Showing papers on "Convective available potential energy published in 2005"

FORECASTER'S FORUM The Use of Moisture Flux Convergence in Forecasting Convective Initiation: Historical and Operational Perspectives

Abstract: Moisture flux convergence (MFC) is a term in the conservation of water vapor equation and was first calculated in the 1950s and 1960s as a vertically integrated quantity to predict rainfall associated with synoptic-scale systems. Vertically integrated MFC was also incorporated into the Kuo cumulus parameterization scheme for the Tropics. MFC was eventually suggested for use in forecasting convective initiation in the midlatitudes in 1970, but practical MFC usage quickly evolved to include only surface data, owing to the higher spatial and temporal resolution of surface observations. Since then, surface MFC has been widely applied as a short-term (0–3 h) prognostic quantity for forecasting convective initiation, with an emphasis on determining the favorable spatial location(s) for such development. A scale analysis shows that surface MFC is directly proportional to the horizontal mass convergence field, allowing MFC to be highly effective in highlighting mesoscale boundaries between different air masses near the earth’s surface that can be resolved by surface data and appropriate grid spacing in gridded analyses and numerical models. However, the effectiveness of boundaries in generating deep moist convection is influenced by many factors, including the depth of the vertical circulation along the boundary and the presence of convective available potential energy (CAPE) and convective inhibition (CIN) near the boundary. Moreover, lower- and upper-tropospheric jets, frontogenesis, and other forcing mechanisms may produce horizontal mass convergence above the surface, providing the necessary lift to bring elevated parcels to their level of free convection without connection to the boundary layer. Case examples elucidate these points as a context for applying horizontal mass convergence for convective initiation. Because horizontal mass convergence is a more appropriate diagnostic in an ingredients-based methodology for forecasting convective initiation, its use is recommended over MFC.

Effects of modifications to the Zhang-McFarlane convection parameterization on the simulation of the tropical precipitation in the National Center for Atmospheric Research Community Climate Model, version 3

Abstract: [1] This study compares the simulation of tropical convection in the National Center for Atmospheric Research Community Climate Model, version 3 (CCM3), using the original and a revised convective parameterization closure in the Zhang-McFarlane scheme. The revised closure couples convection to the large-scale forcing in the free troposphere instead of to the convective available potential energy in the atmosphere as employed in the original closure. In addition, a relative humidity threshold is used for convection trigger. It is shown that the mean precipitation distribution in the tropical regions for both summer and winter is, in general, improved when the new closure is used. During June, July, and August the precipitation in the western Pacific monsoon region is significantly enhanced, alleviating the negative precipitation bias there in the model. The spurious precipitation in the Arabian Peninsula desert is completely eliminated. During December, January, and February the South Pacific Convergence Zone is enhanced considerably. All these changes are desirable in addressing important model deficiencies. The probability distributions of the precipitation intensity from the model simulations are compared with that from the Tropical Rainfall Measurement Mission (TRMM) data. It is shown that over 90% of the CCM3 precipitation is from light rain with rainfall rate less than 1 mm h−1, whereas the simulation with the new closure and the TRMM observations show significant contribution (30–40%) from rainfall rates greater than 2 mm h−1. Precipitation simulation over the western North Pacific summer monsoon region and the Arabian Peninsula was examined in detail to understand the causes of the precipitation biases in CCM3 over these regions. It is demonstrated that the convective available potential energy (CAPE)-based closure limits the CAPE buildup at the beginning of the monsoon season, resulting in the under simulation of the western North Pacific monsoon precipitation. In the Arabian Peninsula the positive feedback between convection and surface evaporation leads to the spurious heavy precipitation center there. In addition to the new closure the use of relative humidity threshold is also found to be important to the improvement of the simulation.

