Showing papers on "Convective available potential energy published in 2002"

Contrasting convective regimes over the Amazon: Implications for cloud electrification

Abstract: [1] Four distinct meteorological regimes in the Amazon basin have been examined to distinguish the contributions from boundary layer aerosol and convective available potential energy (CAPE) to continental cloud structure and electrification. The lack of distinction in the electrical parameters (peak flash rate, lightning yield per unit rainfall) between aerosol-rich October and aerosol-poor November in the premonsoon regime casts doubt on a primary role for the aerosol in enhancing cloud electrification. Evidence for a substantial role for the aerosol in suppressing warm rain coalescence is identified in the most highly polluted period in early October. The electrical activity in this stage is qualitatively peculiar. During the easterly and westerly wind regimes of the wet season, the lightning yield per unit of rainfall is positively correlated with the aerosol concentration, but the electrical parameters are also correlated with CAPE, with a similar degree of scatter. Here cause and effect are difficult to establish with available observations. This ambiguity extends to the “green ocean” westerly regime, a distinctly maritime regime over a major continent with minimum aerosol concentration, minimum CAPE, and little if any lightning.

Contrasting convective regimes over the Amazon: Implications for cloud electrification : Large-scale biosphere-atmosphere experiment in Amazonia (LBA)

Abstract: [i] Four distinct meteorological regimes in the Amazon basin have been examined to distinguish the contributions from boundary layer aerosol and convective available potential energy (CAPE) to continental cloud structure and electrification. The lack of distinction in the electrical parameters (peak flash rate, lightning yield per unit rainfall) between aerosol-rich October and aerosol-poor November in the premonsoon regime casts doubt on a primary role for the aerosol in enhancing cloud electrification. Evidence for a substantial role for the aerosol in suppressing warm rain coalescence is identified in the most highly polluted period in early October. The electrical activity in this stage is qualitatively peculiar. During the easterly and westerly wind regimes of the wet season, the lightning yield per unit of rainfall is positively correlated with the aerosol concentration, but the electrical parameters are also correlated with CAPE, with a similar degree of scatter. Here cause and effect are difficult to establish with available observations. This ambiguity extends to the green ocean westerly regime, a distinctly maritime regime over a major continent with minimum aerosol concentration, minimum CAPE, and little if any lightning.

Direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells

Abstract: Despite the long-surmised importance of the hook echo and rear-flank downdraft (RFD) in tornadogenesis, only a paucity of direct observations have been obtained at the surface within hook echoes and RFDs. In this paper, in situ surface observations within hook echoes and RFDs are analyzed. These “mobile mesonet” data have unprecedented horizontal spatial resolution and were obtained from the Verifications of the Origins of Rotation in Tornadoes Experiment (VORTEX) and additional field experiments conducted since the conclusion of VORTEX. The surface thermodynamic characteristics of hook echoes and RFDs associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and RFDs are favorable (or unfavorable) for tornadogenesis. Tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy, potential buoyancy (as measured by the convective available potential energy), and equivalent potential temperature in the R...

Convective quasi-equilibrium in midlatitude continental environment and its effect on convective parameterization

Abstract: [1] The quasi-equilibrium assumption proposed by Arakawa and Schubert assumes that convection is controlled by the large-scale forcing in a statistical sense, in such a way that the stabilization of the atmosphere by convection is in quasi-equilibrium with the destabilization by the large-scale forcing. The assumption was developed largely based on observations in the tropical maritime environment and has not been evaluated in midlatitudes. This study examines the quasi-equilibrium assumption in midlatitude continental convection environment using summertime observations from the Southern Great Plains of the United States. Two complementary approaches are taken for this purpose. The first one compares the net time rate of change of convective available potential energy to that due to the large-scale forcing. The second one examines the contributions to the net change of CAPE from the boundary layer air and the free tropospheric air above. Results from both the approaches indicate that the quasi-equilibrium assumption is not well suited for midlatitude continental convection. It is shown that the net change of CAPE is comparable to and largely comes from that due to thermodynamic changes of the boundary layer air, while the contribution from the free troposphere above the boundary layer is negligible. The analysis also shows that the role of convective inhibition to suppress convection is the most pronounced when the large-scale forcing in the free troposphere is weak. On the basis of these and other observations, a modification to the quasi-equilibrium assumption is proposed. It assumes that convective and large-scale processes in the free troposphere above the boundary layer are in balance, so that contribution from the free troposphere to changes in CAPE is negligible. This assumption is then tested using the single column model of the NCAR CCM3 by modifying the closure in the CCM3 convection scheme. Such a modification significantly improves the single column model simulation. The applicability of this new quasi-equilibrium assumption to tropical convection environment is also discussed.

