Charles Cohen

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...

Simulation of Tropical Convective Systems. Part I: A Cumulus Parameterization

Abstract A new cumulus parametefization is developed for ffse in mesoscale model simulations of precipitating convective systems. It is designed to estimate convective properties using a cloud model that interacts with the mesoscale model in a physically consistent manner. The cloud model is initiated by and interacts with the grid-scalecirculation without being constrained in instantaneous equilibrium with the grid-scale flow. The parameterization is designed for use in conjunction with explicit moist processes. Simulations of tropical convective lines performed using the scheme are presented in a companion paper (Part II).

The Sensitivity of Simulated Convective Storms to Variations in Prescribed Single-Moment Microphysics Parameters that Describe Particle Distributions, Sizes, and Numbers

The sensitivity of cloud-scale simulations of deep convection to variations in prescribed microphysics parameters is studied, using the single-moment scheme in the Regional Atmospheric Modeling System (RAMS) model. Realistic changes were made to the shape parameters in the gamma distributions of the diameters of precipitating hydrometeors and of cloud droplets, in the number concentration of cloud droplets, and in the mean size of the hail and graupel. Simulations were performed with two initial soundings that are identical except for their temperature. The precipitation rate at the ground is not very sensitive to changes in the value of the shape parameter used for all precipitating hydrometeors (rain, hail, graupel, snow, and aggregates) or to the mean size of the hail and graupel, owing to counteracting effects. For example, with a larger shape parameter value, there is a greater production of precipitation by collection of cloud water, but also a larger rate of evaporation of the liquid precipitation. However, with a larger shape parameter value, the greater production of precipitation by collection and the increased evaporation result in more low-level cooling by the downdraft. Specifying larger hail and graupel results in less low-level cooling by the downdraft. The simulation with the cold initial sounding showed a change in storm propagation velocity when the specified sizes of hail and graupel were increased, but this did not occur when the warm initial sounding was used. With a larger shape parameter for cloud water or with a larger number concentration of cloud droplets, there is less autoconversion and less collection of cloud water and, consequently, much less precipitation at the ground and denser cirrus anvils. While the number concentration of cloud droplets can be forecast in some models with parameterized microphysics, at present the shape parameter for cloud water cannot and must, therefore, be carefully selected.

A Quantitative Investigation of Entrainment and Detrainment in Numerically Simulated Cumulonimbus Clouds

A method is developed that uses numerical tracers to make accurate diagnoses of entrainment and detrainment rates and of the properties of the entrained and detrained air in numerically simulated clouds. These rates and properties are averaged horizontally and over time, and are produced independently of each other. There are no restrictions on the types of clouds to which the procedure can be applied. Cumulonimbus clouds are simulated with a variety of initial thermodynamic soundings. In the simulations, updraft entrainment rates are large near and above cloud base, through the entire depth of the conditionally unstable layer. Stronger updrafts in a more unstable environment are better able to entrain relatively undisturbed environmental air, while weaker updrafts in a less unstable environment can entrain only air that has been modified by the clouds. Smaller convective clouds in more stable environments mix more with their environment but do not necessarily have larger entrainment rates. How much air is entrained depends on the low-level convective available potential energy (CAPE) and on the convective inhibition of the environmental air. Strong updrafts that are produced when the low-level CAPE is large include parcels with a wide range of equivalent potential temperature and are more likely to have an undilute core and to reach or exceed their level of neutral buoyancy than the weaker and more horizontally uniform updrafts that are produced when low-level CAPE is small. These results help to explain previous observations that convective updraft cores are stronger in midlatitude continental clouds than they are in tropical maritime clouds.

The Sensitivity of Simulated Storm Structure, Intensity, and Precipitation Efficiency to Environmental Temperature

Prior parameter space studies of simulated deep convection are extended to embrace shifts in the environmental temperature. Within the context of the parameter space study design, shifts in this environmental temperature are roughly equivalent to changes in the ambient precipitable water (PW). Two series of simulations are conducted: one in a warm environmental regime that is associated with approximately 60 mm of precipitable water, and another with temperatures 8°C cooler, so that PW is reduced to roughly 30 mm. The sets of simulations include tests of the impact of changes in the buoyancy and shear profile shapes and of changes in mixed- and moist layer depths, all of which have been shown to be important in prior work. Simulations discussed here also feature values of surface-based pseudoadiabatic convective available potential energy (CAPE) of 800, 2000, or 3200 J kg 1 , and a single semicircular hodograph having a radius of 12 m s 1 , but with variable vertical shear. The simulations reveal a consistent trend toward stronger peak updraft speeds for the cooler temperature (reduced PW) cases, when the other environmental parameters are held constant. Roughly comparable increases in updraft speeds are noted for all combinations of mixed- and moist layer depths. These increases in updraft strength evidently result from both the reduction of condensate loading aloft and the lower altitudes at which the latent heat release by freezing and deposition commences in the cooler, low-PW environments. As expected, maximum storm precipitation rates tend to diminish as PW is decreased, but only slightly, and by amounts not proportionate to the decrease in PW. The low-PW cases thus actually feature larger environment-relative precipitation efficiency than do the high-PW cases. In addition, more hail reaches the surface in the low-PW cases because of reduced melting in the cooler environments. Although these experiments were designed to feature specified amounts of pseudoadiabatic CAPE, it appears that reversible CAPE provides a more accurate prediction of updraft strength, at least for the storms discussed here.