Effects of Moist Froude Number and CAPE on a Conditionally Unstable Flow over a Mesoscale Mountain Ridge

Abstract: In this study, idealized simulations are performed for a conditionally unstable flow over a two-dimensional mountain ridge in order to investigate the propagation and types of cloud precipitation systems controlled by the unsaturated moist Froude number (Fw) and the convective available potential energy (CAPE). A two-dimensional moist flow regime diagram, based on Fw and CAPE, is proposed for a conditionally unstable flow passing over a two-dimensional mesoscale mountain ridge. The characteristics of these flow regimes are 1) regime I: flow with an upstream-propagating convective system and an early, slowly moving convective system over the mountain; 2) regime II: flow with a long-lasting orographic convective system over the mountain peak, upslope, or lee slope; 3) regime III: flow with an orographic convective or mixed convective and stratiform precipitation system over the mountain and a downstream-propagating convective system; and 4) regime IV: flow with an orographic stratiform precipitatio...

The role of shallow convection in the water and energy cycles of the atmosphere

Abstract: The Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model (AGCM4) is used to study the role of shallow convection in the hydrologic and energy cycles of the atmosphere. Sensitivity tests with AGCM4 show a marked effect of the parameterization of shallow convection in the model. In particular, including the parameterization of shallow convection produces considerably enhanced vertical mixing and decreased stratiform cloud amounts in the lower subtropical atmosphere over the oceans. The differences in simulated stratiform cloud amounts are associated with a change in the globally averaged outgoing shortwave radiative flux at the top of the atmosphere of about 11 W m−2. Additionally, precipitation rates are considerably reduced for stratiform clouds and enhanced for convective clouds in the subtropics, if the parameterization of shallow convection is included in the model. Additional tests show that the simulated responses in cloud amounts and precipitation to the treatment of shallow convection are robust. Additional simulations with modified closures for deep convection and other changes to the treatment of convection in the model still lead to similar responses of the model results.

A convection scheme for data assimilation: Description and initial tests

Abstract: A new simplified parametrization of subgrid-scale convective processes has been developed and tested in the framework of the ECMWF Integrated Forecasting System for the purpose of variational data assimilation, singular vector calculations and adjoint sensitivity experiments. Its formulation is based on the full nonlinear convection scheme used in ECMWF forecasts, but a set of simplifications has been applied to substantially improve its linear behaviour. These include the specification of a single closure assumption based on convective available potential energy, the uncoupling of the equations for the convective mass flux and updraught characteristics and a unified formulation of the entrainment and detrainment rates. Simplified representations of downdraughts and momentum transport are also included in the new scheme. Despite these simplifications, the forecasting ability of the new convective parametrization is shown to remain satisfactory even in seasonal integrations. A detailed study of its Jacobians and the validity of the linear hypothesis is presented. The new scheme is also tested in combination with the new simplified parametrization of large-scale clouds and precipitation recently developed at ECMWF. In contrast with the simplified convective parametrization currently used in ECMWF's operational 4D-Var, its tangent-linear and adjoint versions account for perturbations of all convective quantities including convective mass flux, updraught characteristics and precipitation fluxes. Therefore the new scheme is expected to be beneficial when combined with radiative calculations that are directly affected by condensation and precipitation. Examples are presented of applications of the new moist physics in 1D-Var retrievals using microwave brightness temperature measurements and in adjoint sensitivity experiments. Copyright © 2005 Royal Meteorological Society.

A Numerical Study on the Effects of Taiwan Topography on a Convective Line during the Mei-Yu Season

Abstract: During the morning hours on 23 May 2002, a convective line associated with a mei-yu front brought heavy rainfall along the coast of central Taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy (CAPE) of over 3000 m2 s−2. Doppler radar observations indicated that deep convection was organized into a linear shape with a northeast–southwest orientation along the front about 70 km offshore from Taiwan over the northern Taiwan Strait. The system then moved toward Taiwan at a slow speed of about 4 m s−1. In the present study, the effects of Taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at a 2-km horizontal grid size. Experiments with different terrain heights of Taiwan, including full terrain (FTRN), half terrain (HTRN), and no terrain (NTRN), were performed. The control run using full-terrain and co...

