Dynamics of Convective Clouds and 'CISK' in Vertical Shear Flow-with its Application to Easterly Waves and Squall-Line Systems

Yamasaki, Masanori

doi:10.2151/jmsj1965.62.6_833

Cited by 30 publications

(44 citation statements)

References 56 publications

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“…However, largely because of the uncertainty concerning the validity of the cumulus parameterization used, it was concluded in these studies that it is necessary to understand the cooperative interaction between convection and the large-scale wave in an appropriate way. For this reason, studies with a cloud-resolving model (e.g., Yamasaki, 1975Yamasaki, , 1984 started in the middle of 1970s.…”

Section: Discussion and Remarksmentioning

confidence: 99%

Kelvin Wave-CISK Controlled by Surface Friction

Ohuchi

Yamasaki

1997

Journal of the Meteorological Society of Japan

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As a step toward an understanding of the mechanism of super cloud cluster (SCC), the effect of surface friction on the instability of Kelvin wave is discussed, together with the condition of the vertical profile of parameterized convective heating. In this paper (Part I), linear theory is developed with an equatorial beta-plane model in which the amount of convective heating is assumed to be proportional to the vertical velocity at the top of the boundary layer. Results from the eigenvalue problem suggest that the effect of surface friction is, at least under such parameterization, indispensable for essential understanding of SCC; this study is along the same line as Wang (1988) with respect to the importance of surface friction.Issue is focused on three unstable modes. One is a stationary mode excited with much heating at lower levels, and identified with conditional instability of the first kind (CIFK). The other two are Kelvin modes that contrast with each other; one is destabilized by enough heating at upper levels, even without surface friction (referred to as NFK: Non-Frictional Kelvin mode), and the other is destabilized by moderate heating at lower levels under the control of surface friction (FK: Frictional Kelvin mode). NFK has phase velocities of more than 10ms-1, and its growth rates are on the order of 10-3s-1 at small wavelengths. On the contrary, FK is characterized by smaller phase velocities of less than 10ms-l, and by only slight dependence of its growth rates on wavelength. In addition, the convergence in the boundary layer exhibits a phase shift slightly eastward relative to the convergence aloft; this is the key feature that induces effective feedback between convection and the wave.In view of the large phase velocity and the small preferred scale, NFK may be interpreted as a Kelvin mode of the type that has been treated in many previous studies on SCC. Difficulties in explaining the observed phase velocity and preferred scale of SCC are inherent in NFK. By contrast, FK can give a realistic phase velocity and preferred scale, and clear up major difficulties in the previous studies. Inclusion of surface friction is expected not only to serve as the missing link of the past linear theories of SCC, but also as a basis of future non-linear experiments with explicit moisture processes.

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Section: Discussion and Remarksmentioning

confidence: 99%

Kelvin Wave-CISK Controlled by Surface Friction

Ohuchi

Yamasaki

1997

Journal of the Meteorological Society of Japan

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“…Since the lifetime of mesoscale convection is affected by many factors such as microphysical processes, dynamic and thermodynamic processes, and environmental conditions (Yamasaki, 1983(Yamasaki, , 1984Rotunno et al, 1988), the time scale takes a very wide range from a few hours to more than 10 hours. Nevertheless, a period of a few hours seems to be the basic time scale of mesoscale organized convection (Yamasaki, 1984(Yamasaki, , 1996.…”

Section: Introductionmentioning

confidence: 99%

“…For example, thunderstorms (Byers and Braham, 1949;Browning, 1977) and squall-lines (Hane,1973;Ogura and Chen,1977;Ogura and Liou, 1980;Yamasaki, 1984;Yoshizaki, 1986;Fovell and Ogura, 1988) have been investigated observationally and numerically. Many numerical studies have tried to make clear by what mechanism convection could be long-lasting; that is, maintained longer than the lifetime of an ordinary cumulonimbus (about one hour).…”

Section: Introductionmentioning

confidence: 99%

“…As for organized clouds, Yamasaki (1984) and Fovell and Ogura (1988) simulated squall-lines which lasted for more than several hours, contin-2 The term 'frictional inflow' is different from 'frictional convergence' in that 'frictional inflow' does not necessarily imply convergence. Although horizontal convergence is usually accompanied with mesoscale convection, it is caused by the interaction of convective activity and 'frictional inflow' in most cases.…”

Section: Introductionmentioning

confidence: 99%

“…Simulated squall-lines showed distinct multicellular structure in which each cell tilted upshear. In Yamasaki (1984) the lifetime of mesoscale convection was longer (6 to 8 hours) compared with ordinary time scale of a few hours and the effect of vertical profile of an environmental flow on the time scale of mesoscale convection was emphasized.…”

Section: Introductionmentioning

confidence: 99%

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The Effect of Surface Friction on the Mesoscale Organization of Cumulus Convection in Tropical Cyclones

Nasuno¹,

Yamasaki²

1997

Journal of the Meteorological Society of Japan

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In order to study the effect of surface friction on the properties of mesoscale convection which constitute the rainbands of tropical cyclones, numerical experiments are performed with an axi-symmetric non hydrostatic model. As discussed by Yamasaki (1983), when the tangential wind velocity is not very strong (10 ms-1 or less), the typical time scale of mesoscale convection is a few hours. When the tangential wind intensifies and frictional inflow becomes very significant, the lifetime of mesoscale convection becomes 11-12 hours. In contrast to the convection with time scale of a few hours, it does not decay owing to an outward shift of low-level convergence associated with the intensification of cold downdraft outflow. Its decay is caused by its inward shift due to strong frictional inflow. In this situation, rainwater falling from the cloud, which tilts outward with height at this stage, acts to prevent the low-level warm moist air from flowing into the cloud. A new mesoscale convection is formed at some distance from the old one. Structure, time evolution and the mechanism of the mesoscale convection with the longer lifetime are discussed in detail.

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