Comparison between Observed Convective Cloud-Base Heights and Lifting Condensation Level for Two Different Lifted Parcels

Abstract: Approximately 400 Automated Surface Observing System (ASOS) observations of convective cloud-base heights at 2300 UTC were collected from April through August of 2001. These observations were compared with lifting condensation level (LCL) heights above ground level determined by 0000 UTC rawinsonde soundings from collocated upper-air sites. The LCL heights were calculated using both surface-based parcels (SBLCL) and mean-layer parcels (MLLCL—using mean temperature and dewpoint in lowest 100 hPa). The results show that the mean error for the MLLCL heights was substantially less than for SBLCL heights, with SBLCL heights consistently lower than observed cloud bases. These findings suggest that the mean-layer parcel is likely more representative of the actual parcel associated with convective cloud development, which has implications for calculations of thermodynamic parameters such as convective available potential energy (CAPE) and convective inhibition. In addition, the median value of surface-based CAPE (SBCAPE) was more than 2 times that of the mean-layer CAPE (MLCAPE). Thus, caution is advised when considering surface-based thermodynamic indices, despite the assumed presence of a well-mixed afternoon boundary layer.

The Sensitivity of the Numerical Simulation of the Southwest Monsoon Boundary Layer to the Choice of PBL Turbulence Parameterization in MM5

Abstract: Summertime convection over Arizona typically begins in the early afternoon and continues into the night. This suggests that the evolution of the daytime planetary boundary layer is important to the development of Arizona convection. If numerical models are to provide useful guidance for forecasting convection during the monsoon, then the planetary boundary layer must be simulated as accurately as possible through utilization of the appropriate physical parameterizations. This study examines the most appropriate Pennsylvania State University‐National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) planetary boundary layer parameterization(s) for deterministic and ensemble modeling of the monsoon. The four MM5 planetary boundary layer parameterizations tested are the Blackadar, Burk‐Thompson, Eta, and medium-range forecast (MRF) schemes. The Blackadar and MRF planetary boundary layer schemes correctly predict the development of the deep, monsoon planetary boundary layer, and consequently do a better job of predicting the convective available potential energy and downdraft convective available potential energy, but not the convective inhibition. Because the convective inhibition is not accurately predicted, it is possible that the MM5’s ability to initiate or ‘‘trigger’’ convection might be a limiting factor in the model’s ability to produce accurate quantitative precipitation forecasts during the monsoon. Since the MM5 planetary boundary layer predicted by the Burk‐Thompson and Eta schemes does not accurately reproduce the basic structure of the monsoon planetary boundary layer, their inclusion in a mixed physics ensemble is discussed.

Large-Scale Modes of a Nonrotating Atmosphere with Water Vapor and Cloud–Radiation Feedbacks

Abstract: A minimal model of a moist equatorial atmosphere is presented in which the precipitation rate is assumed to depend on just the vertically averaged saturation deficit and the convective available potential energy. When wind-induced surface heat exchange (WISHE) and cloud–radiation interactions are turned off, there are no growing modes. Gravity waves with wavenumbers smaller than a certain limit respond to a reduced static stability due to latent heat release, and therefore propagate more slowly than dry modes, while those with larger wavenumbers respond to the normal dry static stability. In addition, there exists a stationary mode that decays slowly with time. For realistic parameter values, the effect of reduced static stability on gravity waves is limited to wavelengths greater than the circumference of the earth. WISHE and cloud–radiation interactions both destabilize the stationary mode, but not the gravity waves.

The Impact on Simulated Storm Structure and Intensity of Variations in the Mixed Layer and Moist Layer Depths

Abstract: The sensitivities of convective storm structure and intensity to variations in the depths of the prestorm mixed layer, represented here by the environmental lifted condensation level (LCL), and moist layer, represented by the level of free convection (LFC), are studied using a three-dimensional cloud model containing ice physics. Matrices of simulations are generated for idealized environments featuring both small and large LCL = LFC altitudes, using a single moderately sheared curved hodograph trace in conjunction with convective available potential energy (CAPE) values of either 800 or 2000 J kg−1, with the matrices consisting of all four combinations of two distinct choices of buoyancy and shear profile shape. For each value of CAPE, the LCL = LFC altitudes are also allowed to vary in a separate series of simulations based on the most highly compressed buoyancy and shear profiles used for that CAPE, with the environmental buoyancy profile shape, subcloud equivalent potential temperature, subcl...

