Showing papers on "Convective available potential energy published in 1999"

Observed and model‐simulated diurnal cycles of precipitation over the contiguous United States

Abstract: We analyzed diurnal variations in precipitation, surface pressure, and atmospheric static energy over the United States from observations and NCAR regional climate model (RegCM) simulations. Consistent with previous studies, the mean (1963-1993) pattern of the diurnal cycle of summer U.S. precipitation is characterized by late afternoon maxima over the Southeast and the Rocky Mountains and midnight maxima over the region east of the Rockies and the adjacent plains. Diurnal variations of precipitation is weaker in other seasons, with early to late morning maxima over most of the United States in winter. The diurnal cycle in precipitation frequency accounts for most of the diurnal variations, while the diurnal variations in precipitation intensity are small. The broad pattern of the diurnal cycle of summer precipitation is fairly stable, but the interannual variability in the diurnal cycle of winter precipitation is large. The diurnal cycle of July convective available potential energy (CAPE) is dominated by a solar driven march of a high-CAPE (2-4 kJ kg -1 ) tongue moving from the Southeast into the Northwest, with maximum values in the late afternoon to early evening over most of the United States. The solar driven diurnal and semidiurnal cycles of surface pressure result in significant large-scale convergence over most of the western United States during the day and over the region east of the Rockies at night. The diurnal cycle of low-level large-scale convergence suppresses daytime convection and favors nighttime moist convection over the region east of the Rockies and the adjacent plains. The nocturnal maximum in the region east of the Rockies is also enhanced by the eastward propagation of late afternoon thunderstorms generated over the Rockies. Over the Southeast and the Rockies, both the static instability and the surface convergence favor afternoon moist convection in summer, resulting in very strong late afternoon maxima of precipitation over these regions. RegCM simulations of 1993 summer precipitation with three different cumulus convection schemes (Grell, Kuo, CCM3) all had deficiencies in capturing the broad pattern of the diurnal cycle of precipitation over the United States. The model also overestimated precipitation frequency and underestimated precipitation intensity. The simulated diurnal cycles of surface pressure and CAPE were weak compared to observations. All the schemes produced too much cloudiness over the Southeast for July 1993 which reduced surface solar radiation and thus daytime peak warming at the surface. The model's criteria for onset of moist convection appear to be too weak, so moist convection in the model starts too early and occurs too often with all the three schemes.

Convective Precursors and Predictability in the Tropical Western Pacific

Abstract: Conditions leading to convective outbreak in the Tropics are investigated by multivariate analysis of sounding and satellite data from the tropical western Pacific area. Circumstances that make the prediction problem difficult are discussed and addressed by applying linear “error-in-variables” and nonlinear statistical simulation techniques to a large dataset. Low- to midtropospheric moisture is identified as the dominant factor regulating convective outbreak in this region. Based on the results it is argued that such moisture is particularly important in regulating spontaneous convective outbreak, but instability and near-surface wind speed probably play some role in allowing previous shallow or midtopped cumulus activity to deepen. Mesoscale-mean convective available potential energy sufficient for convection is found to exist almost 90% of the time. Quantitative estimates of noise in the data are obtained and accounted for in reaching these conclusions. The results imply that large-scale mean ...

Increased hurricane intensities with CO2-induced warming as simulated using the GFDL hurricane prediction system

Abstract: The impact of CO2-induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO2 increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO2 conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO2 storms, with SSTs warmer by about 2.2 °C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3–7 m/s (5%–11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO2 sample. In terms of storm tracks, the high CO2 sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO2 storm than for the control, and the high CO2 storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.

A similarity hypothesis for shallow-cumulus transports

Abstract: A similarity theory for cumulus convection is proposed, and applied to the problem of cumulus transports, using data from several large-eddy simulations to test the theory. the parameters in the similarity hypothesis include the cloud-base mass flux, the buoyancy of a parcel undergoing reversible ascent, through the cloud layer and the depth of the cloud layer. Using arguments based on the turbulence kinetic-energy budget a velocity-scale is derived, in addition to the conventional Convective Available Potential Energy scale. This new scale is analogous to the convective velocity-scale used in boundary-layer theories, but incorporates the effects of latent-heat release. the ratio between the cloud-base mass flux and the velocity-scale is found to be a key parameter in describing cumulus convection. It is shown that the similarity hypothesis can be applied to mass-flux schemes to determine the fractional entrainment rate. For this the entrainment rate is assumed to be related to the rate of generation of turbulent kinetic energy in the cloud ensemble. the resulting scaling is tested against fractional entrainment rates diagnosed from the large-eddy simulations.