Sensitivities of Simulated Convective Storms to Environmental CAPE

AbstractA set of 225 idealized three-dimensional cloud-resolving simulations is used to explore convective storm behavior in environments with various values of CAPE (450, 800, 2000, and 3200 J kg−1). The simulations show that when CAPE = 2000 J kg−1 or greater, numerous combinations of other environmental parameters can support updrafts of at least 10 m s−1 throughout an entire 2-h simulation. At CAPE = 450 J kg−1, it is very difficult to obtain strong storms, although one case featuring a supercell is found. For CAPE = 800 J kg−1, mature storm updraft speeds correlate positively with strong low-level lapse rates and reduced precipitable water. In some cases, updrafts at this CAPE value can reach speeds that rival predictions of parcel theory, but such efficient conversion of CAPE to kinetic energy does not extend to all storms at CAPE = 800 J kg−1, nor to any storms in simulations at lower or higher CAPE. In simulations with CAPE = 2000 or 3200 J kg−1, the strongest time-averaged mature updrafts, while ...

A Comparison of Cumulus Parameterizations in Idealized Sea-Breeze Simulations

Abstract Four cumulus parameterizations in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5) are compared in idealized sea-breeze simulations, with the aim of discovering why they work as they do. Compared to simulations of real cases, idealized cases produce simpler results, which can be more easily examined and explained. By determining which features of each parameterization cause them to produce differing results, a basis for improving their formulations and assisting modelers who may design new cumulus parameterizations can be provided. The most realistic results obtained for these simulations are those using the Kain–Fritsch scheme. Rainfall is significantly delayed with the Betts–Miller scheme, due to the method of computing the reference sounding. Another version of this parameterization, which computes the reference sounding differently, produces nearly the same timing and location of deep convection as the Kain–Fri...

The Motion of Simulated Convective Storms as a Function of Basic Environmental Parameters

Abstract Based on results from a three-dimensional cloud-resolving model, it is shown that simulated convective storm motions are affected by thermodynamic as well as kinematic properties of the environment. In addition to the mean wind and its vertical shear, the effect on isolated storm motion of parameters such as bulk convective available potential energy (CAPE), the vertical distribution of buoyancy in the profile, the heights of the lifting condensation level (LCL) and level of free convection (LFC), and cloud-base temperature is considered. Storm motions show at least some sensitivity to all input parameters. Consistent with previous studies, hodograph radius has the most pronounced effect, but the vertical distribution of shear (which also influences the mean wind) affects storm evolution and propagation, even when the effective hodograph radius is unchanged. Among the thermodynamic parameters, the most significant variations occur when the LCL–LFC configuration is modified or when cloud-base temp...

Variability of Updraft and Downdraft Characteristics in a Large Parameter Space Study of Convective Storms

Over 200 convective storm simulations are analyzed to examine the variability in storm vertical velocity and updraft area characteristics as a function of basic environmental parameters. While it is known that bulk properties of the troposphere such as convective available potential energy (CAPE) and deep-layer wind shear exert significant influence over updraft intensity and area, additional parameters such as the temperature at the cloud base, the height of the level of free convection (LFC), and the vertical distribution of buoyancy also have an effect. For example, at low CAPE, updraft strength is strongly related to the vertical distribution of buoyancy, and also to the bulk environmental wind shear. More generally, updraft area and its temporal variability both tend to increase in environments where the LFC is raised. Additionally, in environments with persistent storms, downdraft strength is sensitive to the bulk shear, environmental temperature, and LFC height. Using multiple linear regression methods, the best combinations of environmental parameters explain up to 81% of the interexperiment variance in second-hour mean peak updraft velocity, 74% for midlevel updraft area, and 64% for downdraft velocity. Downdraft variability is explained even less well (49%) when only persistent storms are considered. These idealized simulation results show that it is easier to predict storm updraft characteristics than those of the downdraft.

Properties of Tropical Cloud Ensembles Estimated Using a Cloud Model and an Observed Updraft Population

Abstract A simple cloud model is developed which is designed for both diagnostic studies and mesoscale cumulus parameterization experiments. The cloud model is combined with an observed population of tropical convective updrafts and used to examine the vertical distributions of convective beating and moistening produced by tropical cloud ensembles. Although the cloud model ensembles are dominated by deep cumulonimbi, their vertical beating and moistening profiles differ significantly from those of individual clouds. These profiles and the total rainfall are sensitive to assumptions that affect the vertical mass flux distributions of the clouds. The ensemble heating and moistening profiles are in general agreement with large-scale budget analyses except for a tendency for the former to concentrate more of the heating above 600 mb. Modeled convective properties are found to be highly sensitive to assumptions concerning the convective environment immediately surrounding the updrafts and downdrafts. This has ...