An Explicit One-Dimensional Time-Dependent Tilting Cloud Model

The primary goal of this study is to evaluate the performance of the Explicit One-dimensional (1D) Time-dependent Tilting Cloud Model (ETTM), which will be potentially used in a cumulus parameterization scheme. The Weather Research and Forecasting (WRF) model was used in a cloud-resolving mode to study 3D cloud characteristics under two sheared environments, one from the Rain in Cumulus over the Ocean (RICO) field experiment and the other from the International H 2 O Project (IHOP). Then, WRF 3D simulation results were used to evaluate ETTM performance. WRF simulations were performed with di¤erent radii from 1 km to 10 km of thermal bubbles for initiation. The three-dimensional cloud features were quite di¤erent between RICO and IHOP due to their environments, which were subtropical maritime sounding and mid-latitude continental sounding , respectively. ETTM 1D cloud simulations, corresponding to each of the WRF simulations, were conducted. The simulated 1D clouds were too weak when the original thermal bubbles were similar to those used in the 3D cloud simulations (i.e., no additional moisture within the thermal bubbles). The sensitivity of model results to relative humidity was tested by imposing a lower bound of 88% (ER88) and 95% (ER95) humidity to the thermal bubble in ETTM simulations. When compared with the original simulations, 1D results from ER88 and ER95 showed clear improvements, but they were still underestimated relative to 3D clouds, and the results from IHOP were slightly worse than those from RICO. Sensitivity tests with a zero-degree cloud tilting angle and with a di¤erent radius of the downdraft were also examined. Results show that the downdraft due to the tilting of the cloud slightly improved ETTM's performance in terms of the heat and moisture fluxes, while the influence of using di¤erent downdraft sizes on 1D simulation results is not clear.

Meteorology and Atmospheric Physics, 2013

A one-dimensional Explicit Time-dependent Tilting cloud Model (ETTM) that separates updraft and downdraft columns and takes into account the effect of cloud tilting on precipitation is introduced and incorporated into the Advanced Regional Prediction System (ARPS). Results of the stand-alone ETTM are compared with that of cloud resolving simulations using the ARPS mesoscale model. Inter-comparison is performed by qualitative examination of simulated parameters such as vertical distribution of fluxes of mass, heat, and moisture. Although there is a great degree of similarity between the vertical profiles, ETTM systematically underestimates magnitudes of all fluxes. Sensitivity tests carried with ETTM show that the effect of varying cloud radius and tilting angle is considerable on the simulated cloud behavior. Increasing the cloud radius, results in a corresponding increase in fluxes of mass, heat, and moisture, while increasing the cloud tilt angle has the opposite effect. Since ETTM showed promise as a suitable sub-grid cumulus parameterization scheme; it was incorporated into ARPS as an additional cumulus parameterization scheme (CPS) to be available for the wider community. Results of simulations using ETTM and other CPSs already available in ARPS were compared for 2, 4 and 10 km grid spacings to assess its utility. Simulation results of the 2 km grid showed that at this resolution, the simulated time series of updraft velocities using the new scheme (ETTM) compared well with the results of other schemes in the ARPS model. The simulations with horizontal resolution of 4 km that was compared with the convection resolving reference run (No-CPS-2KM) showed almost consistent results for all schemes except for one using KF scheme. The results of the simulation with the ETTM scheme and other schemes in the model with resolution of 10 km showed that at this resolution, there is not significant difference between the uses of these schemes.

