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
Abstract
Not available
Abstract
Not available
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
The nonlinear Schrödinger (NLS) equation was used to explore possible mechanisms responsible for the intraseasonal oscillation phenomenon occurring in the atmosphere. An NLS equation depicts such a scenario that in a narrowbanded wave packet, the balance between nonlinearity and dispersion results in either a focusing of the wave packet appearing as a solitary envelope or a defocusing in the form of a superlinear envelope. The occurrence of either case depends on the signs of the dispersion coefficient and the Landau constant. Application of this concept to the intraseasonal oscillation was motivated by observational evidence that the large-scale eastward propagation of large cloud clusters with a recurrence period of 40–50 days is concurrently accompanied by mesoscale westward propagations of smaller cloud clusters. The small clusters are actually embedded in the large clusters and exhibit themselves in a well-organized wave train form. In the context of an NLS equation, the eastward propagation of large clusters could be an envelope propagation of the small cluster wave train at the group velocity. In order to examine this hypothesis, a theoretical framework was established in this study. An NLS equation was derived from the nonlinear shallow water equation forced by the internal atmospheric heating, such as that due to cumulus clouds. Two types of wave trains that may resemble dynamical behaviors of the intraseasonal oscillation were investigated: mixed Rossby–gravity wave trains and Rossby wave trains. For mixed Rossby–gravity wave trains, only a superlinear envelope is possible. For Rossby wave trains, the occurrence of a solitary envelope or a superlinear envelope depends on the wavelength. Eventually, the current NLS equation model is still too crude to reproduce quantitative characteristics of the intraseasonal oscillation, for example, the propagation speed of the large cloud clusters.
Abstract
The nonlinear Schrödinger (NLS) equation was used to explore possible mechanisms responsible for the intraseasonal oscillation phenomenon occurring in the atmosphere. An NLS equation depicts such a scenario that in a narrowbanded wave packet, the balance between nonlinearity and dispersion results in either a focusing of the wave packet appearing as a solitary envelope or a defocusing in the form of a superlinear envelope. The occurrence of either case depends on the signs of the dispersion coefficient and the Landau constant. Application of this concept to the intraseasonal oscillation was motivated by observational evidence that the large-scale eastward propagation of large cloud clusters with a recurrence period of 40–50 days is concurrently accompanied by mesoscale westward propagations of smaller cloud clusters. The small clusters are actually embedded in the large clusters and exhibit themselves in a well-organized wave train form. In the context of an NLS equation, the eastward propagation of large clusters could be an envelope propagation of the small cluster wave train at the group velocity. In order to examine this hypothesis, a theoretical framework was established in this study. An NLS equation was derived from the nonlinear shallow water equation forced by the internal atmospheric heating, such as that due to cumulus clouds. Two types of wave trains that may resemble dynamical behaviors of the intraseasonal oscillation were investigated: mixed Rossby–gravity wave trains and Rossby wave trains. For mixed Rossby–gravity wave trains, only a superlinear envelope is possible. For Rossby wave trains, the occurrence of a solitary envelope or a superlinear envelope depends on the wavelength. Eventually, the current NLS equation model is still too crude to reproduce quantitative characteristics of the intraseasonal oscillation, for example, the propagation speed of the large cloud clusters.
Abstract
Combinations of statistical analyses including principal component analysis, and uni- and multivariate singular spectrum analyses, were carried out to characterize the spatial–temporal structures of trend and interannual oscillatory variabilities of precipitation over the major north-flowing river basins in the former Soviet Union.
The series of monthly precipitation were corrected for the biases of precipitation measurement due to the gauge-type change and changes in observing procedures. An upward trend was found in the monthly precipitation series for the last half century. This upward trend was stronger in the North Dvina and Pechora River basins, and in the Ob-Irtysh River basins, but much weaker (still upward, though) in the Yenisey–Lena River basins. The notable increases of precipitation over the southwestern part—the Volga and Ural River basins—were found to be due at least in part to the upward phase of some quasi-century periodicity. Generally speaking, the precipitation increases appeared to be more apparent during the cold seasons in the western half of the sector, while in the eastern part, it appeared to be equally or more notable during summer.
