Several earlier studies showed that the southwest Pacific near New Zealand is a region of blocking in the Southern Hemisphere. In this paper a second region of frequent blocking over the southeast Pacific is confirmed using 25 years of daily data. A comparison between the characteristics of blockings over the southwest and southeast Pacific revealed several interesting differences. Although the highest frequency of blocks occurs in the austral winter in both the regions, the frequency of blocks is greater over the southwest Pacific. However, these blocks are short lived and no block is found that lasts beyond 11 days duration. While over the southeast Pacific blocks are longer lasting with several blocks lasting for more than 11 days duration. Local energetics of two blocks, one with a 21 day duration over the southeast Pacific and another with a 10 day duration over the southwest Pacific are analysed using Mak’s formulation. The contributions from various temporal scale interactions (seasonal, intraseasonal and high frequency components) to the episodal average local energetics of these two blocks are evaluated. It is found that blocking disturbances in both the cases extract barotropically, kinetic energy from the seasonal diffluent jet at almost the same rate. But the redistribution of kinetic energy within the block region is wider for the southeast Pacific block than for the southwest Pacific blocks this seems to maintain the longer duration of the southeast Pacific block. The synoptic eddy straining mechanism proposed by Shutts in manifested in three energetic terms, of which one (V→1.A→2,2) is found to be particularly large for both blocks. The pressure work process and the baroclinic conversion are also important, for both the blocks and the magnitudes are higher for the southeast Pacific block.
An approximated system of equations describing mesoscale (horizontal length scale 105 m) phenomena is constructed. The wind speed of the mesoscale phenomena considered is assumed to be quasi-symmetric along the horizontal stream lines. The quasi-symmetry means that the variation of wind speed in the tangential direction of horizontal stream lines is one order smaller than that of the mesoscale. As a result of the quasi-symmetry, the primary flow is in a generalized gradient balance with geopotential φ. The primary flow (i.e., the generalized gradient flow) is parallel to the iso-φ lines, and determined by the derivatives of φ up to the second. The secondary flow, which constitutes the secondary circulation in the vertical plane perpendicular to the primary flow, is algebraically determined by the derivatives of φ up to the third. The constructed system of equations, which include the primary and secondary flows, exactly conserves the energy, while the potential vorticity is only approximately conserved. The potential vorticity is exactly conserved, when or where the local directional angle of iso-φ lines is vertically and temporally uniform. When or where the flow is large-scale, then or there the present system is reduced to the quasi-geostrophic system. Furthermore, for quasi-circular flows, the present equations are reduced to the balanced vortex equations.
Using an ocean general circulation model (OGCM) forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the NCEP/NCAR reanalysis of 1973-1995, we examined interannual atmosphere-ocean variations in the tropical western North Pacific related to the Asian summer monsoon-ENSO coupling. The OGCM simulation was successful in reproducing the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines that is linked with anomalous tropical convection in that vicinity. A heat budget analysis shows that a longitudinal asymmetry of surface latent heat flux anomalies is crucially responsible for the reinforcement and persistence of the east-west gradient of SST anomalies between the two regions. It is also found that the transition from premonsoon regime to monsoon regime in the tropical Indian Ocean affects the interannual atmosphere-ocean variations in the tropical western North Pacific during the period from the late 1970s to the early 1990s. In the spring before strong Asian summer monsoon, an equatorially asymmetric air-sea coupled mode tends to appear in the tropical Indian Ocean. Simultaneously with the beginning of the strong monsoon regime, the northern Indian Ocean and South China Sea are covered by cool SST anomalies due to enhanced wind speed and evaporation, whereas warm anomalies relevant to a cold episode of ENSO are still maintained in the warm pool region east of the Philippines. The latitudinally asymmetric anomalies of tropical convection and SST become dissipated and convection over the warm pool region of the western North Pacific becomes localized and enhanced with the establishment of east-west gradient of SST anomalies. Due to enhanced convective heating, equatorially asymmetric atmospheric Rossby waves are excited to the west of anomalous convection, which induce low-level westerly anomalies. This dynamic process further facilitates the localization of intense convection through the change in surface latent heat flux and SST. While such a positive atmosphere-ocean feedback system persists in boreal summer, the Pacific and Japan (PJ) teleconnection pattern in response to enhanced convection prevails and brings about unusually hot summers in recent years especially in the vicinity of Japan.
