An amplitude vacillation observed in a laboratory experiment is simulated using a three-dimensional global grid numerical model. The time variations of energy components and internal structures revealed by the numerical solutions show that the amplitude vacillation consists of the stages of the buildup and breakdown of the zonal available potential energy accompanied by the decay and growth of baroclinic waves, respectively. The spectral analysis indicates that the mechanism of this amplitude vacillation is the non-linear interaction between a single dominant wave and the zonal flow. This is in a great contrast to another explanation by the interference of two waves with the same azimuthal wavenumber and different phase speeds (e. g., Buzyna et al., 1989).
The effect of conditional heating on unstable Kelvin waves generated by CISK and evaporation-wind feedback was investigated with multi-level models on the equatorial beta-plane. In agreement with previous analytic results, the scale-selection catastrophe of linear heating is modified but not eliminated by conditional heating. There is only one region of upward motion, but it contracts to the smallest zonal scale allowed by diffusion or model resolution. The pathology is most severe in the CISK-unstable case. With evaporation-wind feedback, Kelvin-wave instability is possible apart from CISK; growth rate in this case increases slowly with decreasing diffusion, but contraction is still observed. We note incidentally that eastward moving evaporation-wind modes can be generated on the beta-plane with no mean wind, contrary to previous assumptions requiring an easterly background flow. While evaporation-wind feedback is an improvement over CISK, as far as scale selection is concerned, zonal phase speeds remain too large compared to the observed propagation of tropical intraseasonal oscillations and superclusters, unless (1) the parameter q controlling the strength of moisture convergence feedback lies close to the CISK-neutral point, (2) the parameter A controlling the strength of evaporation-wind feedback is small, and (3) the heating profile is assumed proportional to vertical velocity. Small positive phase speeds are obtained under these conditions, for example, in a two-level model.
Wave-CISK (conditional instability of second kind) with conditional heating on a rotating sphere is investigated using a global spectral model. With certain initial conditions, an eastward-propagating wavenumber-one mode with a single ascending region is obtained. With more general initial conditions, however, more than one convection events, of nearly equal strength and separated at nearly equal intervals, coxist and interact very little with each other. This happens when the radius for a convection event to affect other such events, which is inversely proportional to the growth rate, is much smaller than the circumference of the earth. In additional to the eastward propagating modes, a westward-propagating mode is also found, whose flow field is dominated by gravity wave response to the cumulus heating. The above results indicate that the wave-CISK with conditional heating does not always select the eastward-propagating wavenumber-one mode. An alternative mechanism that leads to the selectivity of the wavenumber-one mode is suggested.
A retrieval method applicable to precipitating storms, which are transient or include complicated physical processes is presented. An imaginary situation is assumed that environmental fields of temperature and moisture and time sequence of the wind field are known by the observations of upper soundings and dual- (or more) Doppler radars. In this method, equations of thermodynamic and microphysical (scalar) variables are time-integrated by imposing these data. First, the performance of this method is examined by application to the results of numerical simulations of a quasi-periodic multi-cellular storm. It is shown that a time sequence of the wind field is necessary, which is sampled with a time step resolving transient features of the storm, to retrieve scalar fields well. This method is further applied to an aperiodic multi-cellular storm and a quasi-periodic multi-cellular storm having a complicated ice-phase process. Retrieved results attained in both cases are satisfactory.