MJO in the NCAR CAM2 with the Tiedtke Convective Scheme

Abstract: The boreal winter Madden–Julian oscillation (MJO) remains very weak and irregular in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 2 (CAM2) as in its direct predecessor, the Community Climate Model version 3 (CCM3). The standard version of CAM2 uses the deep convective scheme of Zhang and McFarlane, as in CCM3, with the closure dependent on convective available potential energy (CAPE). Here, sensitivity tests using several versions of the Tiedtke convective scheme are conducted. Typically, the Tiedtke convection scheme gives an improved mean state, intraseasonal variability, space–time power spectra, and eastward propagation compared to the standard version of the model. Coherent eastward propagation of MJO-related precipitation is also much improved, particularly over the Indian–western Pacific Oceans. A composite life cycle of the model MJO indicates that over the Indian Ocean wind-induced surface heat exchange (WISHE) functions, while over the western/c...

The Sensitivity of Simulated Storm Structure, Intensity, and Precipitation Efficiency to Environmental Temperature

Abstract: Prior parameter space studies of simulated deep convection are extended to embrace shifts in the environmental temperature. Within the context of the parameter space study design, shifts in this environmental temperature are roughly equivalent to changes in the ambient precipitable water (PW). Two series of simulations are conducted: one in a warm environmental regime that is associated with approximately 60 mm of precipitable water, and another with temperatures 8°C cooler, so that PW is reduced to roughly 30 mm. The sets of simulations include tests of the impact of changes in the buoyancy and shear profile shapes and of changes in mixed- and moist layer depths, all of which have been shown to be important in prior work. Simulations discussed here also feature values of surface-based pseudoadiabatic convective available potential energy (CAPE) of 800, 2000, or 3200 J kg 1 , and a single semicircular hodograph having a radius of 12 m s 1 , but with variable vertical shear. The simulations reveal a consistent trend toward stronger peak updraft speeds for the cooler temperature (reduced PW) cases, when the other environmental parameters are held constant. Roughly comparable increases in updraft speeds are noted for all combinations of mixed- and moist layer depths. These increases in updraft strength evidently result from both the reduction of condensate loading aloft and the lower altitudes at which the latent heat release by freezing and deposition commences in the cooler, low-PW environments. As expected, maximum storm precipitation rates tend to diminish as PW is decreased, but only slightly, and by amounts not proportionate to the decrease in PW. The low-PW cases thus actually feature larger environment-relative precipitation efficiency than do the high-PW cases. In addition, more hail reaches the surface in the low-PW cases because of reduced melting in the cooler environments. Although these experiments were designed to feature specified amounts of pseudoadiabatic CAPE, it appears that reversible CAPE provides a more accurate prediction of updraft strength, at least for the storms discussed here.

A generalization of CAPE into potential-energy convertibility

Abstract: The concept of the potential-energy convertibility (PEC) is proposed as a generalization of convective available potential energy (CAPE). It is defined as a vertical integral of buoyancy weighted by a non-dimensional normalized vertical momentum. This is a measure of convertibility of potential energy into kinetic energy in the sense that the actual conversion rate is recovered when PEC evaluated by the convective-scale local buoyancy and vertical momentum, as available from cloud-resolving model (CRM) simulations, is multiplied by the normalization factor for the vertical momentum. It reduces to CAPE, when the standard parcel-lifted buoyancy and a unit value for the normalized vertical momentum are used. It is equivalent to Arakawas–Schubert's cloud work function, when the buoyancy and the vertical momentum profile for an entraining plume are used. PEC evaluated from locally defined buoyancy and vertical momentum in CRM simulations correlates better with the convective precipitation than CAPE. The evaluation of PEC within a convective parametrization may be possible with an appropriate definition of the effective entrainment rate, for example, which is expected to improve CAPE-based convective parametrizations. Copyright © 2005 Royal Meteorological Society

