Showing papers on "Convective available potential energy published in 2012"

Physical Processes Associated with Heavy Flooding Rainfall in Nashville, Tennessee, and Vicinity during 1–2 May 2010: The Role of an Atmospheric River and Mesoscale Convective Systems*

Abstract: A multiscale analysis is conducted in order to examine the physical processes that resulted in prolonged heavy rainfall and devastating flash flooding across western and central Tennessee and Kentucky on 1–2 May 2010, during which Nashville, Tennessee, received 344.7 mm of rainfall and incurred 11 flood-related fatalities. On the synoptic scale, heavy rainfall was supported by a persistent corridor of strong water vapor transport rooted in the tropics that was manifested as an atmospheric river (AR). This AR developed as water vapor was extracted from the eastern tropical Pacific and the Caribbean Sea and transported into the central Mississippi Valley by a strong southerly low-level jet (LLJ) positioned between a stationary lee trough along the eastern Mexico coast and a broad, stationary subtropical ridge positioned over the southeastern United States and the subtropical Atlantic. The AR, associated with substantial water vapor content and moderate convective available potential energy, supporte...

A mass-flux cumulus parameterization scheme for large-scale models: description and test with observations

Abstract: A simple mass-flux cumulus parameterization scheme suitable for large-scale atmospheric models is presented. The scheme is based on a bulk-cloud approach and has the following properties: (1) Deep convection is launched at the level of maximum moist static energy above the top of the boundary layer. It is triggered if there is positive convective available potential energy (CAPE) and relative humidity of the air at the lifting level of convection cloud is greater than 75%; (2) Convective updrafts for mass, dry static energy, moisture, cloud liquid water and momentum are parameterized by a one-dimensional entrainment/detrainment bulk-cloud model. The lateral entrainment of the environmental air into the unstable ascending parcel before it rises to the lifting condensation level is considered. The entrainment/detrainment amount for the updraft cloud parcel is separately determined according to the increase/decrease of updraft parcel mass with altitude, and the mass change for the adiabatic ascent cloud parcel with altitude is derived from a total energy conservation equation of the whole adiabatic system in which involves the updraft cloud parcel and the environment; (3) The convective downdraft is assumed saturated and originated from the level of minimum environmental saturated equivalent potential temperature within the updraft cloud; (4) The mass flux at the base of convective cloud is determined by a closure scheme suggested by Zhang (J Geophys Res 107(D14), doi: 10.1029/2001JD001005 , 2002) in which the increase/decrease of CAPE due to changes of the thermodynamic states in the free troposphere resulting from convection approximately balances the decrease/increase resulting from large-scale processes. Evaluation of the proposed convection scheme is performed by using a single column model (SCM) forced by the Atmospheric Radiation Measurement Program’s (ARM) summer 1995 and 1997 Intensive Observing Period (IOP) observations, and field observations from the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE) and the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The SCM can generally capture the convective events and produce a realistic timing of most events of intense precipitation although there are some biases in the strength of simulated precipitation.

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

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

Some issues in uncertainty quantification and parameter tuning: a case study of convective parameterization scheme in the WRF regional climate model

Abstract: . The current tuning process of parameters in global climate models is often performed subjectively or treated as an optimization procedure to minimize model biases based on observations. While the latter approach may provide more plausible values for a set of tunable parameters to approximate the observed climate, the system could be forced to an unrealistic physical state or improper balance of budgets through compensating errors over different regions of the globe. In this study, the Weather Research and Forecasting (WRF) model was used to provide a more flexible framework to investigate a number of issues related uncertainty quantification (UQ) and parameter tuning. The WRF model was constrained by reanalysis of data over the Southern Great Plains (SGP), where abundant observational data from various sources was available for calibration of the input parameters and validation of the model results. Focusing on five key input parameters in the new Kain-Fritsch (KF) convective parameterization scheme used in WRF as an example, the purpose of this study was to explore the utility of high-resolution observations for improving simulations of regional patterns and evaluate the transferability of UQ and parameter tuning across physical processes, spatial scales, and climatic regimes, which have important implications to UQ and parameter tuning in global and regional models. A stochastic importance sampling algorithm, Multiple Very Fast Simulated Annealing (MVFSA) was employed to efficiently sample the input parameters in the KF scheme based on a skill score so that the algorithm progressively moved toward regions of the parameter space that minimize model errors. The results based on the WRF simulations with 25-km grid spacing over the SGP showed that the precipitation bias in the model could be significantly reduced when five optimal parameters identified by the MVFSA algorithm were used. The model performance was found to be sensitive to downdraft- and entrainment-related parameters and consumption time of Convective Available Potential Energy (CAPE). Simulated convective precipitation decreased as the ratio of downdraft to updraft flux increased. Larger CAPE consumption time resulted in less convective but more stratiform precipitation. The simulation using optimal parameters obtained by constraining only precipitation generated positive impact on the other output variables, such as temperature and wind. By using the optimal parameters obtained at 25-km simulation, both the magnitude and spatial pattern of simulated precipitation were improved at 12-km spatial resolution. The optimal parameters identified from the SGP region also improved the simulation of precipitation when the model domain was moved to another region with a different climate regime (i.e. the North America monsoon region). These results suggest that benefits of optimal parameters determined through vigorous mathematical procedures such as the MVFSA process are transferable across processes, spatial scales, and climatic regimes to some extent. This motivates future studies to further assess the strategies for UQ and parameter optimization at both global and regional scales.