Diurnal march of the convection observed during TRMM-WETAMC/LBA : Large-scale biosphere-atmosphere experiment in Amazonia (LBA)

Abstract: [1] Radiosonde, satellite data, Tropical Ocean-Global Atmosphere (TOGA) radar 2 km constant altitude plan position indicator (CAPPI), and rainfall collected from the TRMM-Wet Season Atmospheric Mesoscale Campaign (WETAMC)/Large-Scale Biosphere-Atmosphere (LBA) Experiment in Amazonia have been used to investigate the diurnal cycle of the tropical convection. Geostationary Operational Environmental Satellite (GOES 8) images were used to describe the diurnal modulation of the total/high/convective cloud fraction and the diurnal evolution of the size spectrum and initiation/dissipation of the convective systems. Radar 2 km CAPPI were used to describe the diurnal cycle of the rain fraction for different thresholds and the diurnal evolution of the size spectrum and initiation/ dissipation of the rain cells. An average over the four rain gauge networks was applied to describe the average hourly rainfall. The upper air network data set was used to compute the thermodynamic variables: equivalent potential temperature (θ e ), convective available potential energy (CAPE), thickness of positive buoyancy, instability, and convective inhibition. High and convective cloud area fractions reach their maximum some hours after the maximum rainfall detected by rain gauge and radar 2 km CAPPI. The minimum cloud cover occurs only a few hours before the maximum precipitation and the maximum cloud cover occurs during the night. The maximum rainfall takes place at the time of the maximum initiation of the convective systems observed by satellite and rain cells. At the time of maximum precipitation the majority of the convective systems and rain cells are small sized and present the maximum increasing area fraction rate. The diurnal evolution of θ e also presents a very clear diurnal variation, with maximum occurring in the early afternoon. The CAPE is well related to θ e . When θ e is high CAPE is high; the atmosphere is unstable and has a deep layer of positive buoyancy and small convective inhibition. These results suggest the following mechanism controlling the diurnal of convection: In the morning, cloud cover decreases as the solar flux reaching the surface increases and consequently increases θ e . In the early afternoon, convection rapidly develops, high and convective cloud fractions increase rapidly, and the maximum precipitation and initiation is observed. After convection is developed the atmosphere profile is modified, reaching a nearly saturated state; the water vapor flux decreases in the boundary layer which becomes very stable, thereby inhibiting surface fluxes and consequently extinguishing the convection.

Diurnal march of the convection observed during TRMM‐WETAMC/LBA

Abstract: [1] Radiosonde, satellite data, Tropical Ocean–Global Atmosphere (TOGA) radar 2 km constant altitude plan position indicator (CAPPI), and rainfall collected from the TRMM-Wet Season Atmospheric Mesoscale Campaign (WETAMC)/Large-Scale Biosphere-Atmosphere (LBA) Experiment in Amazonia have been used to investigate the diurnal cycle of the tropical convection. Geostationary Operational Environmental Satellite (GOES 8) images were used to describe the diurnal modulation of the total/high/convective cloud fraction and the diurnal evolution of the size spectrum and initiation/dissipation of the convective systems. Radar 2 km CAPPI were used to describe the diurnal cycle of the rain fraction for different thresholds and the diurnal evolution of the size spectrum and initiation/dissipation of the rain cells. An average over the four rain gauge networks was applied to describe the average hourly rainfall. The upper air network data set was used to compute the thermodynamic variables: equivalent potential temperature (θe), convective available potential energy (CAPE), thickness of positive buoyancy, instability, and convective inhibition. High and convective cloud area fractions reach their maximum some hours after the maximum rainfall detected by rain gauge and radar 2 km CAPPI. The minimum cloud cover occurs only a few hours before the maximum precipitation and the maximum cloud cover occurs during the night. The maximum rainfall takes place at the time of the maximum initiation of the convective systems observed by satellite and rain cells. At the time of maximum precipitation the majority of the convective systems and rain cells are small sized and present the maximum increasing area fraction rate. The diurnal evolution of θe also presents a very clear diurnal variation, with maximum occurring in the early afternoon. The CAPE is well related to θe. When θe is high CAPE is high; the atmosphere is unstable and has a deep layer of positive buoyancy and small convective inhibition. These results suggest the following mechanism controlling the diurnal of convection: In the morning, cloud cover decreases as the solar flux reaching the surface increases and consequently increases θe. In the early afternoon, convection rapidly develops, high and convective cloud fractions increase rapidly, and the maximum precipitation and initiation is observed. After convection is developed the atmosphere profile is modified, reaching a nearly saturated state; the water vapor flux decreases in the boundary layer which becomes very stable, thereby inhibiting surface fluxes and consequently extinguishing the convection.

