Nonprecipitating Convection Research Articles

The Strong Impact of Weak Horizontal Convergence on Continental Shallow Convection

Abstract Idealized large-eddy simulation (LES) is a basic tool for studying three-dimensional turbulence in the planetary boundary layer. LES is capable of providing benchmark solutions for parameterization development efforts. However, real small-scale atmospheric flows develop in heterogeneous and transient environments with locally varying vertical motions inherent to open multiscale interactive dynamical systems. These variations are often too subtle to detect them by state-of-the-art remote and in situ measurements, and are typically excluded from idealized simulations. The present study addresses the impact of weak [i.e., O(10−6) s−1] short-lived low-level large-scale convergence/divergence perturbations on continental shallow convection. The results show a strong response of shallow nonprecipitating convection to the applied weak large-scale dynamical forcing. Evolutions of CAPE, mean liquid water path, and cloud-top heights are significantly affected by the imposed convergence/divergence. In contrast, evolving cloud-base properties, such as the area coverage and mass flux, are only weakly affected. To contrast those impacts with microphysical sensitivity, the baseline simulations are perturbed assuming different observationally based cloud droplet number concentrations and thus different rainfall. For the tested range of microphysical perturbations, the imposed convergence/divergence provides significantly larger impact than changes in the cloud microphysics. Simulation results presented here provide a stringent test for convection parameterizations, especially important for large-scale models progressing toward resolving some nonhydrostatic effects.

Role of Surface Friction on Shallow Nonprecipitating Convection

The role of surface friction on shallow nonprecipitating convection is investigated using a series of large-eddy simulations with varying surface friction velocity and with a cloud identification algorithm. As surface friction intensifies, convective rolls dominate over convective cells and secondary overturning circulation becomes stronger in the subcloud layer, thus transporting more moisture upward and more heat downward between the subcloud and cloud layers. Identifying individual clouds, using the identification algorithm based on a three-dimensional topological analysis, reveals that intensified surface friction increases the number of clouds and the degree of tilting in the downstream direction. Highly intensified surface friction increases wind shear across the cloud base and induces cloud tilting, which leads to a vertically parabolic profile of liquid water mixing ratio instead of the classical two-layer structure (conditionally unstable and trade inversion layers). Furthermore, cloud tilting induces more cloud cover and more cloud mass flux much above the cloud base (e.g., 0.8 < z < 1.2 km), but less cloud cover and less cloud mass flux in the upper cloud layer (e.g., z > 1.2 km) because of increased lateral entrainment rate. Similarly, profiles of directly measured entrainment and detrainment rates show that detrainment in the lower cloud layer becomes smaller with stronger surface friction.

A Probabilistic Bulk Model of Coupled Mixed Layer and Convection. Part II: Shallow Convection Case

Abstract The probabilistic bulk convection model (PBCM) developed in a companion paper is here extended to shallow nonprecipitating convection. The PBCM unifies the clear-sky and shallow convection boundary layer regimes by obtaining mixed-layer growth, cloud fraction, and convective inhibition from a single parameterization based on physical principles. The evolution of the shallow convection PBCM is based on the statistical distribution of the surface thermodynamic state of convective plumes. The entrainment velocity of the mixed layer is related to the mass flux of the updrafts overshooting the dry inversion capping the mixed layer. The updrafts overcoming the convective inhibition generate active cloud-base mass flux, which is the boundary condition for the shallow cumulus scheme. The subcloud-layer entrainment velocity is directly coupled to the cloud-base mass flux through the distribution of vertical velocity and fractional cover of the updrafts. Comparisons of the PBCM against large-eddy simulations from the Barbados Oceanographic and Meteorological Experiment (BOMEX) and from the Southern Great Plains Atmospheric Radiation Measurement Program (ARM) facility demonstrate good agreement in terms of thermodynamic structure, cloud-base mass flux, and cloud top. The equilibrium between the cloud-base mass flux and rate of growth of the mixed layer determines the equilibrium convective inhibition and cloud cover. This process is an important new insight on the coupling between the mixed-layer and cumulus dynamics. Given its relative simplicity and transparency, the PBCM represents a powerful tool for developing process-based understanding and intuition about the physical processes involved in boundary layer–convection interactions, as well as a test bed for diagnosing and validating shallow convection parameterizations.