Observations of the convective environment in developing and non‐developing tropical disturbances

Abstract: Analyses of thermodynamic data gathered from airborne dropwindsondes released from the upper troposphere during the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment are presented. We focus on two systems that finally became hurricanes Karl and Matthew, and one system (Gaston) that attained tropical storm status, but subsequently weakened and never redeveloped during five days of monitoring. Data for all events show that the largest values of total precipitable water are collocated with the surface trough and with values of convective available potential energy that seem high enough to support convective organization. These values coincide mostly with low values of convective inhibition. Vertical profiles of virtual potential temperature show little variability between soundings on a particular day, but the system means from day to day show a slight warming. In contrast, vertical profiles of pseudo-equivalent potential temperature, θe, show much more variability between soundings on a particular day on account of the variability in moisture. In all systems, there was is a tendency for the lower troposphere to moisten, but in the non-developing system, the troposphere became progressively drier in the height range between approximately 2 and 9 km during the five days of observations. In the developing systems, the troposphere moistened. The most prominent difference between the non-developing system and the two developing systems was the much larger reduction of θe between the surface and a height of 3 km, typically 25 K in the non-developing system, compared with only 17 K in the developing systems. Conventional wisdom would suggest that, for this reason, the convective downdraughts would be stronger in the non-developing system and would thereby act to suppress the development. Here we propose an alternative hypothesis in which the drier air weakens the convective updraughts and thereby the convective amplification of absolute vorticity necessary for development. Copyright © 2012 Royal Meteorological Society

Trends of Convective Available Potential Energy over the Indian region and its effect on rainfall

Abstract: Many studies in recent times have suggested that long-term changes in Convective Available Potential Energy (CAPE) are associated with the changes in convective activity and atmospheric energy budget and, hence, the changes in CAPE could be used as a potential indicator of climate change. Here, we analyse daily radiosonde data of 32 stations over the Indian region from 1984 to 2008 (25 years), to study the climatology and long-term changes in CAPE, lower troposphere moisture content and upper tropospheric temperature. Using gridded daily rainfall data over this region, the relationship between average CAPE and average rainfall as well as the trends in CAPE and its effect on rainfall pattern are also examined during different seasons of this region. All the coastal and island stations, as well as some stations in the foothills of the Himalayas, show higher values of CAPE compared to other inland stations. This study clearly demonstrates that CAPE values have been systematically increasing over most parts of the Indian region during last 25 years. The increase in CAPE is found to be due to increase in low-level moisture content and decrease in upper level temperature over the last 25 years. The seasonal variations show that the CAPE is higher during the monsoon compared to pre-monsoon or post-monsoon seasons and it suggests that thermodynamic conditions favourable for high CAPE together with large-scale dynamics are necessary for organized monsoon convections over this region. In comparison with a large increase in the all-India average of CAPE during monsoon season, which is about 38 J Kg -1 year -1, the all-India summer monsoon rainfall increases about 1.3 mm year -1. The systematic increasing trend in CAPE may be compensating for weakening of monsoon circulation and, thus, maintaining the monsoon rainfall over the Indian region.

A case of offshore convective initiation by interacting land breezes near Darwin, Australia

Abstract: A case of offshore convective initiation by interacting land breezes near Darwin, Australia is investigated using convection-permitting model simulations, radar-derived precipitation observations, thermodynamic profiles from radiosonde soundings and surface measurements. These analyses elucidate the convergence of two land breezes in the Van Diemen Gulf, one originating from the Tiwi Islands and the other from mainland Australia; the convergence is sufficient to initiate a line of convection that forms parallel to the mainland coast in the early morning. While differing in small-scale features, the modeled system shows reasonably good agreement with the observed precipitation accumulations. However, using simulations with different initialization times and examining a second case, it is shown that the representation of the land-breeze system and subsequent convective initiation is very sensitive to the upstream wind and thermodynamic conditions, making correct simulation of these processes challenging.