Properties of the Convection Scheme in NCEP's Eta Model that Affect Forecast Sounding Interpretation

Abstract: The impact of parameterized convection on Eta Model forecast soundings is examined. The Betts-Miller- Janjicparameterization used in the National Centers for Environmental Prediction Eta Model introduces char- acteristic profiles of temperature and moisture in model soundings. These specified profiles can provide misleading representations of various vertical structures and can strongly affect model predictions of parameters that are used to forecast deep convection, such as convective available potential energy and convective inhibition. The specific procedures and tendencies of this parameterization are discussed, and guidelines for interpreting Eta Model soundings are presented.

Multidecadal trends in tropical convective available potential energy

Abstract: [1] Time series of convective available potential energy (CAPE) calculated from 15 tropical radiosonde stations indicate mostly positive trends in CAPE during 1958–1997. Increases in CAPE are associated with increases in near-surface temperature and water vapor, consistent with previous studies. The predominantly positive trends appear mostly as a shift in the middle 1970s, consistent with the time of an apparent shift of the background state of the climate system, as documented by others. A general circulation model of the atmosphere forced by observed sea surface temperatures does not reproduce these overall increases in CAPE, although it does reproduce the temperature trends. The observed changes imply significant changes to the tropical atmosphere over the last 40 years, and potential limitations of climate model simulations.

Origin of lapse rate changes in the upper tropical troposphere

Abstract: Vertical motions in clouds arise from a variety of thermodynamic processes, including latent heat release, evaporative cooling, melting, and cloud radiative heating. In the Tropics, the net upward vertical mass flux from convective systems should approximately balance subsidence in clear sky regions associated with radiative cooling, provided the exchange of mass with midlatitudes can be assumed small. Tropical climatologies of temperature, water vapor, and ozone are used to calculate the clear sky radiative mass flux, and the derivative of this mass flux with respect to potential temperature, d Mr(u)/du, is used as a proxy for net convective outflow. Convective outflow increases rapidly at 345 K ( ;11.3 km). This corresponds to the pseudoequivalent potential temperature ue at which air parcels near the surface first attain positive convective available potential energy (CAPE). The rate at which dMr(u)/du decreases above 345 K is similar to the rate at which the near surface ue probability distribution function (PDF) decreases. This behavior is referred to as ‘‘scaling.’’ It suggests that the timescale for removal of an air parcel from the convective boundary layer is independent of ue (once it has positive CAPE), and that the residual vertical mass flux from convective clouds can be described as if air parcels detrain near their level of neutral buoyancy (LNB). It is also suggested that the mean tropical temperature profile above 345 K is controlled, not by mixing, but by the need for the vertical variation in net convective outflow to be consistent with the near-surface ue PDF, and that this accounts for the fact that the mean temperature profile above 345 K increasingly deviates from a moist adiabat. It is also argued that there are sufficient high ue air parcels near the surface to sustain the Brewer‐Dobson circulation by detrainment at the LNB followed by radiative ascent into the stratosphere.

