Yoshimitsu Ogura

NUMERICAL SIMULATION OF THE LIFE CYCLE OF A THUNDERSTORM CELL

Abstract A model of cumulus clouds is presented that combines the vertical equation of motion, the equation of mass continuity, the first law of thermodynamics, and the following cloud microphysical processes: condensation of water vapor to produce cloud droplets, conversion of cloud droplets to raindrops, glaciation, sublimation of water vapor, melting of ice crystals, evaporation of cloud droplets, evaporation of raindrops, evaporation of ice crystals, and evaporation of melting ice crystals. The conversion and glaciation processes are parameterized and the drag force is assumed to be provided by the weight of hydrometeors. The result of time integration of the model shows that, with the inclusion of the microphysical processes, some aspects of the three stages of the life cycle of a cumulus cloud as depicted by Byers and Braham in 1949 (developing stage, mature stage, and decaying stage) are simulated qualitatively by the model. The model also shows that the rate of conversion from cloud droplets to ra...

A Comparison Between Axisymmetric and Slab-Symmetric Cumulus Cloud Models

Abstract Axisymmetric and slab-symmetric cumulus cloud models with Kesslers parameterizations for microphysical processes are developed. By using a staggered grid arrangement and applying a modified upstream difference scheme, erroneous behavior in the center of a simulated cloud, which would result with the use of the ordinary upstream difference scheme, is eliminated. A comparison between the present two models of different geometries confirms in general the conclusions reached in previous studies: the updraft in an aixsymmetric model grows more vigorously than in a slab-symmetric model. However, the ratio of the maximum updraft in the slab-symmetric model to that in the axisymmetric model is 0.53 in this study, notably larger than Murrays 0.12. An analysis of the pressure gradient force associated with could motions reveals that the vertical pressure gradient force due to perturbed pressure is: 1) of the same order of magnitudes as that of the thermal buoyancy force in the core region of the cloud; i...

Diagnostic Determination of Cumulus Cloud Populations from Observed Large-Scale Variables

Abstract A method is proposed whereby some properties of cumulus cloud populations are determined from observed large-scale meteorological variables. This method combines large-scale heat and moisture balance considerations with a simple steady-state one-dimensional model for an individual cumulus cloud. When applied to the mean summertime conditions over the Marshall Islands, the method gives a bimodal distribution of the vertical mass flux at the cloud base in terms of the cloud top height: one group of clouds penetrates up to the 200–300 mb level and another group stays below the 600 mb level, while relatively few cloud tops lie. between 400 and 600 mb. The total percentage area covered by all clouds is found to be a few percent and the vertical velocity inside the cloud at the cloud base is estimated to be somewhere near 1 m sec−1 for most clouds. Because the observed large-scale averaged upward velocity at that level is of the order of 0.1 cm sec−1, this result indicates that the total mass entering ...

The Pressure Perturbation and the Numerical Modeling of a Cloud

Abstract The adaptation of the deep convection equations of Ogura and Phillips to moist convection results in an implicit relationship between temperature, potential temperature, pressure, and saturation vapor pressure. Typically, the pressure is treated as a known function of height to eliminate this condition. The effect of this approximation is investigated by numerically integrating the primitive equations in two-dimensional Cartesian coordinates. The model includes precipitation, and the equations are integrated for the life cycle of a strong cumulus cell. The result indicates that the non-dimensional pressure perturbation is notably an order smaller than scale analysis indicates it might be and that the deviation from the environmental pressure can be ignored in determining the saturation vapor pressure. This is partly because the pressure diagnostic equation acts to smooth out the pressure distribution.

A Relationship Between Cloud Activity and the Low-Level Convergence as Observed in Reed-Recker’s Composite Easterly Waves

Abstract The method proposed by Ogura and Cho is applied to the averaged easterly wave disturbances in the western Pacific based on Reed and Recker’s data set to determine the cloud mass flux distribution in various weather conditions, ranging from disturbed conditions in the trough region to a relatively undisturbed condition in the ridge region. The results show that shallow clouds are present practically everywhere in the wave disturbances, even in the ridge region where the large-scale vertical motion is downward. In contrast, deep clouds penetrating into the layer above the 500-mb level are present in a significant amount only in the region near the trough axis (most intense in the region east of the trough axis). More quantitatively, while the total mass flux associated with all cumulus clouds at the cloud base level is not correlated with the low-level, large-scale convergence, the mass flux due to deep clouds is found to increase linearly with the increase of low-level, large-scale convergence. Th...

