory.The establishment of convective instability and conditional symmetry instability,and the centralized release of convective available potential energy(CAPE) are important conditions of the occurrence and development in this rainstorm.

Electrical Behavior of Downburst-Producing Convective Storms over the High Plains

Abstract: A great body of research literature pertaining to microburst generation in convective storms has focused on thermodynamic factors of the pre-convective environment as well as storm morphology as observed by radar imagery. Derived products based on Geostationary Operational Environmental Satellite (GOES) sounder data have been found to be especially useful in the study of thermodynamic environments. However, addressed much less frequently is the relationship between convective storm electrification, lightning phenomenology and downburst occurrence. Previous research in lightning production by convective storms has identified that electrification, phenomenology, and polarity are dependent upon the thermodynamic structure of the ambient atmosphere, especially vertical moisture stratification. Thus, relevant parameters to describe the thermodynamic setting would include convective available potential energy (CAPE), due to its influence on updraft strength, and moisture characteristics of the boundary layer, due to its relationship to precipitation physical processes. It has already been addressed that buoyant energy and moisture stratification are important factors in convective storm development and downburst generation.

The Role of Downward Momentum Transport in the Generation of Convective Downbursts

Abstract: A downburst index has been developed to assess the magnitude of convective downbursts associated with heavy precipitation-producing (HP) convective systems. The index, designed for use during the warm season over the central and eastern continental United States, is composed of relevant parameters that represent the simultaneous physical processes of convective updraft development and downburst generation, incorporating positive buoyant energy or convective available potential energy (CAPE) and the vertical equivalent potential temperature gradient between the surface and the mid-troposphere. In addition to large CAPE and the presence of a vertical equivalent potential temperature gradient gradient, previous research has identified other favorable conditions for downburst generation during cold-season convection events. A primary mechanism in downburst generation associated with synoptically forced convective systems is the downward transport of higher momentum possessed by winds in the mid-troposphere.