A Direct Measure of Entrainment

Abstract A method is introduced for directly measuring convective entrainment and detrainment in a cloud-resolving simulation. This technique is used to quantify the errors in the entrainment and detrainment estimates obtained using the standard bulk-plume method. The bulk-plume method diagnoses these rates from the convective flux of some conserved tracer, such as total water in nonprecipitating convection. By not accounting for the variability of this tracer in clouds and in the environment, it is argued that the bulk-plume equations systematically underestimate entrainment. Using tracers with different vertical profiles, it is also shown that the bulk-plume estimates are tracer dependent and, in some cases, unphysical. The new direct-measurement technique diagnoses entrainment and detrainment at the gridcell level without any recourse to conserved tracers. Using this method in large-eddy simulations of shallow and deep convection, it is found that the bulk-plume method underestimates entrainment by roughly a factor of 2. The directly measured entrainment rates are then compared to cloud height and cloud buoyancy. Contrary to existing theories, fractional entrainment is not found to scale like the inverse of height, the cloud buoyancy, or the gradient of cloud buoyancy. On the other hand, fractional detrainment is found to scale linearly with cloud buoyancy. Finally, direct measurement is used to diagnose the spatial distribution of entrainment and detrainment during the evolution of an individual deep cumulonimbus.

Development and structure of a maritime continent thunderstorm

The evaluation of a maritime continent thunderstorm complex (Hector) occurring over Bathurst and Melville Islands north of Darwin, Australia (12° S, 131° E) is investigated primarily using Doppler radar data. Thunderstorm formation follows the development of sea breeze circulations and a period of shallow non-precipitating convection. Evidence exists for initiation of long-lived and organised convection on the sea breeze fronts, although short-lived, scattered convection is apparent earlier in the day. Merging of the convective systems is observed in regions of enhanced low-level convergence related to sea breeze circulations. The merged convective complex is initially aligned in an almost east-west direction consistent with the low-level forcing. The merged complex results in rapid vertical development with updraughts reaching 40 m s− and echo tops reaching 20 km height. Maximum precipitation production occurs during this merger phase. On the perimeter of the merged convective complex, evidence exists for front-to-rear updraughts sloped over lower-level downdraughts with rear-to-front relative flow and forward propagating cold pools. The mature phase is dominated by this convection and the complex re-orientates in the prevailing easterly vertical shear to an approximate north-south direction, then moves westward off the islands with the classic multicellular squall-like structure. The one-dimensional cloud model of Ferrier and Houze (1989) used with a four class ice formulation reproduced the cloud top height, updraught structure and echo profile very well. To test the importance of ice physics upon thunderstorm development, several sensitivity tests were made removing the effects of the ice phase. All of these model clouds reached nearly 20 km, although simulations without the effects of ice had updraughts reduced from about 40 m s−1 to 30 m s−1. The simulated convection was more sensitive to changes in environmental conditions and parameterised cloud dynamics. The strong intensity of the convection was largely accounted for by increasing equivalent potential temperatures due to diurnal heating of the surface layer. The vertical velocity and radar structure of the island thunderstorm has more similarity with continental rather than oceanic convection. Maximum vertical velocities, in particular are almost an order of magnitude greater than typical of oceanic convection. With the intense updraughts, even in the low shear environment, there is evidence for mesoscale circulations within the convection.

A Theory for Nonprecipitating Convection between Two Parallel Plates. Part II: Nonlinear Theory and Cloud Field Organization

Abstract In Part I, an idealized model of nonprecipitating moist convection in a shallow conditionally unstable layer of viscous and diffusive air between two parallel plates was introduced, and the “linear” instability of an exactly saturated static state maintained by diffusion was investigated. If there are initially many clouds, the “linear” theory predicted that weaker clouds are suppressed by the subsidence warming and drying from the ever-growing stronger clouds, and the average cloud spacing becomes arbitrarily large as time goes on. Each growing cloud is surrounded by compensating subsidence, which decreases away from the cloud with a characteristic decay scale Rs, the subsidence radius, which can be understood from gravity wave arguments. In Part II, fields of finite amplitude clouds are considered. An asymptotic analysis is performed in which the moist Rayleigh number Nc2 exceeds by only a small amount μ the value Nc02 necessary for the onset of convection. This leads to a nonlinear set of “clo...