A Satellite Study of the Atmospheric Forcing and Response to Moist Convection over Tropical and Subtropical Oceans

Abstract: Satellitedataareanalyzedtoexplorethethermodynamicevolutionoftropicalandsubtropicalatmospheres prior and subsequent to moist convection in order to offer an observational test bed for convective adjustment, which is central to the quasi-equilibrium hypothesis. Tropical Rainfall Measuring Mission (TRMM) and Aqua satellite measurements are projected onto a composite temporal sequence over an hourly to daily time scale by exploiting the temporal gap between the local satellite overpasses, which changes from one day to another. The atmospheric forcing and response to convection are investigated separately for deep convective and congestus clouds. In the deep tropics, systematic moisture transport from the atmospheric boundary layer (ABL) to the free troposphere is evident in association with deep convection. The quick ABL ventilationsuggestsaswiftconvectiveadjustmentbutisprecededbyasteadybuildupofABLmoisture,which does not imply continuous adjustment to equilibrium. The evolution of convective available potential energy (CAPE) is controlled not only by the ABL moisture but also largely by a coincident ABL cooling linked with a bipolar anomaly of tropospheric temperature. The ABL moisture and temperature effects together lead to a rapid drop of CAPE for 12 h preceding convection, followed by a restoring phase that emerges as the cool anomaly recovers for a day or two. When moist convection is brought by congestus clouds with no deep convection nearby, CAPE gently increases over a period of 1‐2 days until congestus occurs and then declines as slowly, suggestive of no efficient convective adjustment. The subtropical atmosphere shows no sign of convective adjustment whether or not vigorous convection is present.

The spatiotemporal variability of lightning activity in the Himalayan foothills

Abstract: [1] The spatiotemporal variability of lightning activity in the region of the Himalayan foothills and perpendicular to it has been examined from about 16-year data from the TRMM satellite. The monthly mean flash rate for the period from 1995 to 2010 is maximum in an arc-shaped area along the Himalayan foothills and decreases on both the north and south sides of it. Seasonal variation of mean flash rate changes from annual to semiannual as the area shifts from north to south of the Himalayas and the average elevation of area decreases from 5043 m to 219 m. Further, the mean flash rate is higher in the afternoon and lower from midnight to midday. In the Himalayan foothills, the mean flash rate is highly correlated with surface temperature but poorly correlated with the convective available potential energy (CAPE). Lightning flashes occurring in the high-altitude regions, as compared to the low-altitude regions, are less energetic but more frequent. The empirical orthogonal function (EOF) analysis has been applied to three different data sets of flash rates, surface temperature, and CAPE in the 18.75°N–36°N and 68.75°E–93.75°E area for the period of 1995–2010 to examine the relationship between these parameters in terms of seasonal variations in this region. Spatial distributions of the first mode of flash rate and CAPE and of the first and second modes of surface temperature indicate that the Himalayan range exerts a strong effect on the flash rate distributions in this region. However, while the first mode of spatial distribution of flash rate is in phase with the surface temperature, the time series of CAPE and flash rate are out of phase for the first mode but in phase for their second modes. This result amounts to that 65% of variability in CAPE cannot be associated with the changes in flash rate but 22% of CAPE variability can be associated with changes in flash rate. Higher values of flash rate in the Himalayan foothills are suggested to be associated with the diurnal cycle of a mountain breeze front. Relative roles of convective activities due to solar heating of land, orography of the region, and the synoptic convective systems embedded in monsoon current are discussed to explain the results.

Applications of Full Spatial Resolution Space-Based Advanced Infrared Soundings in the Preconvection Environment

Abstract: Advanced infrared (IR) sounders such as the Atmospheric Infrared Sounder (AIRS) and Infrared Atmospheric Sounding Interferometer (IASI) provide atmospheric temperature and moisture profiles with high vertical resolution and high accuracy in preconvection environments. The derived atmospheric stability indices such as convective available potential energy (CAPE) and lifted index (LI) from advanced IR soundings can provide critical information 1 ~ 6 h before the development of severe convective storms. Three convective storms are selected for the evaluation of applying AIRS full spatial resolution soundings and the derived products on providing warning information in the preconvection environments. In the first case, the AIRS full spatial resolution soundings revealed local extremely high atmospheric instability 3 h ahead of the convection on the leading edge of a frontal system, while the second case demonstrates that the extremely high atmospheric instability is associated with the local developme...

Comparing Aerosol and Low-Level Moisture Influences on Supercell Tornadogenesis: Three-Dimensional Idealized Simulations

Abstract: Four three-dimensional, nested-grid numerical simulations were performed using the Regional Atmospheric Modeling System (RAMS) to compare the effects of aerosols acting as cloud condensation nuclei (CCN) to those of low-level moisture [and thus convective available potential energy (CAPE)] on cold-pool evolution and tornadogenesis within an idealized supercell storm. The innermost grid possessed horizontal grid spacing of 111 m. The initial background profiles of CCN concentration and water vapor mixing ratio varied among the simulations (clean versus dusty and higher-moisture versus lower-moisture simulations). A fifth simulation was performed to factor out the impact of CAPE. The higher-moisture simulations produced spatially larger storms with stronger peak updrafts and low-level downdrafts, heavier precipitation, greater evaporative cooling, and stronger cold pools within the forward and rear flank downdrafts. Each simulated supercell produced a tornado-like vortex. However, the lower-moisture...

