Air Mass Transformation Research Articles

変化する大規模場中の日本海収束雲帯のモデリングによる事例研究

A prediction experiment is performed for a typical case of cold-air outbreak with large temporal variation over the Japan Sea in the Asian winter monsoon situation with a very-fine-mesh, primitive-equation model. The purposes are to elucidate the typical evolution of the Japan-Sea convergent cloud band (CCB) that responds to the large-scale environment varying according to the passage of a short-wave trough accompanied by a marked cold vortex aloft, and to diagnose the physical processes responsible for the formation of a paired middle-level jet and weak-wind zone analyzed along the CCB.The model simulates well the overall orientation and intensity of the CCB. The CCB reaches its peak intensity under the short-wave trough, where the layer of low stability in the troposphere is deepest and the largest air-mass transformation occurs. The CCB weakens as the stability of the lower troposphere increases. Net air-mass transformation over the sea decreases behind the trough and eventually it dies away, even though air-mass transformation in cold advection persists. Although the orientation of the CCB tends to be parallel with the large-scale flow, this is not always the case. This is because each part of it migrates, following its own local flow field which has both temporal and spatial variations according to the phase of travelling short waves. The variation in thermal structure may be primarily attributable to the location and orientation of the CCB which govern the relative influences of each lower-boundary forcing.A paired middle-level jet and weak-wind zone develop along the CCB when the CCB is in the mature stage. A sensitivity experiment suggests that the pair of wind anomalies are closely linked with the CCB. Analysis of the ageostrophic wind reveals that the jet is rather geostrophic and nearly balanced with the mesoscale temperature field featuring the CCB, while the weak-wind zone is highly sub-geostrophic. The former develops in a mass-momentum adjustment process due to concentrated diabatic heating along the CCB, while the latter is produced directly by rapid upward motion with diabatic heating in a baroclinic environment. The diabatic vertical displacement in cold advection explains the wind-speed minimum in the vertical observed along the ascending zone.

A High Resolution Air Mass Transformation Model for Short-Range Weather Forecasting

Abstract This paper describes a high resolution air mass transformation (AMT) model. The model is intended for short-range weather forecasts of the temperature and humidity profiles in the lower atmosphere, the structure of the boundary layer, the boundary layer height, and the amount of boundary layer clouds. The AMT model consists of a one-dimensional, multilayer boundary layer model, which is advected along trajectories from a source region to a receptor point. The trajectories are calculated within a larger scale (limited area) model. The initial profiles for temperature and humidity are obtained from observed radiosondes. The paper describes the physical and dynamical background of the model. With the model we have made case studies of the development of stratocumulus over the North Sea, and have simulated the representation of clear skies over land. The output of the model is compared with the output of the ECMWF model and the current operational bulk AMT model. Sensitivity of the model to boundary ...

Build-up, air mass transformation and propagation of Siberian high and its relations to cold surge in East Asia

The present paper discusses the build-up, the air mass transformation and the propagation of the Siberian high as well as its relations to the development of cold surges in East Asia. It has been found that (1) the genesis and development of the Siberian high result from the combined effects of the mass convergence at middle and upper-level and the radiative cooling; (2) the apparent transformation of the Siberian high over land is observed in winter, which is caused by the upward sensible heat and latent heat flux from the underlying surface; (3) the Siberian high and its attendant cold air outbreaks usually undergo a marked low-frequency, southward propagation with the period of 10–20 days; (4) activity of cold surge over the East China Sea and the South China Sea is closely related to the intensity of the Siberian high. The active cold surge occurs when the Siberian high is usually strong.

Modified Eady Waves and Frontogenesis Part I: Linear Stability Analysis

Baroclinic instability in an inviscid fluid with parabolic potential temperature profiles is investigated. Unlike the classical Eady model, there is no short wave cutoff. In addition to the longwave disturbances, which are similar to the Eady waves, shortwave disturbances can also develop in the lower atmosphere, where the stratification is weaker. The growth rate of the short waves increases with increasing stratification aloft. The results show that shortwave disturbances can penetrate into the upper stable layer. The growth rate and disturbances of those waves may be associated with an effective Burger number, which is defined as(The equation is abbreviated)where h(superscript *)is the height of the maximum vertical heat flux(w(superscript ')θ(superscript '))and λ is the horizontal wavelength. Numerical simulations obtained from a nonlinear mesoscale model in Part Ⅱ also confirm that the short waves can develop into a surface front within a few days. Those short waves may correspond to the medium-scale disturbances observed over the AMTEX (Air Mass Transformation EXperiment) region.