Role of Gravity Waves in Triggering Deep Convection during TOGA COARE

Abstract: The role of gravity waves in the initiation of convection over oceanic regions during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) experiment is investigated. First, an autocorrelation method is applied to infrared temperature observations of convective events from satellite images. It reveals that new deep convective cells often occur a few hours after a previous intense event at a typical distance of a few hundred kilometers. Such fast moving modes (faster than 15 m s 21) are interpreted as the trace of gravity waves excited by previous convection and contributing to trigger further convection. Second, the specific case of 11‐12 December 1992, during which an active squall line is generated after the collapse of a previous mesoscale convective system (MCS) nearby, is analyzed with a nonhydrostatic model. The triggering of the second MCS is well reproduced explicitly, owing to the use of the two-way interactive grid nesting. The convective source appears to emit pulses of gravity waves on a wide range of small scales. On the contrary, the troposphere response to the convective source exhibits a spectral simplicity. A slowly evolving mode, characterized by an ascent in the PBL and a compensating subsidence in the free troposphere, favors shallow convection and inhibits deep convection, respectively. Traveling modes propagating away from the convective source are characterized by a fast mode (;50 m s21) and a slower mode (; 25 ms 21) associated with the convective and stratiform development of the source, respectively, in agreement with previous studies. A budget analysis reveals the different factors leading to the deep convection triggering. First, an active PBL characterized by strong surface fluxes and mean ascent, initiates shallow convection, lasting about 2 h, inhibited above by the subsiding motions maintaining a dry layer. Second, horizontal advection of moist and cooler air at midlevels, and detrainment from cumuli, contribute to destroy the dry air layer capping the shallow convective layer. Finally, vertical advection induced by the gravity waves passage modulates the vapor and temperature evolution. Ascending phases favor moistening and cooling, whereas subsiding phases stop these effects, delaying the deep convection onset. The triggering occurs after a strong subsidence, when the ascent phases of deep and shallow modes are combined.

The response of CAPE and CIN to tropospheric thermal variations

Abstract: In the absence of moist convection, convective available potential energy (CAPE) and convective inhibition (CIN) both respond to changes in the thermal and humidity profiles of the atmosphere. It is shown that these changes can be understood in terms of a direct effect, involving changes in the profile in the absence of parcel changes, and an indirect effect involving changes in air-parcel evolution in a developing convective boundary layer. Succinctly, low-wave-number (deep) changes in the thermal profile maximize the direct influence on CAPE while higher-wave-number (shallower, near-surface) thermal changes maximize the direct influence on CIN. A simple estimate of the direct influence on CAPE, which is independent of the assumptions relating to choice of parcel ascent, is shown to give accurate results. The indirect influence comes about as a result of changes in the stability of the profile just above the inversion: the stability acts as a control on the entrainment of dry air into the boundary layer under conditions of surface heating, so that a boundary layer growing into a stable lower troposphere is more humid than one growing into a less stable profile, with significantly higher equivalent potential temperature. Thus, a profile of relatively high stability just above the inversion, typically exhibiting high CIN, will also tend to allow CAPE to build up in the boundary layer quite rapidly, through a suppression of the entrainment under conditions of surface heating and boundary-layer growth. Thus it may be said that high CIN tends to lead to the accumulation of high CAPE even in the absence of convective downdraught feedbacks. Copyright © 2002 Royal Meteorological Society.

Effective Factors in the Development of Deep Convective Clouds over the Wet Region of Eastern China during the Summer Monsoon Season

Abstract: A two-dimensional cloud-resolving model, including a supply of sensible and latent heat fluxes from the surface, is used to study the development of deep convective clouds over a southern region far from the Meiyu front (wet region) of eastern China. Some deep convective clouds were observed during the latter half of the GAME/HUBEX IOP (GEWEX Asian Monsoon Experiment/Huaihe River Basin Experiment Intensive Observation Periods, GEWEX: Global Energy and Water Cycle Experiment) 1998, although there is less large-scale convergence over this region. Numerical simulations reproduce the development of deep convective clouds, and their generating/decaying time. The development process of a convective mixing layer and generation of shallow convective clouds around the top of this layer are also simulated. The results of sensitivity tests on the surface land-use (i.e., the supply of the sensible and latent heat fluxes) and the relative humidity in the middle troposphere, indicate that there are two effective factors in the development of deep convective clouds over this region. One is the large amount of latent heat flux from the surface, and the other is the moist environment in the middle troposphere. The latent heat flux from the surface supplies water vapor to generate convective clouds. Paddy fields can supply a large amount of latent heat flux into the lower atmosphere and are widely distributed over this region. On the other hand, the moist environment in the middle troposphere can cause shallow convective clouds to become deep because the positive buoyancy in the shallow convective clouds is not lost by the evaporation cooling of the entrained air mass. Additionally, this moist environment in the middle troposphere is formed by the development of shallow convective clouds, which transport water vapor from the convective mixing layer.