The Creation of a High Equivalent Potential Temperature Reservoir in Tropical Storm Humberto (2001) and Its Possible Role in Storm Deepening

Abstract: Thirty global positioning system dropwindsondes (GPS sondes) were used to identify and examine the creation of a reservoir of high equivalent potential temperature (θe) in the nascent eye of Tropical Storm Humberto (2001). The θe did not increase in the high surface wind portion of the storm as it does in mature hurricanes; instead air spiraled into the light-wind center of the developing storm where it was trapped by subsidence under a mesoscale convectively generated vortex (MCV). An energy budget revealed that the inflow column took 7 h to reach the storm center during which a combined average surface enthalpy flux of ~230 W m−2 was diagnosed via the bulk aerodynamic equations. This estimate is close to the 250 W m−2 required for balance based on the energy acquired by the column. The high θe in the lowest kilometer, overlain by a near dry-adiabatic layer under the anvil base, resulted in convective available potential energy (CAPE) exceeding 2500 m2 s−2. This conditionally unstable air later s...

Variability of the thermal structure of the atmosphere during wet and dry spells over southeast India

Abstract: Surface and upper-air observations of meteorological parameters at Gadanki (13.5°N, 79.2°E) are utilized to understand the thermal structure of the atmosphere in dry and wet spells and its effect on draught core statistics. The temperature and humidity variations from wet to dry spell are quite pronounced near the surface and in the atmospheric boundary layer (ABL), but not above the ABL. Interestingly, convective available potential energy (CAPE) is found to be higher during wet spells than during dry spells, in contrast to the earlier studies made elsewhere over warm oceans and tropical land masses. The stratified CAPE values based on rainfall occurrence are also larger during wet spells than during dry spells, irrespective of whether the sounding is made before, during or after the rain occurrence. Surprisingly, large CAPE values are observed after the rain occurrence in both spells. The high CAPE during wet spells (and also after rain occurrence) corresponds well with larger surface equivalent potential temperature (θe). During wet spells, the positive buoyancy is not only larger but also vertically extended to higher altitudes. On the other hand, buoyancy profiles during dry spells are punctuated by several negative buoyancies (associated with stable layers), reducing CAPE values in those spells. A synthesis of all measurements reveals that favourable environmental conditions (high CAPE, humid atmosphere and weak stable layers) allow the convection to grow into a deep system in wet spells. On the other hand, strong stable layers, weak CAPE coupled with relatively less humidity above the ABL inhibits the growth of convective cloud during dry spells. These observations explain the differences in draught core statistics in wet and dry spells. Copyright © 2012 Royal Meteorological Society

Convergence zones and their impact on the initiation of a mesoscale convective system in West Africa

Abstract: During the African Monsoon Multidisciplinary Analysis (AMMA) campaign in 2006, extended surface and boundary-layer measurements were performed to study the influence of soil-moisture patterns on the generation of thermally forced circulations and triggering of deep convection. However, not all processes involved in the triggering of a mesoscale convective system (MCS) could be identified in previous studies. Therefore, COSMO (Consortium for Small-scale Modeling) simulations were carried out investigating possible trigger mechanisms. On 31 July 2006, an MCS was initiated and on 1 August, soil-moisture inhomogeneities resulted in a thermally forced circulation with an associated convergence zone, but no deep convection was triggered. It was found that the MCS on 31 July was influenced by a cyclonic vortex and favoured by the superposition of two convergence zones of different origins. Initiation of the MCS occurred in the simulation when moist monsoon air was transported to the north, associated with a cold pool ahead of another MCS, and reached the convergence zone. On 1 August, the simulation reproduced the thermally forced circulation caused by the soil-moisture pattern, which had been produced by the precipitation of the MCS. However, due to low humidity in the boundary layer and low convective available potential energy, the lifting along the convergence zone did not trigger deep convection. Copyright © 2011 Royal Meteorological Society

A composite stability index for dichotomous forecast of thunderstorms

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

An Extended Model for the Potential Intensity of Tropical Cyclones

Abstract: Emanuel’s theory of hurricane potential intensity (E-PI) makes use of the assumption that slantwise convective instability vanishes in a steady-state vortex of a tropical cyclone. In the framework of an extended mathematical potential intensity model it is shown that relaxing this assumption and including an eye results in a larger maximum wind speed compared to that of the predictions made by E-PI. Previous studies by Bryan and Rotunno demonstrate that the effect of unbalanced flow considerably contributes to maximum winds in excess of E-PI (“superintensity”). The authors argue that the proposed mechanism induced by convective instability provides another possible explanation for simulated and observed tropical cyclones exceeding E-PI in addition to flow imbalance. Further evidence for the relevance of conditional instability in mature tropical cyclones to superintensity is given by the fact that convective available potential energy arises in numerical simulations of tropical cyclones. This is d...