Bivariate conditional sampling of buoyancy flux during an intense cold-air outbreak

Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top. The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux. Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.

Airmass transformation over the South China Sea during a winter MONEX cold surge

AbstractSeveral studies have linked a wintertime cycle of deep, moist near‐equatorial convection to surges of cold air from Asia. According to an early hypothesis two successive streams of surge air reach north Borneo and offshore waters of the South China Sea. The first, which follows a long over‐water trajectory, is warm by the time it arrives off Borneo. The second, which follows a short over‐water trajectory, still is cool when it arrives off Borneo. More recent hypotheses suggest only the second stage of the surge involves an airstream from Asia and drying rather than cooling is the predominant downstream thermodynamic effect on this airstream.By means of trajectory calculations and an airmass transformation (AMT) model we test these ideas for a moderate cold surge from December 1978. In particular, we contrast the roles, in the transformation process, of surface fluxes of latent and sensible heat and of boundary layer entrainment. At no time through the course of the surge did we find boundary layer trajectories of the second (short path) type. Nevertheless, late in the course of the case study surge dry air did appear at the surface off the south‐east coast of Vietnam. Neither sea surface temperature nor initial airmass structure could account for the local nature of the drying. The explanation most consistent both with observations and AMT model results is entrainment of air that had been dried by local subsidence at and above the top of the marine boundary layer.

Airmass transformation over the South China Sea during a winter MONEX cold surge

Several studies have linked a wintertime cycle of deep, moist near-equatorial convection to surges of cold air from Asia. According to an early hypothesis two successive streams of surge air reach north Borneo and offshore waters of the South China Sea. The first, which follows a long over-water trajectory, is warm by the time it arrives off Borneo. The second, which follows a short over-water trajectory, still is cool when it arrives off Borneo. More recent hypotheses suggest only the second stage of the surge involves an airstream from Asia and drying rather than cooling is the predominant downstream thermodynamic effect on this airstream. By means of trajectory calculations and an airmass transformation (AMT) model we test these ideas for a moderate cold surge from December 1978. In particular, we contrast the roles, in the transformation process, of surface fluxes of latent and sensible heat and of boundary layer entrainment. At no time through the course of the surge did we find boundary layer trajectories of the second (short path) type. Nevertheless, late in the course of the case study surge dry air did appear at the surface off the south-east coast of Vietnam. Neither sea surface temperature nor initial airmass structure could account for the local nature of the drying. The explanation most consistent both with observations and AMT model results is entrainment of air that had been dried by local subsidence at and above the top of the marine boundary layer.

Nearly steady convection and the boundary-layer budgets of water vapor and sensible heat

The budgets of water vapor and sensible heat in the convective atmospheric boundary (mixed) layer are analyzed by means of a simple slab approach adapted to steady large-scale advective conditions with radiation and cloud activity. The entrainment flux for sensible heat is assumed to be a linear function of the surface flux. The flux of water vapor at the top of the mixed layer is parameterized by extending the first-order Betts-Deardorff approach, i.e., by adopting linear changes for both the specific humidity and the flux across the mixed layer and across the inversion layer of finite thickness. In this way the dissimilarity of sensible heat and water vapor transport in the mixed layer can be taken into account. The experimental data were obtained from the Air Mass Transformation Experiment (AMTEX). The entrainment constant for sensible heat at the top of the mixed layer was found to have values similar to those observed in other weakly convective situations, i.e., around 0.4 to 0.6. This appears to indicate that the effect of mechanical turbulence was not negligible; however, the inclusion of this effect in the formulation did not improve the correlation. In contrast to the first-order approach, the zero-order approach, i. e., the jump equation commonly used for the flux of a scalar at the inversion, (ovw′c′ ) h = we δc (where w e is the entrainment velocity and δc the concentration jump across the inversion), was found to be invalid and incapable of describing the data.

初夏のモンゴル•華北一帯の乾燥地帯における気団変質と大陸の梅雨前線の構造急変

初夏のモンゴル•華北一帯の乾燥地帯における気団変質と大陸の梅雨前線の構造急変

A new convective adjustment scheme, Part II: Single column tests using GATE wave, BOMEX, ATEX and arctic air-mass data sets

The schemes proposed in part I are tested using single-column data sets from tropical field experiments (GATE, BOMEX, ATEX) and an arctic air-mass transformation. Both the deep and shallow schemes perform well. The sensitivity of the schemes to adjustable parameters is also studied. Preliminary global forecasts show significant improvements in the global surface fluxes and mean tropical temperature tendency over the operational Kuo convection scheme.