Similarity of deep continental cumulus convection as revealed by a three-dimensional cloud-resolving model

Abstract: A three-dimensional cloud-resolving simulation of midlatitude continental convection during the Atmospheric Radiation Measurement (ARM) program summer 1997 intensive observation period (IOP) is used to study the similarity of several second and third statistical moments, and second-moment budgets among five episodes of deep convection. Several parameter scales relevant to deep convection similarity are introduced. The dimensionless vertical profiles of the vertical velocity variance and its third moment, cumulus kinetic energy, the prognostic variables’ variances and fluxes, their budgets, as well as several triple correlations cluster together, confirming the dynamical similarity of the simulated convective events. The dimensionless budgets of several second-order moments, such as convective kinetic energy (CKE), its vertical and horizontal components, variance, and vertical fluxes of the prognostic thermodynamic variables, as well as the momentum flux, are also presented. The most interesting aspect of the simulated CKE budget is that, in contrast to the boundary layer and shallow trade wind cumulus convection, the dissipation term is relatively small compared to the dominant buoyancy production, transport, and pressure correlation terms. The prognostic equation for the bulk CKE, defined as the vertically integrated mean CKE per unit area, is also discussed. It is found that the so-called bulk CKE dissipation timescale ranges in the simulation from 4 to 8 h. Therefore, the bulk CKE, mostly contained in the horizontal branches of mesoscale circulations associated with the deep convective systems, can persist much longer than the lifetime of an individual convective cloud. It is also found that the fraction of the bulk CKE associated with the vertical motions is about the same for all of the events considered, suggesting a strong correlation between the bulk CKE and the strength of the convective updrafts. It is shown that the bulk CKE dissipation timescale is inversely proportional to the square root of the bulk CKE itself. It is also found that the convective velocity scale is closely related to the convective available potential energy (CAPE) of the thermodynamic sounding taken immediately before a particular convective event.

Mobile Mesonet Observations on 3 May 1999

Abstract: Two long-lived tornadic supercells were sampled by an automobile-borne observing system on 3 May 1999. The “mobile mesonet” observed relatively warm and moist air, weak baroclinity, and small pressure excess at the surface within the rear-flank downdrafts of the storms. Furthermore, the downdraft air parcels, which have been shown to enter the tornado in past observational and modeling studies, were associated with substantial convective available potential energy and small convective inhibition. The detection of only small equivalent potential temperature deficits (1–4 K) within the downdrafts may imply that the downdrafts were driven primarily by nonhydrostatic pressure gradients and/or precipitation drag, rather than by the entrainment of potentially cold environmental air at midlevels.

Evolution of the Large Scale Circulation, Cloud Structure and Regional Water Cycle Associated with the South China Sea Monsoon During May-June, 1998

Abstract: In this paper, changes in the large-scale circulation, cloud structures and regional water cycle associated with the evolution of the South China Sea (SCS) monsoon in May-June 1998 were investigated using data from the Tropical Rainfall Measuring Mission (TRMM) and field data from the South China Sea Monsoon Experiment (SCSMEX). Results showed that both tropical and extratropical processes strongly influenced the onset and evolution of the SCS monsoon. Prior to the onset of the SCS monsoon, enhanced convective activities associated with the Madden and Julian Oscillation were detected over the Indian Ocean, and the SCS was under the influence of the West Pacific Anticyclone (WPA) with prevailing low level easterlies and suppressed convection. Establishment of low-level westerlies across Indo-China, following the development of a Bay of Bengal depression played an important role in building up convective available potential energy over the SCS. The onset of SCS monsoon appeared to be triggered by the equatorward penetration of extratropical frontal system, which was established over the coastal region of southern China and Taiwan in early May. Convective activities over the SCS were found to vary inversely with those over the Yangtze River Valley (YRV). Analysis of TRMM microwave and precipitation radar data revealed that during the onset phase, convection over the northern SCS consisted of squall-type rain cell embedded in meso-scale complexes similar to extratropical systems. The radar Z-factor intensity indicated that SCS clouds possessed a bimodal distribution, with a pronounced signal (less than 30dBz) at a height of 2-3 km, and another one (less than 25 dBz) at the 8-10 km level, separated by a well-defined melting level indicated by a bright band at around 5-km level. The stratiform-to-convective cloud ratio was approximately 1:1 in the pre-onset phase, but increased to 5:1 in the active phase. Regional water budget calculations indicated that during the active phase, the SCS was a strong sink (E-P much less than 0) of atmospheric moisture, with the primary source of moisture coming from regions further west over Indo-China and the eastern Indian Ocean. Before onset and during the break, the SCS was a moisture source (E-P greater than ) to the overlying atmosphere. In particular, the SCS provided the bulk of moisture to the torrential rain over the YRV in mid-June 1998.