Links Between Weather Phenomena and Characteristics of Refractivity Measured by Precipitation Radar

Abstract: Refractivity depends on meteorological parameters such as temperature and water vapour pressure and can be measured using a weather radar. A realistic atmospheric simulation from the Meso-NH numerical model is used in order to describe and establish the relation between refractivity and the dynamic and thermodynamic phenomena responsible for the development and propagation of convection. These investigations lead to discussion of the complementarity between the refractivity and the convective available potential energy. The relation observed between the refractivity signal and the meteorological parameters calls the refractivity measurement into question, since it is based on phase differentiation with time and space and can be degraded by phase aliasing problems. These aliasing problems increase with the radar frequency (perceptible in the S-band, serious in the C-band, and more serious in the X-band) and also with the integration range and sampling time. Thus, a statistical approach permits us to simulate the possibility of measuring the refractivity with operational radar during convective events. A typical case in the south-east region of France is selected to simulate measurements by radar (S-band, C-band, X-band) in convective systems, in order to evaluate the measurement feasibility, particularly in terms of phase ambiguity, related to temporal and spatial sampling, of a future implementation of the refractivity measurement over the French operational radar network. This numerical statistical approach is completed with a similar study using in-situ measurements performed at the Trappes station. The seasonal and diurnal dependencies of aliasing are investigated, leading to clarification of the impact of the turbulent fluxes on the refractivity measurement.

Large‐scale potential vorticity anomalies and deep convection

Abstract: The effect on deep convection of large-scale potential vorticity (PV) anomalies and their associated tropospheric stable layers is complex and not well understood. This article examines the meteorological events of 9 July 2007 (IOP7b of the Convective and Orographically-induced Precipitation Study (COPS)), which was dominated by an upper-level PV anomaly that stretched from the UK to southern France and as far northeast as Denmark. Three precipitation regions were identified from the case: lines of intense storms beneath the PV anomaly; less intense, more widespread convective precipitation to the east of the PV anomaly; and, in between, a region of no precipitation. The latter of these coincided with the high-resolution measurements and model analyses from COPS. The extensive and varied data analyzed in this investigation show that convective available potential energy (CAPE) was present in this region (the distribution of CAPE and convective inhibition (CIN) is presented via an innovative, pseudo-3D visualization that allows horizontal and vertical interactions to be considered). However, convection was capped by a complex arrangement of dry layers; the base of the key layer was at 750 hPa. These dry layers descended separately from the upper troposphere, moving around the PV anomaly as it developed from a breaking Rossby wave to the west during the seven days before the IOP. This case adds to other studies that show that descent of complex dry layers is an important mechanism for forming convection-inhibiting atmospheric lids in Western Europe. A simple conceptual model is developed that synthesizes the effect of large-scale PV anomalies on deep convection from a series of consistent case studies. This model has significant implications for storm forecasting and projections of storminess in future climates, as it highlights the importance of thin structures that can advect hundreds of km before having an impact. Copyright © 2012 Royal Meteorological Society

Numerical simulation of the diurnal cycle of rainfall in SW Amazon basin during the 1999 rainy season: the role of convective trigger function

Abstract: In continental areas, the maximum rainfall simulated with the Brazilian developments on the Regional Atmospheric Modeling System (BRAMS) occurs around 4 h earlier than the one observed with rain gauges. This work presents the successful implementation of a new convective trigger function (CTF) in the convective parameterization scheme used in BRAMS that corrects this misfit between model and observations. The importance of the CTF formulation on the diurnal cycle of rainfall over the Amazon Basin is reflected by the following numbers: Over Rondonia (SW Amazonia), the original version of BRAMS simulates the maximum rainfall at 1400 UTC (1000 LST), with the new CTF maximum shifting to 1800 UTC (1400 LST), while the S-band radar rainfall maximum is at 1900 UTC (1500 LST). This is attributed to two factors: (1) the new CTF is now coupled to the sensible and latent heat fluxes at surface; (2) during the early morning, the convective available potential energy is reduced.

Environmental factors influential to the duration and intensity of tornadoes in simulated supercells

Abstract: [1] Simulations were performed using an idealized cloud model to investigate the environmental conditions influential to tornado duration and intensity. The results from twenty-one tornadic simulations are presented—far more than any previous study. The results show a nearly linear relationship between storm relative environmental helicity (SREH) and simulated tornado duration. A multiple linear regression analysis was performed showing that, in addition to SREH, convective available potential energy (CAPE), convective inhibition (CIN), and precipitable water (Pwat) are useful predictor variables for simulated tornado duration. Similar results were found for simulated tornado intensity, as represented by the maximum central pressure drop.

