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G. W. Reuter and N. Aktary

Abstract

Sounding data from central Alberta were analyzed to assess whether the atmosphere indicated deep layers that were susceptible to convective or moist symmetric instability. The observations consisted of 1460 balloon soundings covering the 2-yr period 1990–91. We also identified soundings that supported convective or moist symmetric instability and had precipitation within 12 h after the launch of the soundings. The recorded precipitation amounts were also correlated with the stability properties of the soundings.

During summer, 97% of the soundings were susceptible to convective instability. About a third of these resulted in rainfall events. Soundings with convectively unstable layers were also common during spring (70%) and fall (57%) but rare during winter. Much of the precipitation recorded in spring and summer could be associated with convectively unstable cases.

During winter, 44% of the observed soundings were convectively stable yet indicated the potential for moist symmetric instability, and about half of the total snowfall amounts was associated with these symmetrically unstable soundings. Soundings that were convectively stable yet potentially unstable for moist symmetric instability were encountered less frequently during spring (20%) and fall (26%) and were ram during summer (3%). Mixed-type unstable conditions (i.e., a convectively unstable layer together with a symmetrically unstable layer) were common during spring, summer, and fall and were often associated with significant precipitation amounts. The primary finding was that both convective and moist symmetric instability contributed to the annual precipitation, but a seasonal dependence existed in their relative importance.

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L. Xin and G. W. Reuter

Abstract

A nonhydrostatic axisymmetric cloud model is used to quantify the effects of persistent mesoscale convergence on cumulus development and convective rainfall. The model was initialized by environmental conditions adopted from sounding and Doppler radar velocity data sampled on 19 August 1992 in central Alberta. The sounding showed a moist warm air mass with a moderate amount of convective available potential energy and the wind field had boundary layer convergence but almost no vertical shear in the lowest 5 km. The simulated rainfall intensity and accumulation compared well with radar observations.

The dependence of the convective rainfall on the characteristics of the convergence zone is investigated by intercomparing model simulators with different convergence magnitudes, convergence depths, and convergence profiles. Increasing the magnitude or the depth of convergence causes stronger convection and more precipitation. Rainfall increases monotonically (but nonstrictly linearly) with the convergence magnitude. Doubling the convergence magnitude from 1 × 10−4 to 2 × 10−4 s−1 increases the rainfall by a factor of 2.6, while rainfall increases by only 2.3 times when the convergence is doubled from 1.25 × 10−4 to 2.5 × 10−4 s−1. The nonlinear effects become even more apparent when changing the depth of convergent layers. Even when keeping the vertical mass flux constant, the depth of the convergence affects greatly the timing and amount of the surface rainfall. This is related to the fact that humidity tends to decrease with height and therefore the upward moisture flux is weakest for the deepest convergence layer for a fixed upward momentum flux. The model suggests that rainfall is mostly controlled by the amount of vapor converging into the column below cloud base.

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S. Guan and G. W. Reuter

Abstract

An axisymmetric cloud model is used to investigate the evolution of convective cells and associated rain showers that develop due to the sensible and latent heat released into a calm atmosphere from an industrial cooling complex. The simulated convection is in fair agreement with observations for a particular cloud developingover a large cooling system. Sensitivity experiments were run for two different soundings to quantify how the convection and rainfall depend on the magnitude of the waste energy loss, the area of the source region, and other parameters. The major findings were the following:1 ) Without a sustained waste heat input the model convection ceased. The model convection became progressively more intense and developed more rain when the rate of total waste energy heat released from thepower station was increased. Doubling the total waste energy amount from its control case value caused a 15-fold increase in 3-h rainfall values. However, reducing the control case value of total waste energy release by 20% led to much weaker convection without rain.2) The triggering of cumulus clouds was reduced when the waste energy was released in terms of latent heat rather than sensible heat. As the relative contribution of sensible heat increased, so did the intensity of the circulation and the rainfall.3) The cloud formation depended on the area of the heat source: a wider source tended to delay and to weaken the convection. Specifically, when the area was doubled the simulated cloud appeared 2.5 min laterand the total accumulated rain after 3 h was reduced by 88%. Moreover, the maximum total kinetic energy was reduced by 26%.4) The model results were only slightly sensitive to the radial distribution of the sensible and latent heatfluxes within the source region.

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S. Guan and G. W. Reuter

Abstract

Large oil refineries emit heat, vapor, and cloud condensation nuclei (CCN), all of which can affect the formation of cloud and precipitation. This study quantities the relative contributions of the three factors on cloud development in calm wind conditions using an axisymmetric cloud model. The factor separation technique is applied to isolate the net contributions of waste heat, vapor, and CCN on the rainfall of a cumulus developing in the industrial plume. The mutual-interactive contributions of two or three of the factors are also computed.