Early evolution of vertical vorticity in a numerically simulated idealized convective line

Abstract: The generation and organization of mesoscale convective vortices (MCVs) is a recurring theme in midlatitude and tropical meteorology during the warm season. In this work a simulation of a finite-length idealized convective line in a westerly shear environment is investigated in the absence of ambient vertical vorticity. An asymmetry in average vertical vorticity forms rapidly at early times in the present simulation. This study focuses on the formation and organization of vertical vorticity at these early simulation times. Previous simulations suggest that tilting of either ambient or storm-generated horizontal vorticity is the primary mechanism responsible for the formation, organization, and maintenance of MCVs. This study confirms recent work regarding the generation of vertical vorticity at early times in the simulation. A Lagrangian budget analysis of the vertical vorticity equation, however, shows that vorticity convergence becomes a comparable, and at times dominant, mechanism for the enhancement and long-term organization of vertical vorticity early in the simulation. Despite differences in the initial ambient horizontal vorticity, hodograph, and convective available potential energy, the Lagrangian budget analysis in the present midlatitude case is consistent with the Lagrangian budget results of a previous tropical squall line simulation. The study of idealized convective lines in midlatitude environmental conditions therefore provide valuable insight into understanding vertical vorticity production in tropical squall lines and their potential relevance to tropical cyclogenesis.

Two-dimensional simulation of orographic effects on mesoscale boundary-layer convection

Abstract: In this paper, orographic effects on mesoscale boundary-layer convection are studied through a series of idealized numerical experiments. It is found that hills tend to weaken convective activity at their summits under higher background winds, whilst strong updraughts can be observed at the summits, or slightly downwind of the peaks, under light-wind conditions; these can be associated with so-called convective cores. When the background winds are strong, the effects of the hill length on the results are only significant when the height of the hill reaches 500 m. The combined effect of a sensible-heating maximum on the hill summit and baroclinic tendencies due to the elevated heating is a tendency to produce a convective core under reasonably light-wind conditions. Under higher-wind conditions, the strong ascents and descents on both sides of the hill, as well as the lee-wave dynamics, seem to pose a more important impact on the convective features than the thermal forcing of the hill. A Richardson-number balance has been proposed to explain these two kinds of response. Finally, under stronger-wind conditions, the convective available potential energy (CAPE) is small at the summit and reaches its maximum value in the lee of the hill while, under light-wind conditions, the air parcels at the summit have more buoyancy and the CAPE downwind of the top of the hill is slightly larger than that elsewhere. Copyright © 2002 Royal Meteorological Society

Interactions between Cloud Microphysics and Cumulus Convection in a General Circulation Model

Abstract: In the Colorado State University general circulation model, cumulus detrainment of cloud water and cloud ice has been, up to now, the only direct coupling between convective and large-scale condensation processes. This one-way interaction from the convective to the large-scale environment parameterizes, in a highly simplified manner, the growth of anvils spreading horizontally at the tops of narrow cumulus updrafts. The reverse interaction from the large-scale to the convective updrafts, through which large-scale cloud water and cloud ice can affect microphysical processes occurring in individual convective updrafts, is missing. In addition, the effects of compensating subsidence on cloud water and cloud ice are not taken into account. A new parameterization of convection, called ‘‘EAUCUP,’’ has been developed, in which large-scale water vapor, cloud water, and cloud ice are allowed to enter the sides of the convective updrafts and can be lifted to the tops of the clouds. As the various water species are lifted, cloud microphysical processes take place, removing excess cloud water and cloud ice in the form of rain and snow. The partitioning of condensed vapor between cloud water and cloud ice, and between rain and snow, is based on temperature. The effects of compensating subsidence on the large-scale water vapor, cloud water, and cloud ice are computed separately. Convective rain is assumed to fall instantaneously to the surface. Three treatments of the convective snow are tested: 1) assuming that all snow is detrained at the tops of convective updrafts, 2) assuming that all snow falls outside of the updrafts and may evaporate, and 3) assuming that snow falls entirely inside the updrafts and melts to form rain. Including entrainment of large-scale cloud water and cloud ice inside the updrafts, large-scale compensating subsidence unifies the parameterizations of large-scale cloud microphysics and convection, but have a lesser impact than the treatment of convective snow on the simulated climate. Differences between the three alternate treatments of convective snow are discussed. Emphasis is on the change in the convective, large-scale, and radiative tendencies of temperature, and change in the convective and large-scale tendencies of water vapor, cloud water, cloud ice, and snow. Below the stratiform anvils, the change in latent heating due to the change in both convective and large-scale heatings contributes a major part to the differences in diabatic heating among the three simulations. Above the stratiform anvils, differences in the diabatic heating between the three simulations result primarily because of differences in the longwave radiative cooling. In particular, detraining convective snow at the tops of convective updrafts yields a strong increase in the longwave radiative cooling associated with increased upper-tropospheric cloudiness. The simulated climate is wetter and colder when convective snow is detrained at the tops of the updrafts than when it is detrained on the sides of the updrafts or when it falls entirely inside the updrafts. This result highlights the importance of the treatment of the ice phase and associated precipitation in the convective cloud models used in cumulus parameterizations.