Improved Regional Analyses and Heavy Precipitation Forecasts With Assimilation of Atmospheric Infrared Sounder Retrieved Thermodynamic Profiles

Abstract: This paper describes a procedure to assimilate Atmospheric Infrared Sounder (AIRS)-retrieved thermodynamic profiles into a regional configuration of the Weather Research and Forecasting (WRF) model and validates subsequent precipitation forecasts over the eastern half of the continental U.S. Quality indicators were used to select the highest quality temperature and moisture profiles for assimilation throughout the entire atmosphere in clear and partly cloudy regions and above cloud top in cloudy regions. Separate error characteristics for land and water profiles were also used in the assimilation process. Assimilation of AIRS profiles produced analyses with a better validation to in situ observations than the short-term WRF forecast first-guess field. The AIRS-enhanced initial conditions improved simulation of a severe weather event over Texas and Louisiana from February 12-13, 2007. For this event, assimilation of AIRS profiles produced a more unstable boundary-layer air mass in the warm sector ahead of an advancing midlatitude cyclone, resulting in enhanced convective available potential energy in the model. The simulated squall line and precipitation totals from a forecast initialized with AIRS-enhanced initial conditions more closely reflected ground-based observations than one initialized with a no-AIRS control forecast. The impact of the improved initial conditions through the assimilation of AIRS profiles was further demonstrated through an evaluation of a 37-day period from the winter of 2007. The unstable environment over the Gulf of Mexico and coastal region with the AIRS-enhanced initial conditions resulted in 20%+ improvements in the 6-h accumulated precipitation forecasts out to 48 h over that period.

Modelling the re-intensification of tropical storm Erin (2007) over Oklahoma: understanding the key role of downdraft formulation

Abstract: This article reports on the inland re-intensification of tropical storm (TS) Erin (2007). In this research, the physical processes that resulted in the re-intensification of TS Erin over Oklahoma, USA, on 19 August 2007 was determined and a sensitivity study on microphysics, planetary boundary layer and convective parameterisation schemes was performed in the mesoscale modelling system, MM5. Also, we diagnosed and explained the remarkable difference between model behaviour of the original KainFritsch 1 (KF1) scheme and its revised counterpart (KF2). The numerical results showed only modest sensitivity to the selected microphysics schemes the relatively simple ‘Simple Ice’ and the advanced Reisner-Graupel. We found a relatively high sensitivity to the selected boundary layer parameterisation. Enhanced mixing in the medium range forecast (MRF) scheme leads to a relatively small convective available potential energy (CAPE), a deeper boundary layer and a lower dew point temperature, thus to a relatively stable environment. Therefore, MRF forecasts less precipitation (up to 150 mm) than the local mixing scheme, ETA. Model results appeared most sensitive to the selected convection schemes, that is, Grell, KF1 and KF2. With Grell and KF1, Erin intensifies and produces intense precipitation, but its structure remains close to a mesoscale convective system (MCS) or squall line rather than of the observed tropical cyclone. Both schemes also simulate the most intense precipitation too far south (100 km) compared to observations. On the contrary, KF2 underestimates precipitation, but the track of the convection, the precipitation and the pressure distribution are relatively close to radar and field observations. A sensitivity study reveals that the downdraft formulation is critical to modelling TS Erin’s dynamics. Within tropical cyclogenesis, the mid-level relative humidity (RH) is generally very high, resulting in very small downdrafts. KF2 generates hardly any downdrafts due to its dependence on mid-level RH. However, KF1 and Grell generate much stronger downdrafts because they both relate the downdraft mass flux (DMF) to vertical wind shear. This larger DMF then completely alters TS Erin’s dynamics. A modified version of the Grell scheme with KF2 RH-dependent downdraft formulation improved the simulation considerably, with better resemblance to observations on trajectory, radar reflectivity and system structure. Keywords: Erin, tropical storm, re-intensification, MM5, parameterisation, downdraft, convection, precipitation efficiency (Published: 14 August 2012) Citation: Tellus A 2012, 64 , 17417, http://dx.doi.org/10.3402/tellusa.v64i0.17417

Tornadoes in the New York Metropolitan Region: Climatology and Multiscale Analysis of Two Events

Abstract: This paper describes the climatology of tornadoes around New York City (NYC) and Long Island (LI), New York, and the structural evolution of two tornadic events that affected NYC on 8 August 2007 and 16 September 2010. Nearly half (18 of 34 events from 1950 to 2010) of NYC–LI tornadoes developed between 0500 and 1300 EDT, and August is the peak tornado month as compared to July for most of the northeast United States. A spatial composite highlights the approaching midlevel trough, moderate most unstable convective available potential energy (MUCAPE), and frontogenesis along a low-level baroclinic zone. Shortly before the early morning tornadoes on 8 August 2007, a mesoscale convective system intensified in the lee of the Appalachians in a region of low-level frontogenesis and moderate MUCAPE (~1500 J kg−1). Warm advection at low levels and evaporative cooling within an elevated mixed layer (EML) ahead of the mesoscale convective system (MCS) helped steepen the low-level lapse rates. Meanwhile, a s...