Convective cloud downdraft structure: An interpretive survey

Observational and modeling studies dealing with different aspects of convective clouds are reviewed and interpreted to construct a generalized description of the structure, dynamics, and thermodynamics of convective cloud downdrafts. Observational studies reveal that downdraft speeds and sizes range from typical values of several meters per second and several hundred meters in nonprecipitating cumulus congestus clouds to typical values of 5–10 m s−1 and several kilometers in precipitating cumulonimbi. Maximum measured downdraft speeds appear to be limited to ∼20 m s−1. Different types of downdrafts appear to exist within precipitating convective clouds. Penetrative downdrafts common to nonprecipitating convective clouds and upper regions of precipitating convective clouds exhibit maximum horizontal dimensions of ∼1 km. These downdrafts emerge when subsaturated environmental air is entrained or mixed into the cloud. A second type, cloud edge downdrafts, appear in both observations and in cloud model results. Although their driving mechanisms are not fully understood, such downdrafts may be forced by cloud edge evaporational cooling and localized updraft mass flux compensation. Overshooting downdrafts comprise a third type and are typically associated with intense convection in which updraft air surpasses an equilibrium level of neutral buoyancy, cools upon further ascent, and then descends but remains within a few kilometers of cloud top. Finally, the precipitation‐associated downdraft is one forced at low levels by precipitation loading, evaporation, and melting. This downdraft may attain relatively large scales, of the order of the horizontal dimension of precipitating regions within the lowest several kilometers. Such large scales provide a clear distinction from (penetrative‐type) downdrafts of ∼1 km maximum scale within nonprecipitating convection. There is evidence from both observational and modeling studies that downdraft dynamical and thermodynamical processes are strongly influenced by static stability, wind shear profiles, cloud microphysical processes, and precipitation characteristics. However, the degree to which downdraft structure depends on such interrelated controlling factors has not yet been determined.

Use of a Detailed Planetary Boundary Layer Model for Parameterization Purposes

Use is made of a three-dimensional model of the planetary boundary layer to investigate features of non-precipitating convection and its parameterization. The fine-grid mesh (50 m) model was first developed by J.W. Deardorff (1973) and modified by G. Sommeria (1976) to include possible cloud formation; the model and data set are briefly described in Section 2. In Section 3 the model data are utilized in order to test the validity of certain hypotheses concerning individual cloud features as well as those of a cloud population. These hypotheses, for the most part unverified in the real atmosphere, are frequently used in cumulus parameterization schemes. Sections 4 and 5 present the results of three recent cumulus parameterization schemes by Betts (1975, 1976) and Fraedrich (1976), and tests are made of these schemes with the present model. In order to illustrate the versatility of such a model when compared to the possibilities of intensive studies using atmospheric data, an attempt is made in Section 6 to link turbulent quantities with directly observable cloud features. The turbulent fluxes of heat and moisture, for example, are not easily observable but are important to cloud development and evolution. The empirical relations presented in this section present a reasonable alternative to atmospheric observations for the estimation of various important turbulent quantities. The study presented here emphasizes that despite many drawbacks, a numerical model is a useful tool to increase our understanding of a particular atmospheric phenomenon and can be used as a complement to observations for the improvement of parameterization methods in larger scale models.

Turbulence In Cumulus Clouds

Abstract The nature of turbulence in small cumuli is explored in a time-dependent model of moist non-precipitating convection. The problem is solved in a two-dimensional slab-symmetric geometry, but the turbulence is taken to be three-dimensional. Transport equations for the Reynolds stress and energy decay rate describe all scales of turbulence and hence no “seeding” of grid-resolvable scales with a random field is required. Results show the calculated turbulence to be highly anisotropic and in- homogeneous. Most of the turbulent energy resides in the vertical component of the Reynolds stress near cloud top, Consequently, vertical mixing is expected to be much more important than horizontal, in agreement with some recent experimental data.

A Two-Scale Model of Moist, Non-Precipitating Convection

A two-scale model of convective disturbances is developed in which the larger scale describes the disturbance as a whole and the smaller scale consists of convective turbulence. A novel formulation for the turbulence is developed in the context of a one-dimensional model for the disturbance scale flow. The convective turbulence scheme assumes that vertical mixing occurs when and only when an unstable lapse rate is present. The intensity of the mixing is explicitly computed via a linearized model for the turbulence, coupled with scaling arguments. The model is ultimately applied to moist convection over a heated island. Intense vertical mixing occurs near the top of the resulting cloud. This mixing has a strong effect on the mean cloud circulation, resulting in inflow top and bottom and outflow around the middle. This is in marked contrast to the ascending bubble structure that occurs when such mixing is suppressed. The results confirm the arguments of Fraser (1968).

Direct Testing of a Three-Dimensional Model of the Planetary Boundary Layer Against Experimental Data

Results from a detailed three-dimensional model of the atmospheric boundary layer are compared with observational data in a case of nonprecipitating convection in a tropical boundary layer. The model is a slightly improved version of the one developed by Sommeria (1976) in collaboration with J.W. Deardorff. The experimental data come from the NCAR 1972 Puerto Rico experiment which provided a good set of aircraft turbulence measurements in the fair weather mixed layer over the tropical ocean. The comparison involves statistical properties of the turbulent field as well as some structural features in the presence of small clouds.