On 4 August 1998, a torrential rainfall was observed in the Niigata area, in central Japan. The rainfall was induced by a quasi-stationary rainband aligned in the east-southeast direction from the west of Sado Island. The rainband lasted for several hours with the characteristic features of a back-building formation, in which new convective cells were successively generated in an upstream direction. The rainfall area and intensity simulated by a nonhydrostatic mesoscale model with a 5-km horizontal grid agreed well with the observation. The rainband with a back-building nature was reproduced successfully by a nonhydrostatic model with a 2-km grid. The simulation results showed that the heavy rainfall formed as a result of strong convective instability enhanced by low-level humid and middle-level dry inflows from the southwest into the weak convergence region associated with the Baiu frontal zone. Sensitivity experiments indicated that neither the terrain of Sado Island, nor the evaporative cooling of raindrops affected the formation of the rainband. Although moved to the downstream when the low-level wind convergence was not enhanced enough, the simulated rainband stagnated after the convergence became so strong that the whole low-level air could be lifted upward while crossing the convergence zone. Therefore, the quasi-stationary rainband was maintained by the convection-enhanced low-level wind convergence existing upstream of the rainband. Over the convergence zone associated with the quasi-stationary rainband, new convective cells successively formed and separated from the zone after having developed higher. This successive formation of new cells could maintain a back-building formation. Such a back-building formation was found at several points over the wind convergence zone. These meso-β-scale convective systems with a back-building nature produced the precipitation region with a length over 200 km.
A multilayer energy budget model within and above a rice canopy is developed to show physical and physiological effects of the vegetation on the energy exchange processes and to evaluate the characteristics of the energy budget. Micrometeorogical observations within and above a rice canopy are analyzed to optimize model parameters and to test the model. Model parameters of the inclination factors of a leaf Fl(z) and a panicle Fp for radiation, the effective drag coefficient cd, the transfer coefficient for sensible heat ch and stomatal resistance rs(z) are optimized by fitting the modeled vertical profiles of downward shortwave radiation S↓(z), mean horizontal wind speed, air temperature, plant surface temperature and water vapor pressure to the observations in the morning, near noon and in the afternoon on 4 September 1998, when the growing stage of rice was early ripening. The optimized model parameters demonstrate that the radiation exchange processes depend on the solar altitude and the vertical distribution of the plant area density and its components, i.e. the morphology of the canopy, and that panicles are as important as leaves for the energy exchange processes in the paddy field, while culms are unimportant. The optimized vertical profiles of the stomatal resistance rs(z) show inverse correlations with Fl(z)S↓(z), and positive correlations with vapor pressure deficit VPD(z). Thus, the model demonstrates that the stomatal resistance is also influenced by the morphology of the canopy, and interacts with VPD adjacent to the leaves. According to the calculated vertical profiles of sensible and latent heat fluxes, the negative sensible heat flux in the afternoon originates mainly from lower canopy layers, which occurs even under the condition of higher rs(z).
A regional climate model, NIES/CCSR RAMS, was used with the ECMWF objective analysis data as boundary conditions to reproduce the precipitation pattern over East Asia in June, including that accompanying the Baiu front. In the control experiment, the simulated precipitation pattern was unrealistic, mainly in two respects: precipitation over the Baiu front was too weak, and strong precipitation was found over the North Pacific subtropical high at around 20°N. According to the ECMWF data, strong potential instability is maintained over the subtropical high, under which condition, the cumulus parameterization used in the model predicts strong precipitation to stabilize the atmosphere. Because the lower free atmosphere is very dry over the subtropical high, it is conjectured that the development of deep cumulus convection is suppressed by this dry air in the real atmosphere. When the cumulus parameterization was modified to include this assumption, the intensity of precipitation over the Baiu front as well as the unrealistic precipitation pattern over the subtropical high was clearly ameliorated. The intensification of precipitation over the Baiu front is considered to be due to the increased water vapor transport to the Baiu front, and the decreased stability with respect to the moist convection around the Baiu front. In semi-prognostic experiments with CCSR/NIES AGCM, this modification was found to be also effective in many other parts of the globe. Because problems in precipitation distribution similar to that discussed in the present study are found in many other climate models, the modification of cumulus parameterization suggested here could also be effective in those models.
An analysis was performed of the observed sea breeze circulation penetrating from Osaka Bay to the inland Kyoto basin. A weak wind region (wind speeds of 2 m s-1 or less) of greater than 1000 m in depth, was found just ahead of the inland moving sea breeze front. The formation of the weak wind region was discussed from the view point of the interaction between the sea breeze and heat island circulations. Similar weak wind regions have also been reported to exist in association with the sea breezes observed over the northern periphery of the Tokyo urban area.