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Masashi Nagata and Yoshi Ogura

Abstract

This paper presents results of simulations for a case of heavy precipitation that occurred on 23 July 1982 over western Japan. Special emphasis is placed on synoptic- and subsynoptic-scale processes that led to heavy precipitation and also its linkage with the low-level jet (LLJ).

The model result recaptures the major observed features of this event reasonably well. The sequence of events revealed by the model starts with the formation of a localized surface warm front caused by the deformation field that is associated with an eastward traveling, nondeepening meso-αscale low. It is followed by the initiation of both concentrated convective precipitation at the surface front and stratiform grid-scale precipitation along the sloping frontal surface. The simulations with different model physics reveal significant roles that diabatic heating processes play in the linkage between heavy precipitation and the LLJ. While condensation heating produces a cyclonic circulation with failing pressure manifested as a mesoscale trough over the front, evaporative cooling from stratiform raindrops generates a marked frontogenetic forcing and creates a cold pool beneath the sloping frontal surface. An anticyclonic outflow from the cold pool is accompanied by convergence on its southwestern flank, which further enhances and concentrates the convective activity and the mesoscale trough. The supergeostrophic LLJ is formed in this situation, where an air parcel crosses height contours into low pressure with large angles due to a combination of an alongfront flow in the southwestern part of the anticyclonic outflow anomaly induced by the evaporative cooling and a cross-front flow in the upper branch of the direct secondary circulation associated with the warm-frontogenetical processes.

Isentropic and isobaric ageostrophic motion diagnoses show that the inertial advective component, mainly arising from the horizontal displacement, is the dominant part in the ageostrophic wind in the entrance region of the LLJ, supporting the conclusion that the rapid parcel acceleration itself occurs almost adiabatically through the horizontal displacement crossing into low pressure.

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Masanori Yoshizaki and Yoshi Ogura

Abstract

The Big Thompson storm occurred on 31 July–1 August 1976 over Big Thompson Canyon, Colorado, when a secondary cold frontal surge was accelerated and reached the foothills of the Front Range. Two- and three-dimensional moist compressible cloud models developed by Ogura and Yoshizaki are applied to this storm event. Adopting highly simplified terrain shapes, this study addresses two aspects of the storm. One is the distinct characteristics of the storm structure, as schematically depicted by Carasena et al.; the other is that heavy precipitation occurred in the basin area rather than over the mountain peak area.

When the model was initialized in such a way that moisture-rich, low-level, strong easterlies impinged upon the orography, the model predicted the development of a storm that not only caused heavy precipitation at the right location relative to the mountain peak, but also reproduced the observed storm in many aspects, both in two- and three-dimensional (2D and 3D) simulations. The major qualitative differences between 2D and 3D simulators is that the model storm in two dimensions is highly transient and exhibits the distinct multicellular structure, whereas the model storm in three dimensions tends to be quasi-stationary. This difference was attributed to the weakness of the induced low pressure inside the storm in three dimensions. Qualitatively, the precipitation accumulation in three dimensions is found to be substantially larger than the 2D counterpart In three dimensions, the low-level easterlies ahead of the mountainous area are deflected as they approach the valley to flow nearly parallel to the elevation contours in each side of the valley and these two airflows eventually converge along the valley to produce heavy rainfall. An interesting finding in the model is the creation of a cold air pool beneath the storm in the situation where the cloud base height is lower than the maximum terrain. The model storm slants severely downstream (particularly in 2D simulations) and precipitating particles fall through the cloud layer, thusthus enhancing evaporating cooling.

When the initial distribution of moisture is assumed to be uniform horizontally in the model, the first deep convection (and consequently heavy preciptation) occurs only at or near the mountain peak, in disagreement with the observations.