Turbulence in a Convective Marine Atmospheric Boundary Layer

The structure and kinetic energy budget of turbulence in the convective marine atmospheric boundary layer as observed by aircraft during a cold air outbreak have been studied using mixed layer scaling. The results are significantly different from those of previous studies under conditions closer to free convection. The normalized turbulent kinetic energy and turbulent transport are about twice those found during the Air Mass Transformation Experiment (AMTEX). This implies that for a given surface heating the present case is dynamically more active. The difference is mainly due to the greater importance of wind shear in the present case. This case is closer to the roll vortex regime, whereas AMTEX observed mesoscale cellular convection which is closer to free convection. Shear generation is found to provide a significant energy source, in addition to buoyancy production, to maintain a larger normalized turbulent kinetic energy and to balance a larger normalized dissipation. The interaction between turbulent pressure and divergence (i.e., pressure scrambling) is also found to transfer energy from the vertical to the horizontal components, and is expected to be stronger in roll vortices than in m esoscale cells. The sensible heat flux is found to fit well with a linear vertical profile in a clear or subcloud planetary boundary layer (PBL), in good agreement with the results of Lenschow et al., (1980). The heat flux ratio between the PBL top and the surface, derived from the linear fitted curve, is approximately -0.14, in good agreement with that derived from the lidar data for the same case. Near the PBL top, the heat flux profiles are consistent with those of Deardoff (1979) and Deardorff et al. (1980).

Comparisons of Aircraft and Tower Measurements around Tarama Island during the AMTEX ‘75

Abstract During the field observation period of the Air Mass Transformation Experiment in 1975 (AMTEX ‘75), one of the boundary-layer research flights of the National Center for Atmospheric Research (NCAR) Electra aircraft was conducted in the vicinity of Tarama Island. A 50-m onshore observation tower was located on the southwestern coast of Tarama Island, and a 12-m offshore observation tower on the northern reef. We compare and discuss mean profiles of dry- and wet-bulb temperature, wind speed and direction, and turbulent fluxes of latent and sensible heat and momentum observed by these three observing platforms. We found that the 50.m onshore tower, which was about 3.5 km downwind of the shoreline, and the airplane flight legs over the island at 140 and 160 m height were in the island-modified air. This resulted in large differences between ocean and island measured fluxes, smaller differences in the variances and almost no differences in the means. Thus, we found that for the conditions of this exper...

Simple formulation of heat flux in the unstable atmospheric boundary layer

A formulation for the magnitude of the upward heat flux is developed for the common situation where information on temperature fluctuations is available but measurement of vertical motion fluctuations are unavailable or of inadequate quality. A relationship is also developed which allows inference of the vertical structure of the heat flux from the stability. This development uses data from the Air Mass Transformation Experiment and a number of existing studies in the literature.

Simulations of Cold Surges over Oceans with Application to AMTEX'75

Comparative numerical experiments and observational verifications for cold surges over the ocean are conducted using a primitive equation planetary boundary layer (PBL) model. The comparative experiments illustrate the importance of large-scale environmental conditions on air mass transformation. Such factors as baroclinity, static stability, moisture content, upwind inversion strength and height exert strong controls on the downwind evolution of the PBL and clouds. Most results are obtained for clouds in the cumulus mode, but two comparisons are made with simpler stratocumulus simulations. The PBL flow is influenced by the synoptic geostrophic wind distribution and the PBL depth is also sensitive to large-scale vertical velocity. The responses of this advective PBL to stronger wind are found to be different from that of a horizontally homogeneous PBL. A simulation of an intense cold air outbreak observed during the Air Mass Transformation Experiment, 1975 (AMTEX'75) is performed. Downwind variations of dynamic and thermodynamic PBL properties and cloud distribution and type are well reproduced. The steep sea-surface temperature gradient produces strong PBL baroclinity and a strongly divergent PBL flow. The simulated large cross-isobar angle associated with intense cold air advection and vigorous momentum mixing is in favorable agreement with both observation and theory.

