Large-eddy simulation of convective boundary layer is performed without and with density stratification using a fully compressible nonhydrostatic model. Effect of density stratification in the boundary layer, where Bousinessq model has normally been applied for the analysis, is investigated by comparing the two cases. This investigation also reveals the difference between Bussinesq model and the fully compressible model, which is hardly used in boundary layer analysis. Although the potential temperature with identical neutral distribution is used for the both cases, the boundary layer height of the stratified case is limited to less than half height of the non-stratified case. The cause is considered to be derived from suppression force to the buoyancy. The suppression force is analyzed in terms of the forces working vertically. It is found that vertical pressure gradient shows symmetrical distribution to that of buoyancy force, and the fact indicates that hydrostatic relation between density and pressure is quickly recovered due to the Brunt-Väisälä oscillation. The effect is also explained by the positive Brunt-Väisälä frequency derived from vertical density gradient in the stratified case. As the results, convective boundary layer with density stratification becomes more stable than that expected from the neutral potential temperature profile. The results of the present study indicate that small density stratification cannot be neglected even for simulating convective boundary layer, and Boussinesq approximation which neglects density gradient may cause significant errors. The present results also suggest a problem that a planetary boundary layer scheme based on Boussinesq model will be inconsistent with a mesoscale model which mainly employs fully compressible nonhydrostatic equations.
A surface anticyclone develops in April over the Yellow-East China Seas. The Yellow-East China Sea anticyclone (YESA) is confined in the marine atmospheric boundary layer but highly influential during the onset of sea fog season along the Chinese coast. This paper investigates the mechanism for YESA formation using atmospheric reanalysis, satellite observations and model experiments. Our analysis indicates that YESA is composed of three parts: (1) the westerlies to the north are the surface extension of the westerly wind jet; (2) the southerlies on the Chinese coast are due to the thermal wind between the warm continent and cool Yellow-East China Seas; (3) the northeasterlies to the south are due to the thermal wind between the cool East China Sea and warm Kuroshio Current. The regional atmospheric model successfully simulates the YESA under realistic boundary conditions. In an experiment where the Bohai-Yellow Seas are replaced with flat land, the surface anticyclone is pushed south and forms offshore along the new land-sea boundary, consistent with the thermal high mechanism over the cool sea surface temperature.
In Taiwan, comparing with the major hazardous Mei-Yu fronts and typhoons in summer, a synoptic condition with the relatively weak high and low pressure systems at certain locations in the autumn season may bring torrential rainfall to northern Taiwan. A detailed dual-polarimetric/Doppler radar analysis was carried out for one case under this condition to reveal the mesoscale precipitation mechanisms and microphysical characteristics over terrain. The high pressure system moving eastward off China and the low pressure system over the ocean in the southeast of of south-eastern Taiwan formed a convergent zone at the low levels, resulting in a sequence of convective activities. These convective cells moved westward, and became more organized and intense in the Mt. Datun area and the estuary of Tamsui River near the lee side of Mt. Datun. Moderate intensity convective cells were embedded in the wide, long-lasting stratiform regions. In the mountain area with intense precipitation, terrain-induced upward motion of the cells enhanced condensation, significantly increased drop counts, and acted as a feeder. The older cells in this convective system continued to provide lighter hydrometeors in the upper layer and formed a widespread stratiform region as a seeder. The wider spectrum of drop size distribution set the stage for collision and coalescence process, resulted in the larger drops formed at the low level of the mountain area. Along with the increasing concentration of raindrops, the total effect finally caused heavy rainfall over the mountain area.
Features of the Meiyu frontal rain zone (MFZ, in 110–125°E) and Baiu frontal rain zone (BFZ, in 125–140°E) in the 20th Century simulation (20C3M) and 21st Century projection (Special Report on Emission Scenarios (SRES) A1B), obtained by 22 climate models contributed to the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) are studied. Characteristics of the MFZ and BFZ in the 20th century climate simulation are compared with two sets of observed precipitation data. The latitude of the zone and the precipitation in the zone (“precipitation in FZ”) reproduced by the models are examined, using the 20-year (1980–1999) averaged values for May, June and July. The “precipitation in FZ” in each month obtained from the multi-model ensemble average (MEA) coincides approximately with the observation data. However, MEA reproduces MFZ and BFZ to the north of their observed latitude in May. MEA reasonably reproduces the latitude BFZ for June and July, and the latitude of MFZ only for June. The standard deviation (STD) of the “precipitation in FZ” does not change widely from zone to zone, and from month to month. However, STD of the latitude of zones varies widely. The STD of latitude of MFZ and BFZ became larger in July. The STD of the latitude of MFZ is larger than that of BFZ. The low and medium horizontal resolution models with moist convective adjustment scheme tend to reproduce MFZ and BFZ in the northern latitude, while the models with mass flux scheme and models with Arakawa-Schubert scheme tend to reproduce MFZ and BFZ in the southern latitude. Features of the MFZ and BFZ in the 21st century climate projection by the models are examined using the 20-year (2080–2099) averaged values for May, June and July. The characteristics of the models in regard to MFZ and BFZ in 20th century simulation are commonly found in the 21st century projection. The MFZ and BFZ shift slightly northward, and the “precipitation in FZ” decreased slightly from the 20th century to 21st century. However, these changes are very small, as compared with the respective STD. The change of MFZ and BFZ under the climate changes in the 21st century can not be definitely concluded by models of CMIP3.
The initial value problem of diabatic Rossby waves is analytically solved in a vertical-zonal two-dimensional quasi-geostrophic system on an f-plane. The given basic state is in thermal-wind-balance, and dry baroclinic instability is excluded. The diabatic heating (or cooling) is proportional to the vertical velocity on the middle level and generates potential vorticity (PV) disturbances on the lower and upper levels of the atmosphere. The PV disturbance propagates eastward (westward) on the lower (upper) level relative to the fluid, while the prescribed basic zonal flow advects the PV disturbance toward the opposite direction. For a zonal wavelength shorter than a marginal one, which becomes shorter as the heating becomes stronger, the advection effect is dominant, and consequently, the lower and upper PV disturbances move downstream and away from each other without effective interaction or growth. On the other hand, for a zonal wavelength longer than a marginal one, the propagation effect is dominant, and consequently, the lower and upper PV disturbances tend to move upstream and away from each other. However, this leads to strong mutual interactions between the lower and upper PV disturbances. As a result, the PV disturbances grow exponentially and attain a phase-locked upshear-tilted vertical structure. The behaviour of PV disturbances can be qualitatively explained on the basis of PV thinking.