The development of mixed layers and the formation of convective snow clouds over the Sea of Japan were simulated by using a triply-nested two-dimensional dynamic cloud model with a recently developed microphysical parameterization. Two case studies were made using this model. One is the convective snow storm associated with a typical cold airmass outbreak during the period of Feb. 2-4, 1989, that occurred during the period of an intensive field experiment. A detailed microphysical comparison with the observational results was possible for this case. The other is the snow storm which accompanied an extremely cold airmass outbreak during the period of January 24-26, 1990. Through a comparison between these two cases, the effect of temperature contrast on the mixed-layer development and snow cloud formation was investigated. The model simulated mixed-layer development through heat and moisture supply (total heat fluxes of 439W/m2 and 895W/m2 for the 1989 and 1990 cases) from the warm sea surface and subsequent convective transport. The simulations showed good agreement with observational results in terms of cloud fetch distance, cloud top and base heights and air temperature increase in the mixed layer. From the viewpoint of cloud dynamics and microphysics, the model simulated observed updraft velocity, cloud water content and snow water content well. The model also simulated the drizzle formation in snow clouds and high concentrations of ice crystals in supercooled cloud water regions. A major defect of the simulations was the underestimation of the number concentrations of ice crystals by a factor of 6. In general, clouds form 50-150km leeward of the continental coast (depending on the air-sea temperature contrast) and gradually develop in height and convective activity. Over Japan, snow clouds strengthen around 30km off the coast and then gradually weaken because of negligible heat and moisture supply over the land. In clouds, ice crystals first appear through the freezing of cloud droplets and grow through vapor deposition and accretion of cloud droplets. The dominant precipitation type changes from graupel (over the sea and the coastal area) to snow (over the mountain area). Over mountain slopes, a marked seeder-feeder mechanism operates between upper decaying snow clouds and lower clouds that form due to terrain-induced updrafts. Similar, but less intense microphysical interaction (‘natural seeding’) occurs between snow clouds at different stages over the ocean. With colder air outbreaks, convective activities are stronger and mixed layers deeper. Higher concentrations of ice crystals are produced through deposition/sorption nucleation and the ratio of graupel/snow decreases because of competitive consumption of cloud water among the crystals.
Non-linear evolution of a barotropically unstable circumpolar vortex, which is an idealization of the polar night jet in the stratosphere, is investigated by numerical time-integrations of a full non-linear vorticity equation over wide ranges of several external parameters of the jet. Some non-linear features which cannot be expected in the linear stability analysis are obtained in the time-integrations. Two types of stabilization processes are found in diagnosis of the non-linear evolution with daily maps of the absolute vorticity field: The negative gradient of absolute vorticity on the equator-ward side of the jet disappears owing to the vorticity mixing by the growth of thin vortex filaments in middle latitudes, while the negative gradient on the poleward side of the jet disappears owing to the equator-ward displacement of the polar fluid with low absolute vorticity. Decrease of dominant wavenumber is observed in the non-linear phase of the evolution for wide parameter ranges. An eastward-propagating wave of zonal wavenumber 2 becomes dominant after the stabilization of the circumpolar vortex that is unstable in middle latitudes, even if the exponential growth rate of this wave is not the largest in the linear stability analysis.
Observed winds in coastal zones are dominated mainly by two wind components: the wind component induced by land/sea breeze (LSC: Land/Sea breeze Component) and the wind component associated with the synoptic pressure gradient (SWC: Synoptic Wind Component). This study attempts to investigate the relationships between the Surface Geostrophic Wind (SGW), and the actual surface wind in the coastal zones of Japan, separating the surface wind into two components: the SWC and the LSC. It is possible to separate the SWC from observed wind data through vector accumulation over numerous stations (226 AMeDAS stations) because the Japan Islands are surrounded by seas. Also, extraction of the LSC can be achieved by assuming the main land/sea breeze directions at every station. The calculated mean ratios of the SWC speed to the SGW speed at 1500 JST and 0600 JST are 0.22 and 0.15, respectively. The mean angle of deflection between the SWC and the SGW is 51 degrees measured counterclockwise from the SGW direction to the SWC direction, and the difference between the angles in the daytime and nighttime is very small, while the angle greatly changes with SGW speed. When the SGW speed is small, the deflection angle shows a large value, around 68 degrees. As the SGW speed increases, the deflection angle decreases. The LSC speed shows 1.6ms-1 for sea breeze at 1400 JST, and 0.8ms-1 for land breeze at 0600 JST when the SGW speed is near 0ms-1. However, as expected, these wind components decrease as the SGW speed increases. Eventually, the LSC vanishes when the SGW is over 14ms-1.
