Search Results

You are looking at 1 - 10 of 40 items for

  • Author or Editor: Han-Ru Cho x
  • Refine by Access: All Content x
Clear All Modify Search
Han-Ru Cho

Abstract

Results from a linear analysis and numerical integrations of a two-dimensional model of moist frontogenesis show that mesoscale organizations of precipitation in midlatitude cyclones may be caused by a positive feedback process between potential-vorticity anomalies and latent heat release in condensation. The instability analysis indicates that the instability is a mesoscale process due to the small magnitude of the temperature perturbations associated with potential-vorticity anomalies at the small end of the mesoscale and the small vertical velocity induced by beating at the synoptic scale. Numerical model results show considerable mesoscale organizations in the condensation field can be created by this instability process.

Full access
Han-Ru Cho

Abstract

Not available

Full access
Han-Ru Cho

Abstract

Some statistical properties of homogeneous and stationary shallow cumulus cloud fields are studied. The work is motivated by the observation that the number of cumulus clouds in a large-scale tropical weather system is usually very large. It seems necessary to introduce statistical methods in solving the problem of cumulus parameterization. The study is based upon the assumption that a shallow cumulus cloud field in a homogeneous and stationary state is composed of a large number of statistically independent clouds or cloud groups. The physical basis for this assumption is discussed. Two statistical properties of cloud fields are investigated. The spatial distribution of independent cloud groups is first derived and is given by the Poisson distribution. This distribution is found to agree very well with data of some cloud radar observations. Certain constraints imposed on cloud fields by large-scale circulations and surface conditions are then discussed. The cloud group distribution as a function of vertical moisture transport rates of cloud groups at the cloud-base level is also derived. It is shown that under certain conditions this distribution also gives the size distribution of cloud groups. A comparison of this derived size distribution with the size distribution obtained from cloud radar observations is made. The agreement between the derived and the observed distributions appears quite encouraging.

Full access
Han-Ru Cho

Abstract

Based upon Reed-Recker's (1971) composite easterly wave and the associated cumulus cloud population determined by Cho and Ogura (1974), the effects of cumulus cloud on the large-scale moisture distribution are studied in this note. It has been found that the moisture contents in the upper troposphere increase almost linearly with increasing deep cloud activity. There is also a marked increase in moisture content in the lower troposphere when the weather changes from a relatively suppressed condition to a relatively disturbed condition. However, further intensification of deep cloud activity does not lead to any significant increase in moisture content in the lower troposphere. In the region of the wave where deep cloud activities are developing, the rate of moisture supply of the troposphere exceeds that of precipitation and the atmosphere is moistened. In other regions, where deep cloud activities are decaying, the rate of precipitation exceeds that of the moisture supply and the moisture content is depleted. An empirical method to determine the partition of moisture supplied to an air column into precipitation and storage is suggested.

Full access
Han-Ru Cho

Abstract

The horizontal momentum exchange process between cumulus clouds and their environment are examined. A general formula is also derived for the rates of entrainment and detrainment of momentum of a cumulus cloud ensemble. These general entrainment and detrainment rates are found to depend on the cloud mass entrainment/detrainment rate and cloud dynamic properties in a way quite different from that assumed in previous parameterization studies. The formula is applied to two simple cloud models. It is shown that even in the context of these simple models, the general formulation gives substantially different results.

Full access
Han-Ru Cho

Abstract

Effects of the life cycles of cumulus clouds, on the large-scale heat and moisture equations are derived by introducing a cloud distribution function which describes the cloud fractional coverage distribution in terms of cloud-top height as well as cloud age. It is found, in addition to the heating and drying effects described in previous formulations using a steady-state cloud model, that cumulus clouds have a moistening effect due to the finite life spans of cumulus clouds. The magnitude of this effect is inversely proportional to the lifetimes of clouds. The formulation has been applied to a typical trade wind weather situation observed during Phase III of BOMEX to determine the total cloud mass flux, the fractional cloud coverage distribution, as well as the mean lifetime of the shallow cumulus population. It is found that cumulus clouds covered about 2% of the total observational area. The mean life span of the cloud population is of the order of 9 min. The atmospheric air near the cloud-base level was recycled about three times a day by cumulus clouds. This recycling rate decreases rapidly with height. The recycling rate is about 0.5 day−1 at the bottom of the trade inversion, and decreases to zero near the top of the inversion layer.

Full access
Yoshimitsu Ogura and Han-Ru Cho

Abstract

No abstract available.

Full access
Han-Ru Cho and Lawrence Cheng

Abstract

Dynamic processes of cumulus clouds which may produce significant horizontal eddy transport of vertical vorticity in the tropical atmosphere are discussed. It is shown in this paper that horizontal eddy transport of vorticity by cumulus convection, if it exists, must be due entirely to the irrotational component of the horizontal wind produced by clouds. The ability of clouds to produce this eddy transport depends critically on the presence of cyclonic-anticyclonic vortex couplets in the cloud circulations. The generation processes of these vortex couplets are also discussed.

A formula is derived to parameterize this eddy transport process in the large-scale mean vorticity equation. The formulation is tested using. GATE data. GATE-A/B-scale mean vorticity budgets are analyzed for two 1-day periods during Phase III of the experiment. The agreements between the theoretically predicted and the observed apparent vorticity sources are found to be much improved by including the effects of cloud horizontal eddy transport of vertical vorticity.

Full access
Han-Ru Cho and Zuohao Cao

Abstract

The effects of different moisture distributions on the generation of moist potential vorticity (MPV) in extratropical cyclones are examined by numerical simulations. These sensitivity experiments show that low-level negative MPV generation depends on the initial distribution of potential temperature and moisture gradients. The generation of negative MPV depends on the baroclinic and moisture gradient vectors. It can also depend on the locations of condensation. Although condensation has no direct effect on MPV production, MPV can be generated in the neighborhood of condensation where the gradients of potential temperature and moisture have significantly changed. It is found in the sensitivity experiments that the bent-back warm front is the most favored regions for negative MPV generation because strong deformation of moisture and potential temperature fields occur. After the cyclones mature, negative MPV moves into the warm core. It can be transported upward through the enhanced vertical motion induced by latent heat release.

Full access
Han-Ru Cho and Yoshimitsu Ogura

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. This would suggest that the generation and maintenance of shallow clouds and deep clouds are controlled by separate mechanisms. The daily rainfall estimated on the basis of the computed convective mass flux is compared with observations with fairly good agreement.

Full access