WMSI - A New Index For Forecasting Wet Microburst Severity

Abstract: A new index to assess the potential and severity of wet microbursts is currently under development. The index, designated as the Wet Microburst Severity Index (WMSI), accounts for the physical processes of convective storm development and downburst generation by incorporating such parameters as convective available potential energy (CAPE), to represent the process of updraft formation, and Theta-e Deficit (TeD), to represent downburst development. Since convective storm updrafts require buoyant energy, a very important parameter used in the analysis of convection is CAPE, which is easily computed from Geostationary Operational Environmental Satellite (GOES) sounding data. Since updraft strength is proportional to CAPE, large CAPE would result in strong updrafts that could lift the precipitation core within a convective storm to the mid-level dry air layer. CAPE also plays a major role in the formation of precipitation. The strong updrafts resulting from large CAPE will increase the size of precipitation particles, which, in turn, will then enhance the effect of precipitation loading. The amount of mid-level water vapor relative to the low-level water vapor in the atmosphere as indicated by a sounding profile is important in the determination of the strength of downdrafts that occur in convective storms. This condition is modeled by the Theta-e Deficit (TeD), represented by the algorithm TeD = theta-e max - theta-e min , where (theta-e max ) refers to the maximum value of theta-e at the surface and (theta-e min ) refers to the minimum value of theta-e in the mid-levels of the troposphere. TeD serves as an important indicator of the difference in water vapor concentration (plus thermal energy cpT) between the surface and mid-levels. Large TeD values imply the presence of relatively dry air at mid levels that will result in evaporative cooling and the generation of large negative buoyancy as the dry air is entrained into the convection cell. The WMSI algorithm is given as the following expression: WMSI = (CAPE)(theta-e max - theta-e min )/1000. WMSI has been implemented as a new GOES sounder-derived product in the suite of GOES microburst products during the 2003 convective season. This paper will outline the development of the WMSI algorithm and provide examples of the new WMSI product, in which index values at each sounding retrieval location are plotted on GOES imagery. Product validation will entail comparison of the nondimensional GOES WMSI values to measured convective wind gusts at the surface for each wet microburst event. Validation data for the 2003 convective season will be presented. WMSI values were manually calculated for five events in the southeastern United States during the 2002 convective season and compared to measured surface wind gusts. WMSI correlated well to surface wind gusts and a value of approximately 30 was determined to be the threshold for the occurrence of severe winds (> 50 knots)

Simulation of the fine structure of the 12 July 1996 Stratosphere‐Troposphere Experiment: Radiation, Aerosols and Ozone (STERAO‐A) storm accounting for effects of terrain and interaction with mesoscale flow

Abstract: [1] Vertical mixing of chemical tracers and optically active constituents by deep convection affects regional and global chemical balances in the troposphere and lower stratosphere. This important process is not explicitly resolved in global and regional models and has to be parameterized. However, mixing depends strongly on the spatial structure, strength, and temporal evolution of the particular storm, complicating parameterization of this important effect in the large-scale models. To better quantify dynamic fields and associated mixing processes, we simulate a thunderstorm observed on 12 July 1996 during the STERAO-A (Stratosphere-Troposphere Experiment: Radiation, Aerosols, and Ozone) Deep Convection field project using the Goddard Cloud Ensemble (GCE) model. The 12 July STERAO-A storm had very complex temporal and spatial structure. The meteorological environment and evolution of the storm were significantly different than those of the 10 July STERAO-A storm extensively discussed in previous studies. Our 2-D and 3-D GCE model runs with uniform one-sounding initialization were unable to reproduce the full life cycle of the 12 July storm observed by the CHILL radar system. To describe the storm evolution, we modified the 3-D GCE model to include the effects of terrain and the capability of using nonuniform initial fields. We conducted a series of numerical experiments and reproduced the observed life cycle and fine spatial structure of the storm. The main characteristics of the 3-D simulation of the 12 July storm were compared with observations, with 2-D simulations of the same storm, and with the evolution of the 10 July storm. The simulated 3-D convection appears to be stronger and more realistic than in our 2-D simulations. Having developed in a less unstable environment than the 10 July 1996 STERAO-A storm, our simulation of the 12 July storm produced weaker but sustainable convection that was significantly fed by wind shear instability in the lower troposphere. The time evolution, direction, and speed of propagation of the storm were determined by interaction with the nonuniform background mesoscale flow. For example, storm intensity decreased drastically when the storm left the region with large convective available potential energy. The model appears to be successful in reproducing the rectangular four-cell structure of the convection. The distributions of convergence, vertical vorticity, and position of the inflow level in the later single-cell regime compare favorably with the airborne Doppler radar observations. This analysis allowed us to better understand the role of terrain and mesoscale circulation in the development of a midlatitude deep convective system and associated convective mixing. Wind, temperature, hydrometeor, and turbulent diffusion coefficient data from the cloud model simulations were provided for off-line 3-D cloud-scale chemical transport simulations discussed in the companion paper by DeCaria et al. (2005).