Vertical velocity characteristics of deep convection over Darwin, Australia

Abstract: Continuous vertical velocity measurements using a 50-MHz wind profiler located at Darwin in northern Australia during periods of active convection have been analyzed. This dataset is dominated by continental-type convection. Numerous examples of shallow, deep, and decaying convection were seen and it is shown that only the deep systems have substantial tilts to the draft structure. The most intense updrafts occur above the freezing level, but shallow convection also produces large-amplitude vertical motions. The strength of these updrafts in this dataset is very similar to other tropical, oceanic data. That observation is consistent with the idea that the magnitude of the updrafts is much less in the Tropics than for intense midlatitude convection because the convective available potential energy is distributed over a much deeper layer in the Tropics, although more intense updrafts may be present at other tropical locations, such as the Tiwi Islands north of Darwin. The size of the cores, however...

Three-Dimensional Cloud-System Modeling of GATE Convection

Abstract: Deep convection and its associated mesoscale circulations are modeled using a three-dimensional elastic model with bulk microphysics and interactive radiation for a composite easterly wave from the Global Atmospheric Research Program Atlantic Tropical Experiment. The energy and moisture budgets, large-scale heat sources and moisture sinks, microphysics, and radiation are examined. The modeled cloud system undergoes a life cycle dominated by deep convection in its early stages, followed by an upper-tropospheric mesoscale circulation. The large-scale heat sources and moisture sinks associated with the convective system agree broadly with diagnoses from field observations. The modeled upper-tropospheric moisture exceeds observed values. Strong radiative cooling at the top of the mesoscale circulation can produce overturning there. Qualitative features of observed changes in large-scale convective available potential energy and convective inhibition are found in the model integrations, although quantitative magnitudes can differ, especially for convective inhibition. Radiation exerts a strong influence on the microphysical properties of the cloud system. The three-dimensional integrations exhibit considerably less sporadic temporal behavior than corresponding two-dimensional integrations. While the third dimension is less important over timescales longer than the duration of a phase of an easterly wave in the lower and middle troposphere, it enables stronger interactions between radiation and dynamics in the upper-tropospheric mesoscale circulation over a substantial fraction of the life cycle of the convective system.

Motion characteristics of thunderstorms in southern Germany

Abstract: The motion of thunderstorms in southern Germany was investigated. The thunderstorms were observed by a lightning position system during the summer months of the years 1992–1996. On average every second day thunderstorms were observed somewhere in southern Germany. In general thunderstorms approached from westerly and south-westerly directions. The average speed was 13 m s−1. No significant relation between the occurrence of thunderstorms and the large-scale synoptic pattern described by the Grosswetterlagen (large-scale weather pattern) was found. Thunderstorms were observed during almost all Grosswetterlagen. The reduction to eight weather patterns based on the low-level flow in southern Germany showed that thunderstorms are likely when the flow has a westerly (43%) or easterly direction (20%). Three distinct groups of different lighting patterns could be identified: stationary, moving thunderstorms and thunderstorm lines. The convective available potential energy (CAPE) and the wind shear were retrieved from radio soundings from Munchen and Stuttgart. On average CAPE was 583 J kg−1 for stationary thunderstorms, 701 J kg−1 for moving thunderstorms and 876 J kg−1 for thunderstorm lines. The corresponding average bulk Richardson numbers were 37, 22 and 21. The steering level was found to be at about 6 km m.s.l. However, it should be noted that in most cases the soundings do not completely describe the local environment of thunderstorms, since radio soundings are only available twice a day. Copyright © 1999 Royal Meteorological Society