Radar investigations of cloud merger

Abstract: Presented are the results of radar investigations of cloud merger during their natural cycle of development and after the seeding. The observations were carried out in the southwest of the Kingdom of Saudi Arabia in 2008. It is revealed that the merging was observed under conditions of very unstable atmosphere, when the convective available potential energy exceeded 3000 J/kg. The cloud merger impacts significantly their development. The most considerable changes due to the cloud merging were observed for the cloud mass and precipitation flow increased as a result of this process by two or three times. It is discovered that the impact of the merger of clouds on their characteristics exceeds significantly the effect of seeding carried out for the feeder clouds.

Some Essential Issues Connected with Severe Convective Weather Analysis and Forecast

Abstract: Many weather forecasters seem to have acquaintance with most of basic concepts or fundamental theories which are connected with severe convection,but some of them are misapplied frequently by some forecasters when they are engaged in severe convective weather analysis or forecasting argumentation.Due to the above problem,some basic concepts and fundamental theories should be explained from the view of forecasting application.The following issues are discussed in this paper.They are the relationship between humidity and water vapor content,the role of clod air during the precipitation process,the fundamental theories connected with thermal and dynamic instability,the sounding analysis related to instability parameters, the relationship between helicity or moist potential vorticity and instability,the relationship among the convergence line,lifting velocity and convective vertical movement,and the essential connection between the synoptic patterns and severe convective phenomena.

An observational analysis of a derecho in South China

Abstract: Derechos occur frequently in Europe and the United States, but reports of derechos in China are scarce. In this paper, radar, satellite, and surface observation data are used to analyze a derecho event in South China on 17 April 2011. A derecho-producing mesoscale convective system formed in an environment with medium convective available energy, strong vertical wind shear, and a dry layer in the middle troposphere, and progressed southward in tandem with a front and a surface wind convergence line. The windstorm can be divided into two stages according to differences in the characteristics of the radar echo and the causes of the gale. One stage was a supercell stage, in which the sinking rear inflow of a high-precipitation supercell with a bow-shaped radar echo induced a Fujita F0 class gale. The other stage was a non-supercell stage (the echo was sequentially kidney-shaped, foot-shaped, and an ordinary single cell), in which downbursts induced a gale in Fujita F1 class. This derecho event had many similarities with derechos observed in western countries. For example, the windstorm was perpendicular to the mean flow, the gale was located in the bulging portion of the bow echo, and the derecho moved southward along with the surface front. Some differences were observed as well. The synoptic-scale forcing was weak in the absence of an advancing high-amplitude midlevel trough and an accompanying strong surface cyclone; however, the vertical wind shear was very strong, a characteristic typical of derechos associated with strong synoptic-scale forcing. Extremely high values of convective available potential energy and downdraft convective available potential energy have previously been considered necessary to the formation of weak-forcing archetype and hybrid derechos; however, these values were much less than 2000 J during this derecho event.

Convective Squalls over the Eastern Equatorial Atlantic

Abstract: The Congo Basin and the adjacent equatorial eastern Atlantic are among the most active regions of the world in terms of intense deep moist convection, leading to frequent lightning and severe squalls. Studying the dynamics and climatology of this convection is difficult due to a very sparse operational network of ground-based observations. Here, a detailed analysis of recently available high temporal resolution meteorological observations from three oil platforms off the coast of Angola spanning the three wet seasons from 2006/07 to 2008/09 is presented. The annual cycle of squall days as identified from wind data closely follows that of convective available potential energy (CAPE) and therefore mirrors the cycle of wet and dry seasons. The diurnal cycle of squall occurrence varies from station to station, most likely related to local features such as coastlines and orography, which control the initiation of storms. An attempt to classify squalls based on the time evolution of the station meteorology and satellite imagery suggests that microbursts account for at least one-third of the strong gusts, while mesoscale squall lines appear to be quite rare. On a daily basis the probability of squall occurrence increases with increasing values of CAPE, downdraft CAPE, and 925–700-hPa wind shear, and decreases for high convective inhibition, all calculated from vertical profiles of temperature and humidity at the nearest grid point in the NCEP–NCAR and ECMWF reanalysis datasets. Both the climatological results and the stability indices can be used for local forecasting to avoid squalls impacting on operations on the offshore platforms.