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Hiroaki Watanabe and Yoshi Ogura

Abstract

An exceptionally heavy rainstorm hit the coastal area of the western part of Japan on 23 July 1983. The 6-h rainfall accumulation exceeded 300 mm locally. At its peak period, the hourly precipitation rate was as high as 90 mm. Yet the area of heavy precipitation was limited in its extent so that the Maddox criteria for midlatitude Mesoscale Convection Complex was not met in terms of size, when viewed in satellite data. The precipitation occurred in the warm, moist southwesterly sector of a weak, eastward propagating medium scale cyclone that developed along the Baiu front. Prior to the onset of heavy precipitation, the atmosphere was very moist up to the 600 mb level, and became convectively very unstable.

The main topographic feature in the area where heavy precipitation occurred is a mountain ridge that runs approximately parallel to the coastline, with modest elevations of generally less than 1000 m except for mountain peaks. An analysis of raingage records over land clearly indicates that the rainfall accumulation was maximized in the coastal area rather than in the mountainous area. A detailed analysis of PPI radar data reveals that, during the heavy precipitation period, convective cells formed in succession over the sea about 50 km off the coast. As they moved eastward and approached the coastline, they developed rapidly and organized into a band structure. They then weakened on the downwind side of the mountains.

A one-layer model developed by Danard is applied to investigate the topographic effect on the surface flow in the situations under study. The model result indicates that the surface flow over land is deflected mainly by the effect of the topographic barrier and partly by the increased surface friction over land. A convergence zone forms over the coastal strip and the adjacent sea between this deflected flow and the relatively undeflected flow over water. A local maximum of convergence is located just over the area of the maximum rainfall accumulation. This feature accounts for the enhancement of traveling convective cells over the coastal strip and suggests that even a mountain ridge of modest height could enhance precipitation significantly.

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Yoshi Ogura and Diane Portis

Abstract

A cold front which passed through the dense network of the SESAME-AVE (Severe Environmental Storms and Mesoscale Experiment–Atmospheric Variability Experiment) on 25–26 April 1979 was investigated. Rawinsonde data collected from 23 special stations and 19 National Weather Service stations at three-hour intervals for a 24-hour period were used along with hourly surface data, radar summary charts and GOES-East satellite images. Severe storms formed along the surface front during this period. The analysis focused on the vertical circulation across the frontal surface at low levels.

The major features of the cold frontal system that emerged from an analysis of this unique data set include a familiar direct vertical circulation, with moist warm air ascending just above the surface front. However, the upgliding motion was intercepted by a secondary circulation at middle levels. The analysis result was compared with model predictions of Hoskins and Bretherton (1972) as calculated by Blumen (1980). Several features of the observed front were found to agree qualitatively well with the model prediction. These include: a) Both the horizontal temperature gradient and the vertical component of vorticity have their maxima near the ground surface; b) The horizontal gradient of potential temperature is smaller in the warm air region than in the cold air region; c) The temperature inversion layer representing the frontal surface is located behind and below the axis of the maximum cyclonic relative vorticity. However, the model is found to be less successful in predicting the low-level convergence field; the observed surface convergence and cyclonic vorticity are of the same order of magnitude and concentrated in zones of approximately the same width of 300 km. The observed maximum ascending motion is located at low levels, rather than in middle levels as predicted. The subsidence in the cold air region is also much stronger than the model prediction.

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Yoshi Ogura and Masanori Yoshizaki

Abstract

When the western coast of India lies in the path of the low-level west-southwest wind crossing the Arabian Sea during the summer monsoon season, deep convection frequently develops over the ocean off the coast. In such a situation, the maximum rainfall occurs near the coast, not over the Western Ghats. In order to study the physics underlying orographic-convective precipitation over this area, a two-dimensional compressible moist cloud model is applied. The model is written in terrain-following coordinates and includes the Coriolis force and a planetary boundary layer parameterization. The initial fields of thermodynamic variables are specified using observed data gathered upstream of the offshore precipitating systems over the Arabian Sea. Two wind profiles are considered: vertically uniform and nonuniform flows. The latter profile represents a monsoonal westerly jet at low levels and easterlies in the layer above 5 km. Three cases are considered for each wind profile by including or omitting moisture in the atmosphere and heat and moisture fluxes from the ocean.