On the Interaction between the Subcloud and Cloud Layers in Tropical Regions

Abstract A model for the subcloud layer in tropical regions is developed to investigate the effect of the mass exchange between the subcloud and the cloud layers on the heat and moisture budgets in the subcloud layer. This mass exchange occurs in association with the updraft inside ascending cumulus clouds and the induced downdraft outside these clouds. The model is first applied in discussing the moisture balance in the subcloud layer in weak traveling easterly waves, using Reed and Recker’s composite data. In the trough-axis region the upward transport of moisture due to ascending cumulus clouds and the downward transport at the cloud base level due to the induced downdraft is found to be much larger in magnitude than the large-scale horizontal convergence of moisture in the subcloud layer or the evaporation from the ocean surface. Steady-state solutions of the model are obtained which describe the relations between the large-scale convergence, the vertical convective mass flux, the energy flux at the o...

Tornadoes in Chiba Prefecture on 11 December 1990

Abstract On the evening of 11 December 1990, two supercell storms bit the Chiba Prefecture, southeast of Tokyo, and spawned two tornadoes in Mobara and Kamogawa. The Mobara tornado caused the most severe tornado damage since 1960 in Japan over a damage swath of 6.5 km in length and 500 m in average width. A detailed damage survey revealed that the tornado moved north-northeastward at a speed of about 16 m s−1. The maximum wind speed near the ground, estimated from damage to structures, was more than 78 m s−1. The storms were initiated in the warm sector of a developing extratropical cyclone, about 6–7 h prior to the tornadogenesis. They moved straightforwardly northeastward at a speed of about 16 m s−1 throughout their life cycles including their supercell phases. The mesocyclone in the Mobara storm had been detected by a single-Doppler radar for 44 min. Vertical vorticity of the mesocyclone amplified to 2 × 10−2 s−1 almost simultaneously between 1 and 5 km AGL, about 20 min prior to the tornadogenesis. A...

A Determination of the Trade-Wind Cumuli Population Using BOMEX Data and an Axisymmetric Cloud Model

Abstract A cumulus cloud population is determined over the trade-wind region under an undisturbed weather condition from the large-scale heat and moisture budgets. The BOMEX data during the period 22–23 June 1969 are used. Six classes of clouds (A–F) are considered. Each class of clouds changes the large-scale mean temperature and humidity through condensation, evaporation and convective transport processes. The change due to each class of clouds is determined using the time-dependent, two-dimensional, axisymmetric cloud model developed by Soong and Ogura (1973). It is found that the cloud population, consisting of 66.4 A clouds, 8.9 D clouds and 1.6 E clouds km−2 DAY−1, satisfies approximately the large-scale heat and moisture budget requirements. The cloud top heights of the three classes of clouds are 0.8, 1.6 and 2.0 km, respectively, and the cloud radii at the cloud base level during the mature stage are 62.5, 150 and 225 m, respectively. Consequently, the fractional area coverage of the predicted cl...

Non-Stationary Finite-Amplitude Convection in a Thin Fluid Layer Bounded by a Stably Stratified Region

Abstract Numerical integrations are performed for the equations governing two-dimensional convection flows in a fluid confined between two horizontal plates. A situation considered here is that local heating at a time-independent rate is provided at the middle level of the fluid so that the upper half of the fluid is destabilized and the lower half stabilized. It is shown that steady-state solutions are obtained when the Rayleigh number (R) is 1.1 times Rc (critical Rayleigh number at which convection sets in according to the linearized theory). For three cases where R=1.5 Rc, 2 Rc and 3 Rc, time-dependent solutions are obtained which describe extremely regular and repeatable convection flows. The flow pattern is such that plume-like cells generated by heating move horizontally, merge with neighboring plumes, and new plumes are generated. This process is repeated. Time-dependent but irregular solutions are obtained for R=5 Rc and beyond.