A forecasting aspect of thundersquall over Calcutta and its parameterisation during pre-monsoon season

Abstract: Severe thunderstorms accompanied by squalls are the most hazardous weather phenomena during pre-monsoon season in north-eastern region of India. An attempt has been made in this paper to study some parameters for forecasting thundersqualls over Calcutta (Airport) during pre-monsoon season. Parameterisation of thermodynamic components alongwith the synoptic support during thundersqualls over Calcutta has been discussed here. A forecasting aspect for propagation speed of thunderstorm cell at Calcutta in pre-monsoon season has been examined with respect to radar-echo positions, mid-level winds and convective available potential energy (CAPE). Occurrences of multiple thundersqualls over Calcutta Airport within a few hours’ interval have been discussed and examined through hodograph analysis, radar observations and synoptic situations.

Tropical convective variability in the CAPE phase space

Abstract: SUMMARY The relationship between pseudo-adiabatic and reversible convective available potential energy (CAPE) in the tropical atmosphere is examined using the Tropical Ocean‐Global Atmosphere programme (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) radiosonde dataset. The effect of the ice phase and the dependence of the various CAPE quantities on surface variabilities is also examined. CAPE distribution is qualitatively different for the regimes with positive and negative reversible CAPE (rCAPE). The former is characterized by a strong linear relationship (synchronic line) with a slightly weaker increase of rCAPE than that of pseudo-adiabatic CAPE, with its evolution primarily dictated by the surface moisture. This synchronic line also penetrates into the negative rCAPE regime, where CAPE distributes diffusively between this synchronic line and a line with zero reversible CAPE. These more complex distributions of CAPE in the negative rCAPE regime are explained by their dependence on the level of neutral buoyancy.

The Two-Dimensional Response of a Tropical Jet to Propagating Lines of Convection

Abstract: This paper uses simple one- and two-dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet, representative of the African easterly jet. The results are used to infer changes in the environment of the forcing region, which would in reality tend to influence the evolution of the storm through convective mechanisms, which are not resolved here. From linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing: this result is confirmed in two-dimensional nonlinear simulations. When linear wave modes are computed in a basic state that includes the jet structure, small sensitivities to the basic-state jet are found. The two-dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing. Principally, it is found that the modes of response to the forcing may be associated with characteristic changes in the basic-state shear, convective available potential energy (CAPE), and convective inhibition (CIN), which would be expected to have significant influence on the convective system itself.

Numerical simulations and doppler radar data analysis of a hail process in huaihe river basin

Abstract: A hail process in Huaihe River Basin observed by CINRAD Doppler radar on 12May 2000 hasbeen simulated by using nonhydrostatic mesoscale numerical model MM5The simulated wind fieldis analyzed and compared with Doppler radar wind data,both of them show that there was ameso-β cyclone around the hail regionResults show that this meso-β cyclone existed below 3 kmin the troposphere and its cyclonic circulation was very obvious below 1 kmThe temperature andmoisture fields from the simulation are also analyzedFurthermore,the storm-relativeenvironmental helicity and CAPE(convective available potential energy)are discussed