An Analysis of a Meso-β System in a Mei-yu Front Using the Intensive Observation Data During CHeRES 2002

Abstract: The conventional and intensive observational data of the China Heavy Rain Experiment and Study (CHeRES) are used to specially analyze the heavy rainfall process in the mei-yu front that occurred during 20–21 June 2002, focusing on the meso-β system. A mesoscale convective system (MCS) formed in the warm-moist southwesterly to the south of the shear line over the Dabie Mountains and over the gorge between the Dabie and Jiuhua Mountains. The mei-yu front and shear line provide a favorable synoptic condition for the development of convection. The GPS observation indicates that the precipitable water increased obviously about 2–3 h earlier than the occurrence of rainfall and decreased after that. The abundant moisture transportation by southwesterly wind was favorable to the maintenance of convective instability and the accumulation of convective available potential energy (CAPE). Radar detection reveals that meso-β and-γ systems were very active in the MαCS. Several convection lines developed during the evolution of the MαCS, and these are associated with surface convergence lines. The boundary outflow of the convection line may have triggered another convection line. The convection line moved with the mesoscale surface convergence line, but the convective cells embedded in the convergence line propagated along the line. On the basis of the analyses of the intensive observation data, a multi-scale conceptual model of heavy rainfall in the mei-yu front for this particular case is proposed.

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

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

The role of relative humidity on shallow cumulus dynamics; results from a Large Eddy Simulation model

Abstract: Shallow cumulus clouds play an important role in the vertical transport of heat, moisture and momentum in the atmosphere. However, due to their relatively small horizontal scales they cannot be resolved explicitly in climate or weather forecasting models. The mass flux parameterization is used to solve this problem. Our question is whether atmospheric relative humidity influences the cloud dynamics and hence should be incorporated in parameterizations. To this end we have used a large eddy simulation model (LES) to see how the cloud dynamics are affected when the mean atmosphere specific humidity and mean temperature profiles are changed in such a way that the mean buoyancy remains constant. We have also used the LES model to see what effect the surface fluxes of latent and sensible heat have on the cloud dynamics. In particular, we have varied these surface fluxes such that the surface buoyancy flux, to a good approximation remains identical, i.e. the convective velocity scale is identical. Our results show that mean atmospheric relative humidity does influence the cloud dynamics. The difference in cloud mass flux between the simulations is primarily determined by the cloud fraction, which is related to the relative humidity; more moisture leads to larger cloud fractions. Cloud vertical velocities need not be scaled; they seem to have the same profiles. More humid environments lead to increased total and liquid water content, increased entrainment and detrainment rates and at the same time more turbulent kinetic energy is generated, thus the buoyancy flux is also affected. When trying to scale the latter quantity following Grant and Lock (2004) we found that the convective available potential energy (CAPE) must be submitted to severe scrutiny since CAPE is largely affected by the mean atmospheric relative humidity.