A rotor circulation was directly observed near the Baiu front in the lower troposphere by three-dimensional Döppler measurements with the MU radar at Shigaraki, Japan (35°N, 136°E). The temporal and vertical scale of the rotor were ∼50min and ∼2km, and the stratification observed by radiosondes was statically stable. The synoptic meteorological analysis suggests that the rotor existed just below and between Baiu-frontal banded precipitation clouds which were organized in a meso-α-scale cyclone. Precipitation echoes observed simultaneously by C/Ku-band radars were quite weak in the downdraft in front of the rotor, and became significant and tall up to ∼9km altitude at the back of the rotor circulation. The rotor was identified with a meso-β-scale depression observed by the routine meteorological network, which had a horizontal scale of ∼40km in the zonal direction and 150-200km in the meridional direction, and moved from west to east at ∼50km/h. Based on brief discussions, we conclude that the rotor circulation was locally developed from an orographic disturbance by shear instability which was occasionally induced in a weak statically-stable layer maintained by (conditional) symmetric instability.
The characteristics of the inter-decadal variations of Baiu-precipitation after 1900, and their relationships to the large-scale characteristics were analyzed with the use of the data of precipitation amounts, surface temperature and sea-level pressure. The Baiu precipitation averaged over Japan tended to show small amounts from 1924 to 1944 (called “Period I”). But it increased around 1950 and showed relatively large amounts from 1952 to 1972 (called “Period II”). This inter-decadal variation had large amplitudes in the south-western part of Japan. The following meteorological components showed significant changes between the two periods; 1) the meridional temperature gradient on the western side of Japan (increased), 2) the sea-level pressure between 30°N and 40°N in Japan (decreased) and 3) the precipitation amount at Manila, Philippines (decreased). No significant changes were found in the meridional pressure gradient on the north-eastern side of Japan and in the first EOF component of the surface temperature in Japan, although they were correlated with year-to-year variations of Baiu precipitation. The results of the analysis suggest that the increment of Baiu precipitation from Period I to Period II was associated with long-term variations of sub-tropical circulation. An hypothetical process is proposed for the increment of Baiu precipitation around 1950.
Two different cumulus parameterization schemes, one developed by Kuo and the other by Betts-Miller, are used to simulate the orographic-convective rainfall associated with the Western Ghats for two days during which monsoon rainfall was moderate to heavy. A ten-layer primitive equation limited area nested grid model is used to perform numerical simulations. It is found that predicted rainfall near the Western Ghats with the Kuo scheme agrees well with the observations. With the Betts-Miller scheme, model failed to predict rainfall over this region. To find out uncertainties in the adjustment parameters used in the Betts-Miller scheme, five sensitivity experiments are performed. Different values are assigned to the two adjustment parameters, namely the relaxation time scale and the saturation pressure departure, in each of the sensitivity experiments. Results from these sensitivity studies indicate that specification of relaxation time scale depends on the model horizontal resolution. Relaxation time scale needs to be smaller as the model horizontal resolution increases. Also, rainfall predictions are less sensitive to different values of relaxation time scales than those for the saturation pressure departure. Variations in the prescribed thermodynamic reference profiles caused by small prescribed changes in the values of saturation pressure departure led to improvements in the rainfall predictions. It was also found that there exists a lower limit on the values of relaxation time scales and saturation pressure departures for the monsoon region beyond which predicted rainfall rates do not show further improvement.
The interactions of the sea breeze with urban heat islands (UHI) are examined with use of a two-dimensional numerical model. The strength of the interaction depends on the intensity of the sea breeze, the intensity of the heat island circulation that develops over the inland side of the urban area, and the penetrating speed of the sea breeze. These factors are basically controlled by three characteristics of the urban area-the size or width, the distance from the sea, and the intensity of the UHI