Analysis and Numerical Simulation Research on Severe Surface Wind Formation Mechanism and Structural Characteristics of a Squall Line Case

Abstract: An unusually severe squall line resulted in significant losses of lives and property on June 3,2009,in Henan,China.To better understand the characteristics and production mechanism of the squall line,the data of satellite,radar,intensive surface observation,and the mesoscale Weather Research and Forecasting(WRF) model were used to investigate the atmospheric background,macrostructure,and microstructure of the squall line and the formation mechanism of the damaging surface wind.The results show that a northeast cold vortex was the main influencing system of the squall line.The transversal trough located at the back of the northeast cold vortex induced a strong cold airflow,which met with a relatively weak southwesterly warm and moist airflow to produce convection.The system further developed in the study region as a severe squall line.The atmosphere contained weak southwesterly winds and water vapor at low layers;thus,the atmospheric environment of the squall line formation was dryer.The atmosphere was conditionally unstable with a convective available potential energy(CAPE) index of approximately 1300 J/kg and adequate wind shear.A relatively cold and moist high with thunderstorms and a strong cold pool on the surface field occurred concurrently with the squall line to produce severe surface wind in addition torather than heavy rain.The results of the WRF model showed that although the maximum downdraft of the squall line was only-13 m/s,the surface outflow wind speed was 35 m/s,which exceeds the maximum downdraft by a factor of 2.7.Further investigation revealed that the cooling processes of rain evaporation and graupel melting are the major contributors to the decrease in surface temperature and strong wind production.Among them,the cooling rate due to rain evaporation was approximately-3 K/min while that due to graupel melting was approximately-0.7 K/min.Therefore,the key factor to influence the cold pool intensity was rain evaporation;this cold pool played a critical role in the formation of the severe surface winds during the squall line event.

A Modeling Study of the Impacts of Pearl River Delta Urban Environment on Convective Precipitation

Abstract: Using a 2-km-resolution WRF model,along with its single-layer urban canopy model and Thompson bulk-parameterization microphysics scheme,numerical simulation of a convective rainstorm is executed over Guangzhou city in the Pear River Delta(PRD) to investigate the impacts of urban environments(including urban land use changes and urban air pollution-enhanced cloud droplet number concentration) on the convective precipitationResults indicate that urban heat island and dry island effect produced by the land surface can lead to an enhanced urban boundary layer,which is conducive to air confluence and unstable energy increase in the areas near the citySimulation shows that to the north and south of Guangzhou city,caused by the urban land surface,there exists respectively a convergence zone with enhanced CAPE(Convective Available Potential Energy) valuesThe simulated radar echoes being initiated within these zones and consistent well with observations reflect that the urban land surface plays a more direct role in convection initiation and developmentOnce convection develops,sensitive experiments show that enhanced cloud droplet number concentration might lead to more precipitationIn case with high concentration(polluted cases),precipitation is increased by up to 20%Diagnostic analysis suggests that the increased precipitation results from that more rain water and supercooled cloud water produce in the convective cloudsThese rain and cloud water,carried upward by strong upward motion intensified by more latent heat release during this process,can be transported to higher altitudes,where ice formation is enhanced through freezing or through interaction with other already existing ice species,and that finally results in surface precipitation increase

A Study of Convective Rainstorms along the East Slope of Western Sichuan Plateau

Abstract: With conventional rain gauge data,radio-soundings,and NCEP 1°×1° reanalysis data,12 rainstorm events along the east slope of the western Sichuan Plateau during 2004-2010 were analyzed.The weather patterns and environmental parameters during the formation and development of the convective rainstorms were investigated and the role of the western Sichuan Plateau in the formation of the convective rainstorms was evaluated.A set of idealized numerical simulations of the area's topography were carried out to verify the results based on observations and the WRF mesoscale numerical model and the mean vertical profile calculated from plateau-upstream radio-soundings of the 12 convective rainstorms.The results are as follows:The plateau low pressure system moving into the Sichuan Basin is the main weather occurrence affecting the topographically convective rainstorm formation and development.With a low system moving into the Sichuan Basin,the southwest(south) flow in the lower troposphere evolves into a southeast(east) flow,the occurrence of which is the main cause of the topographically convective rainstorm formation.The air column contains sufficient moisture and convective available potential energy(CAPE) for the formation and development of the topographically convective rainstorm.The CAPE shows a clear diurnal variation with a peak phase during 1400LST to 2000LST,which is beneficial to the convective rainstorm formation and development.Due to the topographic Fr of the lower southeast(east) flow being slightly less than 1,the forced ascent and blocking of the around-flow may coexist when the air flows up the plateau.The air flow climbing in the topographical buffer zone and the cyclonic shear induced from the around-flow are both favorable for the initiation and development of the convective rainstorms.Lastly,a physical concept model is developed for the convective rainstorm formation in the buffer zone of the western Sichuan Plateau.