Showing papers on "Convective available potential energy published in 1995"

Sensitivity of climate simulations to the parameterization of cumulus convection in the Canadian climate centre general circulation model

Abstract: A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective‐scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub‐cloud layer, acts to remove CAPE at an exponential rate with a specified...

Three‐dimensional model studies of exchange processes of ozone in the troposphere over Europe

Abstract: A three-dimensional Mesoscale Chemistry Transport (MCT) model driven by meteorological data from a Numerical Weather Prediction (NWP) model is used to calculate ozone and other chemical species over Europe over a 10-day period. The meteorological model uses cloud water as one of the prognostic variables and has an advanced treatment of stratiform and convective cloud and precipitation processes. In this way an improved resolution in time and space of cumulus cloud episodes is obtained with a better simulation of the convective transport as a consequence. It should also improve the computations of the photolysis rates which are highly dependent on cloud cover and cloud optical depth. The 10-day period (July 1-10, 1991) is characterized by warm weather and frequent occurrence of cumulus convection. The model results are compared to ground-based ozone measurements and ozone profiles. The results presented suggest that physical processes, especially convection, may dominate in the vertical distribution of ozone in the free troposphere, that sinking air which compensates for convective updrafts is important for the tropospheric ozone budgct, and that transport of O 3 precursors from the boundary layer into the free troposphcre by convection enhances the rate of O 3 production significantly. However, considerable unccrtainty of the absolute magnitude of the convective mixing exists.

A climatological study of tropical thunderstorm clouds and lightning frequencies on the French Guyana Coast

Abstract: The climatological conditions that are favorable to thunderstorm cloud development in the coastal zone of French Guyana are investigated. The analysis reveals that both the thunderstorm occurrence and the mean monthly lightning frequency are correlated to the Intertropical Convergence Zone annual oscillation, thus increasing from March to August when the South-East trade-wind flux drives continental air masses towards French Guyana and decreasing from September to February when French Guyana is under the influence of maritime air masses driven by the North-East trade-wind. The study of the lightning frequency values, observed during individual thunderstorm events, as a function of the cloud-top height shows that these values are intermediate to the lightning frequency values deduced from the parameterizations proposed by Price and Rind (1992) for typical continental and maritime thunderstorm clouds of a similar height. This behavior suggests that the air masses in which thunderstorm clouds develop in the French Guyana coast are mixed air masses with intermediate characteristics. Further, the absence of correlation between the lightning frequency values and both the cloud-top heights and the corresponding convective available potential energy (CAPE) values indicates that these parameters can not be used to predict lightning activity in these clouds. Most probably, the aerosol concentrations in the subcloud layer and consequently the cloud droplet concentrations play an important role in the development of lightning activity in the considered clouds.

Quasi-Lagrangian Measurements in Convective Boundary Layer Plumes and Their Implications for the Calculation of CAPE

Abstract: Measurements were made to determine the level of origin of air parcels participating in natural convection. Lagrangian measurements of conservative variables are ideal for this purpose. A simple remotely piloted vehicle was developed to make in situ measurements of pressure, temperature, and humidity in the convective boundary layer. These quasi-Lagrangian measurements clearly show that convective plumes originate in the superadiabatic surface layer. The observed boundary layer plumes have virtual temperature excesses of about 0.4 K in a tropical region (Orlando, Florida) and of about 1.5 K in a desert region (Albuquerque, New Mexico). The water vapor contribution to parcel buoyancy was appreciable in Orlando but in Albuquerque was insignificant. These observations indicate that convective available potential energy should he determined by adiabatically lifting air parcels from the surface layer, at screen level.

A comparative study between two cases of extreme rainfall events in Catalonia

Abstract: A diagnostic study of the synoptic aspects of two cases of heavy rain producing floods in Catalonia (northeast corner of the Iberian peninsula) is presented. The diagnosis consists in the determination of the area where large scale circulation induces favorable conditions for development of thunderstorms. We identify these conditions as upward quasi-geostrophic forcing, convergence of water vapor at low levels and convective instability in the lower troposphere. A composite chart showing where the three synoptic mechanisms overlap determines the area. Local studies of instability are carried out from upper air data given by the radiosonde ascents of Palma de Mallorca, looking for both the Convective Available Potential Energy and Bulk Richardson number and its temporal evolution during the events.

The Importance of Understanding Mesoscale Model Parameterization Schemes for Weather Forecasting

Abstract: A severe weather outbreak that occurred on 21–23 November 1992 in the southern United States is used to illustrate how an understanding of model parameterization schemes can help in the evaluation and utilization of mesoscale model output Results from a mesoscale model simulation show that although the model accurately simulated many of the observed mesoscale features, there are several aspects of the model simulation that are not perfect Through an understanding of the model parameterization schemes, these model imperfections are analyzed and found to have little effect on the overall skill of the model forecast in this case Mesoscale model output also is used to provide guidance to evaluate the severe weather threat By using the model output to produce hourly calculations of convective available potential energy (CAPE) and storm relative environmental helicity (SREH), it is found that regions with positive CAPE, SREH greater than 150 m2 s−2, and model-produced convective rainfall correspond

Rotating convective disturbances in the trades

Abstract: Observations of rotating convective disturbances in the trade-wind region just east of Hawaii are presented. Rotation results from stretching of vertical ambient vorticity in the convective regions. Surface friction is shown to play a significant dissipative role in these disturbances. Ekman pumping associated with the rotation is probably only of secondary importance. Convection is found to be associated with weak local maxima in the boundary-layer equivalent potential temperature. This suggests that convection occurs where such anomalies make the buoyancy of surface parcels larger than average. Convective available potential energies are found to be small, which means that such equivalent-potential-temperature anomalies are important in determining parcel buoyancy. It is hypothesized that mesoscale circulations induced by the ascending motion of convection reinforce the anomalies, resulting in the intensification of the circulations. The equivalent potential temperature of the boundary layer results from a balance between surface fluxes and entrainment from above. Ascending motion apparently reduces entrainmcnt, thus allowing the equivalent potential temperature to rise, whereas subsidence has the opposite effect.

The sensitivity of surface energy balance to convective parameterization in a general circulation model

Abstract: This study investigates the interaction between convection and the surface energy fluxes, and its sensitivity to convective parameterization schemes using a general circulation model. Two simulations of the global circulation averaged annually from 1 June 1985 to 31 May 1986 are performed, with particular emphasis on the tropical Pacific. In the control simulation, a convective scheme that parameterizes convection based on low-level moisture convergence is used. A second experiment employs a parameterization scheme that uses the time rate of change of convective available potential energy (CAPE) to determine convection. When the low-level moisture convergence is used as the closure for convective parameterization, convection and low-level convergence occur near the landmass of Southeast Asia. The large-scale circulation is such that a fairly strong surface wind that provides moisture to fuel convection is located in the western tropical Pacific warm pool regions, giving rise to relatively high latent heat flux there. When the time rate of change of CAPE is used to close convective parameterization, convection and its associated low-level large-scale convergence and weak surface wind speed occur in the warm pool region, resulting in low latent heat flux there. Response of surface solar radiative flux to convection is found to bemore » the largest in the surface energy budget. In the experiment, more clouds are produced over the tropical oceans, leading to less solar radiation received on the surface. More clouds in the experiment also lead to less net emission of longwave radiation from the ocean surface due to the cloud greenhouse effect, albeit the magnitude is much smaller than that for solar radiation. The large changes in surface latent heat and solar radiative fluxes from the control run to the experiment suggest that the surface energy balance in the atmospheric general circulation model is highly sensitive to convective parameterization. 44 refs., 10 figs.« less