Numerical simulations of convective equilibrium under prescribed forcing

Abstract: Some characteristic properties of simulated moist convective equilibria are examined for different imposed cooling rates These numerical simulations extend the work of Vallis et al and were motivated by the need to show that their conclusions (concerning the upscale energy cascade) were not sensitive to vertical resolution Although kinetic energy does cascade to large scales, much of the large-scale motion in the model is associated with horizontally-divergent winds, and the energy spectra may be better explained as a direct consequence of the generation of convective lines These lines have a typical spacing of about 60 km which leads to a local maximum in the kinetic-energy spectrum In addition, the design of our experiments was found to match that envisaged in recent idealized ‘heatengine theories’ of radiative-convective equilibria, thereby providing an opportunity to evaluate their utility It is shown that a variant of these scaling theories appears to fit the statistical properties of our simulations quite well and provides expressions for the convective available potential energy, cloud mass flux and the fractional area occupied by convective updraughts Scaling arguments also suggest that the convective line separation is of the order of the convecting layer depth divided by the square root of the updraught fractional area—consistent with the wavelength of the local maximum in the energy spectrum

Variability of Water Vapor, Infrared Radiative Cooling, and Atmospheric Instability for Deep Convection in the Equatorial Western Pacific

Abstract: In the troposphere of the equatorial western Pacific, the water vapor variability dominates the temperature variability in changing the clear-sky infrared (IR) cooling rate. The large water vapor variability, especially its bimodal distribution at certain levels of the upper troposphere, leads to distinct structures of the clear-sky IR radiative cooling rate. The IR cooling rate, its maximum normally in the upper troposphere (∼300 hPa) and minimum in the lower troposphere (∼650 hPa), tends to become vertically uniform when the upper troposphere is abnormally dry. A local, maximum IR cooling rate may occur in the boundary layer when the lower troposphere becomes extraordinarily dry. The changes in IR cooling due to the water vapor variability affect the rate of generation of convective available potential energy (CAPE) and the conditional instability for deep convection. Little or no mean rainfall over an area of roughly 3 × 105 km2 is observed when either the rate of generation of CAPE suffers fr...

Storm tracking and monitoring using objective synoptic diagnosis and cluster identification from infrared Meteosat imagery : A case study

Abstract: The present paper investigates the potential of combining image processing techniques based on cluster analysis of infrared (IR) Meteosat images with dynamic meteorological theory on synoptic systems. From this last point of view the highest probability of deep convective development is favoured where the overlapping of four mechanisms acting at synoptic scale is produced: upward quasi-geostrophic forcing, convergence of water vapour at low levels, convective instability in the lower troposphere and great convective available potential energy. Cloud tracking is performed over sequences of Meteosat IR images by using a shape parameterisation approach after appropriate filtering for non-significant clouds and automated identification of convective systems. The integrated methodology is applied to the case study of the heavy rainfall event which produced floods in the South of France and the North of Italy on September 27–28th, 1992. The analysis focuses on the monitoring and explanation of the zones most affected by heavy rainfall with the aim of investigating possible improvements of the predictive potential of cloud tracking and allowing identification of the areas which most lend themselves to flash floods for use in operational flood forecasting applications.

Vortex motions in the atmospheric convective boundary layer

Abstract: Large vortices with scales ranging from hundreds meters to tens of kilometers are generallyfound in the atmospheric convective boundary layer(CBL).These vortices play important roles in the vertical transport of momentum,heat,water vaporand other tracers in the boundary layer.On the basis of the view of interaction between theconvection in CBL and the gravity waves in the upper stable layer the authors developed aconvection-wave theory on the formation of large vortices.According to the theory thewavenumber spectrum of the large vortices mainly depends on the atmospheric conditions in bothof the upper and lower layers,such as wind speed,wind direction shear,stratification as well astemperature jump.In the present paper satellite image and weather data in a case of cold air outbreak over warmocean are analyzed to study every stage of the convective processes,such as cloud street,convective cell as well as their transformation.According to the theory the wavenumbercompositions for cloud street and convective cell are calculated,respectively,on the basis of theatmospheric conditions at every stage.The distributions of vertical motions,convergent band anddisturbed interface are obtained and compared with the cloud patterns in the convective processes.Thus the study seems to offer a likely explanation for the origin of large vortices in CBL.