暖かい海上における気団変質過程の数値実験

The interaction between small-scale convections and large-scale flow in the airmass transformation processes is investigated by making use of a numerical model. In the model a uni-directional large-scale wind is assumed. Fine resolution is used in the cross-stream and vertical directions in order to treat explicitly the convections transporting heat, mois-ture and momentum in the transformed layer, while coarse resolution is used in the down-stream direction to represent the slow evolution of the large-scale flow. The average profiles of potential temperature, specific humidity and velocity indicate that the transformed layer forms a moist convectively mixed layer characterized by a multiple-layer structure. When the sea surface temperature rises in the downstream direction, a pressure drop in that direction is produced by the convective heating, and the acceleration due to the pressure gradient force is balanced mainly with the deceleration due to the frictional force induced by the convective momentum flux. Because the excess of the pressure gradient force over the frictional force produces a mean downward motion, the development of the mixed layer is suppressed. Because the suppression is brought about by the convective heating which develops the mixed layer, the process can be called a negative feedback process.

Heat Transport in the Atmospheric Boundary Layer

Abstract The structure of turbulence and transport of heat is examined from data obtained from 11 aircraft soundings executed in heated boundary layers during the Air Mass Transformation Experiment. Various influences on the turbulent transport are revealed by analyzing properties of the joint frequency distribution in polar coordinate. Such an analysis allows determination of a correlation coefficient and fluctuation amplitude as a function of properties of different types of motion comprising the total flow. In particular, the contribution of various events to the total flux is revealed without a priori specification of sampling criteria. In the present case, shear generation of turbulence and broken stratocumulus at the boundary layer top modify the profiles of virtual and sensible heat flux. The upward virtual heat flux shows the usual linear decrease with height but vanishes and becomes downward at the relatively low level of 60% of the boundary layer top. The vertical structure of the heat flux is e...

The effects of mesoscale convective cells on the surface wind field over the ocean

Near-surface wind velocities were measured on an array of anchored spar buoys in the East China Sea in February 1975 as a part of the Japanese AMTEX '75 (Air Mass Transformation Experiment), a subprogram of GARP. These data were used to determine the effects of atmospheric convection associated with mesoscale convective cells (MCC) on near-surface winds over the ocean. When MCC were present, a peak occurred in the near-surface wind spectrum in the so-called spectral gap.

An Air Mass Transformation Model for Short-Range Weather Forecasting

Abstract A model has been constructed for the purpose of forecasting at specific places the potential temperature, the specific humidity and the depth of the boundary layer. The model, an Air Mass Transformation model (AMT model), consists of a trajectory model and a one-dimensional boundary layer model. The horizontal and vertical advection of air masses is described by trajectories. The trajectories are computed from the analyzed geostrophic wind fields of a large-scale grid-point model. The lowest trajectory advects the air in the boundary layer. The air-mass transformations in the boundary layer are described by a simple boundary layer model. During unstable conditions, a slab model is used; during stable conditions, linear potential temperature profiles and humidity profiles are assumed. Along the lowest trajectory, the surface fluxes of heat and water vapor are computed from a simple parameterization scheme. The initial conditions for the state of the boundary layer are obtained from an analysis sch...

Numerical Study of an AMTEX'75 Oceanic Cyclone

Abstract The diabatic effects of latent heat release, boundary layer moisture and heat flux from the ocean surface and large-scale forcing due to upper-level systems are three physical processes affecting oceanic cyclogenesis. Detailed analyses of a major winter storm which occurred during the initial phase of the Air Mass Transformation Experiment in 1975 (AMTEX '75) indicated that the role of these three processes was vital to the Cyclone's development. To gain further insight into their influence, a control and three numerical experiments were performed using a multi-level moist primitive equation model with fine vertical resolution in the boundary layer. The simulation which included complete physics faithfully reproduced the major feature of the observed system. It was found that latent heating had a profound impact on the middle-level baroclinicity, the intensity and phase speed of the storm, and the vertical coupling within the simulated system. Without the surface moisture and heat source, the eff...

The Midwest Snow Storm of 8–11 December 1977

Abstract Conventional surface and upper-air aerological data are combined with radar and aircraft measurements to give a description of a major winter storm that deposited over 69 cm of new snow at Muskegon, Michigan, between 8 and 11 December 1977. It is shown that most of this snow occurred during four distinct episodes, three of which were related to air-water temperature contrasts as Arctic air flowed over the relatively warm surface of Lake Michigan. These four episodes have been identified as 1) pre-cold frontal, 2) post-cold frontal, 3) secondary trough, and 4) mesolow phases. During the pre-cold front phase heavy snow was associated with an advancing synoptic-scale trough. Little direct effect of the lakes was detected. Following passage of the cold front, strong northwesterly winds across Lake Michigan resulted in strong air-mass transformation with light steady snow along the downwind shoreline. The evidence suggests the presence of horizontal-roll convection as the dominant organizational mode ...