Among six cases considered, results from the moist and nonuniform wind profile case with heat and moisture fluxes from the ocean are found to be the most consistent with observations of precipitation rate, preferred location of rainfall, and lack of high-level clouds in the downwind side of the mountain. When fluxes from the ocean are excluded, the predicted rainfall accumulation is about the same. However, the maximum rainfall rate occurs over the mountain peak area, in disagreement with the observation. When fluxes from the ocean are included, but with the vertically uniform basic flow, the predicted maximum rainfall occurs at the coast. However, its rate is about half that observed. It is thus concluded that, in order to account for the observed features of rainfall over the Arabian Sea and the Ghat Mountains during the summer monsoon season, two factors, the strongly sheared environment and fluxes of latent and sensible heat from the ocean, are essential. These factors were not considered by Smith and Lin nor Grossman and Durran.

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Yoshi Ogura and Yi-Leng Chen

Abstract

No abstract available.

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Yoshi Ogura and Jih-Yih Jiang

Abstract

The two-dimensional version of the cumulus ensemble model developed by Soong and Ogura is applied both to a prestorm situation and to the mature stage of the extratropical mesoscale convective system (MCS) that developed on 10–11 April 1979 (AVE-SESAME-79 I) over the central United States. The objective is to investigate the statistical properties of convection, developing in response to an imposed large-scale forcing, and the thermodynamic feedback effect of clouds on the large-scale environment in midlatitudes. The result is compared to that recently obtained by Tao for a tropical rainband.

The outstanding result of the model integration for 17 h of physical time is that statistical properties of clouds averaged horizontally over 128 km of the model domain undergo temporal variations for a given time-independent large-scale forcing, rather than settling down into a steady state. When applied to a prestorm situation, the model predicts heavy precipitation that continues to fall for the first 5 h, followed by a 4 h period without precipitation. A second burst of deep convection then occurs. An analysis of the result reveals that the pause of precipitation occurs when the subcloud layer is dried up primarily due to the net vertical transport of moisture associated with clouds. Convection again starts developing when the moisture in the subcloud layer is replenished by the imposed large-scale forcing. The precipitation rate averaged over the precipitation period is found to exceed the supply of moisture by the large-scale forcing. The result implies that the fraction of moisture convergence in a vertical air column that contributes to moisten the environmental atmosphere in Kuo's cumulus parameterization scheme can be negative.

Further, the result indicates the following: 1) The updraft mass flux increases with height until it reaches the local maximum at 350 mb, indicating that the cloud population is dominated by deep clouds, in contrast to the bimodal or broad spectral distribution of clouds observed in the tropics. 2) The cloud heating effect does not balance the large-scale cooling effect, reflecting the fact that the storage and horizontal advection terms are not negligibly small compared to the vertical advection term in the large-scale heat budget; and 3) The net vertical fluxes of heat and moisture are not negligibly small cormpared to condensation and evaporation processes at upper levels in the heat and moisture budgets, reflecting the fact that the atmosphere considered here is more unstably stratified and updrafts are stronger than the tropical counterparts.

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King-Sheng Tai and Yoshi Ogura

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FGGE revel III-b data provided by the European Centre for Medium-Range Weather Forecasts and the outgoing longwave radiation data measured by satellites are used to investigate observationally relationships between deep cloud activity and large-scale meteorological fields during the Northern Hemisphere summer (May–September) of 1979 over the eastern Pacific. Aside from the summer monsoon area over southern and eastern Asia, the eastern Pacific is the area where strong, deep convection frequently develops in the northern summer. This is also one of the tropical ocean areas that have been least explored meteorologically.

A power spectral analysis using the Maximum Entropy Method is made for the meridional wind component at 850 mb at various grid points in the analysis domain for each mouth from May through September. The waves with a period of 4–6 days are not only stronger in July and August than those of other months, but active in the regions of 100°–130°W in the eastern Pacific and 130°–160°E in the western Pacific within the zone of 5°–15°N. These waves possess a wavelength of 3000–3500 km and travel westward with a speed of 5–7 m s−1. Deep convection is found to occur at or slightly behind the wave trough axis.