The simulations for midlatitude and tropical conditions indicate that the sensible heat provides the major stimulus for cloud development and rain formation. The pure contribution of the industrial CCN is to enhance the condensation, causing an increase in the mass of total cloud water. The simulation results indicate that mutual interactions between waste heat and industrial CCN are large for both cases considered.

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L. Xin and G. W. Reuter

Abstract

The volume velocity processing (VVP) technique is used with a simulated wind field to determine the accuracy of kinematic quantities for different numbers of wind parameters and different sizes of analysis volumes. Accurate estimates of divergence, deformation, and vertical shear are obtained if the VVP method contains seven wind parameters and the analysis volumes have a range of about 20 km and an azimuthal extent of about 40°. The seven-parameter VVP method is applied to a convective storm in central Alberta, Canada. The analysis showed that low-level convergence and moderate vertical shear preceded the enhancement of precipitation, while low-level divergence suppressed the convection.

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G. W. Reuter and M. K. Yau

Abstract

The entrainment mechanisms of deep convective clouds in South Africa are simulated by using both axially and slab-symmetric cumulus models having very high spatial and temporal resolutions. The simulated clouds on three different days show a structured organization with small-scale features such as in-cloud downdrafts. The mixing processes are examined by analyzing the time variation of dynamic and thermodynamic quantities along computed parcel trajectories. The calculations indicate that most of the entrainment occurs at the cloud top. Evaporative cooling and perturbation pressure cause downdrafts that transport highly diluted air from the cloud top down to lower levels. The penetrative downdrafts are mainly located near the edges of the cloud.

In the presence of wind shear the downshear portion of the clouds becomes cooler and more diluted, and it is associated with stronger downdrafts compared to the upshear side. The asymmetric organization is attributed to turbulent exchange of horizontal momentum near the cloud top.

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G. W. Reuter and M. K. Yau

Abstract

No abstract available.

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G. W. Reuter and M. K. Yau

Abstract

Seven precipitation bands observed during the Canadian Atlantic Storms Program (CASP) are studied to assess the importance of slantwise convective instability. Using three-hourly radiosonde data from regular and special stations, vertical cross sections of θe *(saturated equivalent potential temperature) and M,(absolute angular momentum) and θe(equivalent potential temperature) and M surfaces are constructed. A comparison of the slopes of constant Me and θe * surfaces indicates the presence of potential and conditional slantwise instability. This study focuses on the time evolution of the stability field and on the adjustment to its neutral state.

Consistent results were found in seven cases analyzed. The atmosphere is shown to contain shallow layers of air that are slightly unstable for conditional slantwise convection, particularly in regions having pronounced windshear. In the upper levels, the potential for instability usually remains only a potential because the lack of moisture precludes the actual release of energy. On the other hand, in the lower part of the atmosphere saturation is often realized and the instability is released leading to heavy precipitation which is sometimes organized in multiple bands. Our results also demonstrate that the atmosphere is undergoing an adjustment toward a state of conditional neutrality with respect to slantwise convection in saturated regions. The adjustment time of less than three hours is consistent with Enianuel's hypothesis of rapid adjustment.

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G. W. Reuter and M. K. Yau

Abstract

Atmospheric stability properties for cumulus and slantwise convection in oceanic midlatitude cyclones are analyzed using dropsonde observations from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). Vertical cross sections perpendicular to the low-level wind shear are selected in the frontal regions for four ERICA storms. To assess the stability properties for conditional symmetric instability (CSI), a sounding analysis is carried out along surfaces of constant absolute angular momentum M. The buoyancy of the parcel along the slanted M surface is determined, both with and without the water loading effect. Our analysis suggests that a systematic bias toward overestimation of slantwise instability occurs when the loading effect is neglected.

The major finding of our analysis is that the lower-tropospheric air on the warm side of the warm-frontal zone is stable or neutral with respect to vertical cumulus convection but unstable for slantwise convection. Convective instability, however, is found in the warm sector near the surface low of explosive cyclones during their period of most rapid growth. Our analysis shows that conditional slantwise instability, throughout a deep layer, can occur even in a slowly developing cyclone. Observed precipitation events were consistent with the occurrence of slantwise and cumulus convection.

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G. W. Reuter and M. K. Yau

Abstract

Aircraft measurements are analyzed to determine the mode of entrainment in cumulus congestus clouds. The data were collected in 15 developing cloud towers sampled in southern Africa on three different days. The Paluch method, which is based on a comparison of the total water mixing ratio and wet equivalent potential temperature in the cloudy air and nearby environment, was used to identify the origin of the in-cloud air at the level of observation. The results indicate clearly the occurrence of cloud top mixing, but the presence of lateral entrainment cannot be ruled out. Strong downdrafts were found near the cloud edges, which suggests that the diluted air from the cloud top can penetrate several kilometers downward.

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