Journal of Geophysical Research, 1998

We have generalized the Arakawa-Schubert cumulus parameterization to allow multiple cloud-base levels. A spectrum of cloud-top levels is allowed for each cloud-base level. We use a linear (rather than exponential) mass flux profile, because this greatly simplifies the determination of the entrainment rate and reduces the computational requirements of the parameterization. Cloud-top entrainment is included. The linear mass flux profile and the multiple cloud bases are separately tested in a general circulation model. Results show that the linear mass flux profile tends to lead to smoother precipitation distribution with a decrease in the depth and intensity of deep convection and that cumuli with cloud bases above the boundary layer tend to occur at night over land. Atmospheric Research Program) Atlantic Tropical Experiment (GATE). Cloud coverage over the aircraft box approached 50%, and cumulus towers were accompanied by altostratus, altocumulus, and cirrus clouds. They used aircraft, radar, satellite, and ship data to construct cloud maps, which show that many of the clouds had high bases; for example, one cumulonimbus cloud had a base slanting from 5 to 8 km. The cloud tops reached 13 km. Warner et al. concluded that such high-based convective clouds are symptomatic of convective instability released by uplift from below. Cloudiness during GATE was also studied by Holle et al. [1979], using hourly daytime whole-sky photographs taken aboard four U.S. ships. Clouds with bases higher than 2 km were defined as upper clouds. The average total cloud cover was 77%, and 41% of these were upper clouds. Elsewhere in the tropics, Warner and Grumm [1984] used a variety of observations, including cloud photographs from aircraft, to study cloud distributions associated with a monsoon depression on July 7, 1979, in the Bay of Bengal. They found a dense overcast based at about 7.5 km above sea level, with contiguous cumuli. Turning to middle latitudes, Colman [1983] studied an intense and persistent outbreak of elevated convection, beginning over northeast Texas on March 5, 1982, and continuing into March 6. Colman's analysis indicated that at the midlevels the convective line was organized ahead of a cyclonic circulation center. Temperature and moisture advection generated convective instability aloft. Meanwhile, strong frontogenesis occurred at 850 mbar, and the associated frontal circulation Copyright 1998 by the American Geophysical Union. Paper number 98JD00346. 0148-0227/98/98JD-00346509.00 apparently triggered the convection. In this type of convection, no convective updraft air comes directly from the PBL. Modeling studies also suggest the possible importance of convection starting above the PBL. For example, Randall et al. [1989] studied interactions among radiation, convection, and large-scale dynamics by using a general circulation model (GCM). In their study the penetrative cumulus convection originating in the PBL was represented through the cumulus parameterization of Arakawa and Schubert [1974, hereinafter referred to as AS], which they called "CUP." Manabe's moist convective adjustment parameterization, called "MSTADJ," was used as a supplementary parameterization for moist convection which originates above the PBL; this procedure was devised by the modeling group at UCLA, during the 1970s, and is described by Suarez et al. [1983]. (We emphasize that MST-ADJ is used only to represent convection that originates above the PBL so that it does not duplicate any part of what CUP does.) Randall et al. [1989] reported that MSTADJ was very active in their simulations. Here we show similar results from the current Colorado State University (CSU) GCM, which is descended from the UCLA GCM and is described further later in this paper. Figure 1 shows the frequency of occurrence (hereinafter referred to as "incidence") of penetrative cumulus convection and moist convective adjustment, in a run with the current CSU GCM, for January and July. MSTADJ occurs as frequently as CUP, even in the tropics, in both January and July. The studies discussed above strongly suggest that a cumulus parameterization should allow the possibility of convection originating at any and all possible levels. The main purpose of this paper is to report the development of such a parameterization, based on the approach of AS. As summarized in Table 1, some existing parameterizations do allow convection to originate at any level. Very few of these allow penetrative convection from multiple levels, however. In addition, the literature contains very little information about model-simulated convective activity that originates aloft. Adding multiple cloud-base levels obviously increases the complexity and computational cost of a parameterization and would have been prohibitively difficult in the case of the alreadycomplex AS parameterization had we not made major simplifications of the approach taken by AS and Lord et al. [1982]. We viewed simplification of the AS parameterization as a necessary preliminary step and approached it in two separate ways. The first approach was adoption of the prognostic closure 11,341 11,342 DING AND RANDALL: MULTIPLE CUMULUS CLOUD BASES Weather Rev., 108, 169-194, 1980. Xie, P., and P. A. Arkin, Analyses of global monthly precipitation using gauge observations, satellite estimates, and numerical model predictions, J. Clim., in press, 1996. Zhang, G. J., and N. A. McFarlane, Sensitivity of climate simulations to the parameterization of cumulus convection in the Canadian Climate Centre general circulation model, Atmos. Ocean, 33, 407-446, 1995.

A one-dimensional prognostic cloud model has been developed for possible use in a Cumulus Param-eterization Scheme (CPS). In this model, the nonhydrostatic pressure, entrainment, cloud microphysics, lateral eddy mixing and vertical eddy mixing are included, and their effects are discussed. The inclusion of the nonhydrostatic pressure can (1) weaken vertical velocities, (2) help the cloud develop sooner, (3) help maintain a longer mature stage, (4) produce a stronger overshooting cooling, and (5) approximately double the precipitation amount. The pressure perturbation consists of buoyancy pressure and dynamic pressure, and the simulation results show that both of them are important. We have compared our simulation results with those from Ogura and Takahashi's one-dimensional cloud model, and those from the three-dimensional Weather Research and Forecast (WRF) model. Our model, including detailed cloud microphysics, generates stronger maximum vertical velocity than Ogura and Takahashi's results. Furthermore, the results illustrate that this one-dimensional model is capable of reproducing the major features of a convective cloud that are produced by the three-dimensional model when there is no ambient wind shear.

Journal of the Atmospheric Sciences, 2005

This paper is the first in a two-part series in which the life cycles of numerically simulated shallow cumulus clouds are systematically examined. The life cycle data for six clouds with a range of cloud-top heights are isolated from an equilibrium trade cumulus field generated by a large-eddy simulation (LES) with a uniform resolution of 25 m. A passive subcloud tracer is used to partition the cloud life cycle transport into saturated and unsaturated components; the tracer shows that on average cumulus convection occurs in a region with time-integrated volume roughly 2 to 3 times that of the liquid-water-containing volume. All six clouds exhibit qualitatively similar vertical mass flux profiles with net downward mass transport at upper levels and net upward mass flux at lower levels. This downward mass flux comes primarily from the unsaturated cloud-mixed convective region during the dissipation stage and is evaporatively driven. Unsaturated negatively buoyant cloud mixtures domina...