The structure of easterly waves obtained by a composite technique is similar to those of African waves observed in GATE Phase III and easterly waves in the western Pacific determined by Reed and Recker. These similarities include the amplitude of the wave-related meridional wind (4 m s−1) and a cold core of temperature anomaly pattern at low levels. The manner in which the position of the surface confluence line shifts latitudinally with the passage of waves is remarkably similar to that found in GATE Phase III by Chen and Ogura. However, the structure at the upper levels does not show a distinct secondary maxima of the wave-related perturbations, which is a significant feature in the easterly waves observed in other areas.

The positions of both the axis of the strongest deep cloud activity, inferred from the satellite IR data, and the surface wind confluence line exhibit seasonal variation in the eastern Pacific. Nonetheless, they are closely collocated. Further, they are located over the area of maximum sea surface temperature (SST), suggesting a close relationship among SST, the ITCZ, and the confluence of the surface wind.

At 850 and 700 mb, most of the tropical Pacific is dominated by easterly flow. However, both latitudinal and vertical shears of the easterly flow are much weaker than those observed over the eastern Atlantic and western Africa, suggesting that dry barotropic instability alone cannot account for the formation of easterly waves in the eastern Pacific.

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Yoshi Ogura and Ming-Tao Liou

Abstract

On 22 May 1976 a well-organized squall line with a width of more than 100 km passed through the mesonetwork of the National Severe Storms Laboratory in central Oklahoma. This squall line was evolving from its mature stage to the decaying stage when it passed the network. A total of 81 rawinsonde observations were made at nine stations within the network. These, as well as other observations, were analyzed to depict the kinematic and thermodynamic structure of the squall line. Composite and objective analysis techniques were used to arrive at the time-averaged structure on a vertical plane transverse to the squall line.

The resulting air flow relative to the traveling squall line is consistent with the distribution of the thermodynamic variables and radar reflectivity. A familiar upshear tilt of the updraft is clearly indicated; the upper-level divergence maximum is located ∼70 km to the rear of the low-level convergence maximum. An interesting feature revealed by the analysis is that the horizontal momentum is approximately conserved following air-parcel motions in the updraft before the air reaches the upper-level outflow layer. Consequently, the large momentum originating in the low-level inflow layer is carried upward and meets with the air entering from the rear at middle levels, thus producing a second local convergence maximum. The upper-level updraft associated with this convergence apparently sustains the broad structure of the squall line. Below this updraft, a strong downdraft exists which is fed by a potentially cool dry air that enters the system from the rear at middle levels and is cooled by evaporation. The results of the analysis are compared to the structure of the tropical squall line delineated by other authors and some similarities between them are pointed out, including an “onion” shape formed by the temperature and the dew point curves.

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Wen-Yih Sun and Yoshi Ogura

Abstract

The level 3 turbulence closure model proposed by Mellor and Yamada (1974) is modified 1) to incorporate the formulations for the turbulence third-order moments and pressure terms proposed by Zeman and Lumley (1976) and 2) to introduce turbulence length scales which depend upon the stratification of the atmosphere. The vertical heat and moisture fluxes and the temperature-humidity covariance are determined from differential equations. The model includes two other differential equations, one for the turbulence kinetic energy and the other for virtual potential temperature variance. All other turbulence variables are determined from algebraic equations.

The model is used to simulate the daytime evolution of the planetary boundary layer observed on day 33 of the Wangara boundary-layer experiment. The calculated vertical profiles of the mean wind, temperature and humidity are found to be in good agreement with the observations. The calculated vertical distributions of turbulence variables, including kinetic energy, temperature variance, heat and moisture fluxes, temperature and moisture variance, molecular dissipation, and some third-order moments, compare favorably with those estimated from other numerical models, aircraft observations and laboratory experiments.

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