Forecasting Convective Downburst Potential Over The United States Great Plains

Abstract: A favorable environment for downbursts associated with deep convective storm systems that occur over the central and eastern continental United States includes strong static instability with large amounts of convective available potential energy and the presence of a mid-tropospheric layer of dry air. However, previous research has identified that over the central United States, especially in the Great Plains region, an environment between that favorable for wet and dry microbursts may exist during the convective season, resulting in the generation of hybrid type microbursts. Hybrid microbursts have been found to originate from deep convective storms that generate heavy precipitation, with sub-cloud evaporation of precipitation a significant factor in downdraft acceleration. Accordingly, a new GOES sounder derived product, the GOES Hybrid Microburst Index, is under development and is designed to assess the potential for convective downbursts that develop in an intermediate environment between a wet type, associated with heavy precipitation, and a dry type associated with convection in which very little to no precipitation is observed at the surface.

Convective Circulations in an Idealized Fluid System

Abstract: We investigate the role of boundary layer forcing and surface heterogeneities on the intensity and spectral distribution of the convective circulations of an idealized convective system. Our ultimate goal is to further the understanding of atmospheric convection. However, we depart from realistic atmospheric convection and study an idealized convective system known as the Rayleigh-Benard model in two dimensions. We extended the classical Rayleigh-Benard model to include the effects of boundary heterogeneities. These effects are included, in particular through a sinusoidally variable surface temperature. In this idealized model, the Rayleigh number plays the role of convective available potential energy (CAPE) in atmospheric convection, while the boundary heterogeneities in the temperature play the role of boundary layer forcing. In particular, we study the effects of boundary forcing on the intensity and spectral distribution of convective circulations in great detail. We consider the problem in the linear and weakly nonlinear regimes. In the linear regime, we find an analytical solution for Rayleigh-Benard convection with boundary forcing. We show that the inclusion of periodic boundary forcing causes discontinuities in the linear solution when critical conditions are approached. In the nonlinear regime, we find the solution by direct numerical simulation. The nonlinearities not only remove the discontinuities, but also lead to the appearance of non-trivial modes in the solution. The classical modes appear when the Rayleigh number is supercritical and the amplitude of the boundary forcing is small. Modes governed by boundary forcing dominate when its amplitude is large. Non-trivial modes with wavenumbers different from either the classical or the boundary modes appear only for intermediate values of the boundary forcing. The transitions between regions dominated by the classical Rayleigh forcing, mixed forcing, and boundary forcing depend on the Rayleigh

A Numerical Investigation of a Slow-Moving Convective Line in a Weakly Sheared Environment

Abstract: A series of three-dimensional, cloud-resolving numerical simulations are performed to examine a slowpropagating, quasi-two-dimensional convective system in a weakly sheared environment during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere (TRMM-LBA) field campaign. The focus is on the kinematics and thermodynamics, organization mechanisms, and dynamical effects of low-level shear, ice microphysics and tropospheric humidity. The control simulation, which is initialized with the observed sounding and includes full microphysics, successfully replicates many observed features of the convective system, such as the linear structure, spatial orientation, life cycle, and sluggish translation. The system at the mature stage displays a line-normal structure similar to that associated with squalltype convective systems, but the corresponding mesoscale circulation and thermodynamic modification are much weaker. Ice-phase microphysical processes are not necessary to the formation of the convective system, but they play a non-trivial role in the late evolution stage. In contrast, the low-level shear, albeit shallow and weak, is critical to the realistic realization of the convective line. The tropospheric moisture above the planetary boundary layer has an important impact on the behavior of convective organization. In particular, a dry layer in the lower troposphere significantly suppresses convective development and inhibits the generation of organized convection even though the convective available potential energy is substantial. The free-atmosphere humidity has received little attention in previous studies of organized convection and warrants further investigation.