On the interannual timescales, signals of 4–5-yr and quasi-biennial oscillations were found in the space–time-dependent precipitation series. The 4–5-yr oscillation was quite apparent over the entire Northern Eurasian sector, being stronger over the southeastern and western parts. This oscillation appeared to propagate eastward. The quasi-biennial oscillation was generally weaker; it was very weak during the 1955–65 period. This oscillation was relatively stronger in the western half of the sector and weaker over the eastern half.
Abstract
Combinations of statistical analyses including principal component analysis, and uni- and multivariate singular spectrum analyses, were carried out to characterize the spatial–temporal structures of trend and interannual oscillatory variabilities of precipitation over the major north-flowing river basins in the former Soviet Union.
The series of monthly precipitation were corrected for the biases of precipitation measurement due to the gauge-type change and changes in observing procedures. An upward trend was found in the monthly precipitation series for the last half century. This upward trend was stronger in the North Dvina and Pechora River basins, and in the Ob-Irtysh River basins, but much weaker (still upward, though) in the Yenisey–Lena River basins. The notable increases of precipitation over the southwestern part—the Volga and Ural River basins—were found to be due at least in part to the upward phase of some quasi-century periodicity. Generally speaking, the precipitation increases appeared to be more apparent during the cold seasons in the western half of the sector, while in the eastern part, it appeared to be equally or more notable during summer.
On the interannual timescales, signals of 4–5-yr and quasi-biennial oscillations were found in the space–time-dependent precipitation series. The 4–5-yr oscillation was quite apparent over the entire Northern Eurasian sector, being stronger over the southeastern and western parts. This oscillation appeared to propagate eastward. The quasi-biennial oscillation was generally weaker; it was very weak during the 1955–65 period. This oscillation was relatively stronger in the western half of the sector and weaker over the eastern half.
Abstract
The effects of cumulus heating and the possibility of CISK in extratropical latitudes are explored by combining the Eady model of baroclinic instability with wave-CISK. For perturbations about a state of rest, there is no intrinsic time scale, however the equations can be solved for geopotential tendency. It was found that CISK disturbances could not grow unless the heating exceeded a threshold value. This value is reduced when the boundary layer moisture content is increased, the static stability decreased, or the heating distributed lower in the troposphere.
Perturbations about a baroclinic basic state, for small heating rates resemble a baroclinic wave with heating acting mainly to reduce the effective static stability, resulting in faster growth and shorter wavelengths. However, as the heating is increased, the instability gradually takes on the characteristics of the pure CISK disturbance. The transition level shows the same dependencies on model parameters as for the threshold in the CISK calculation, but occurs at a somewhat lower level.
Comparison of model results with observations of three comma clouds and three polar lows seemed to suggest that two of the comma clouds originated as baroclinic waves which owed their short wavelengths to the effective reduction in stability caused by cumulus heating, while two of the polar lows seemed to have resulted from CISK. The remaining disturbances, one polar low and one comma cloud, appeared to correspond to values in the transition region.
Abstract
The effects of cumulus heating and the possibility of CISK in extratropical latitudes are explored by combining the Eady model of baroclinic instability with wave-CISK. For perturbations about a state of rest, there is no intrinsic time scale, however the equations can be solved for geopotential tendency. It was found that CISK disturbances could not grow unless the heating exceeded a threshold value. This value is reduced when the boundary layer moisture content is increased, the static stability decreased, or the heating distributed lower in the troposphere.
Perturbations about a baroclinic basic state, for small heating rates resemble a baroclinic wave with heating acting mainly to reduce the effective static stability, resulting in faster growth and shorter wavelengths. However, as the heating is increased, the instability gradually takes on the characteristics of the pure CISK disturbance. The transition level shows the same dependencies on model parameters as for the threshold in the CISK calculation, but occurs at a somewhat lower level.
Comparison of model results with observations of three comma clouds and three polar lows seemed to suggest that two of the comma clouds originated as baroclinic waves which owed their short wavelengths to the effective reduction in stability caused by cumulus heating, while two of the polar lows seemed to have resulted from CISK. The remaining disturbances, one polar low and one comma cloud, appeared to correspond to values in the transition region.
