The “three-part ultra-simple” gravity wave parameterization of Warner and McIntyre is compared with the “Doppler-spread” parameterization of Hines. The two parameterizations are tested on a background state at rest with constant buoyancy frequency, as well as on background states defined by the CIRA86 data at 70N in July and January. To achieve as clean a comparison as possible between the two parameterizations, two approaches are taken. The first approach is to adjust the free parameters to obtain the same source level momentum fluxes, and as similar a source spectrum shape, as is possible. The second approach is to adjust the source level momentum fluxes to obtain the same momentum fluxes at mesospheric altitudes. The resulting vertical profiles of the momentum fluxes, of the wave-induced forces, and of the energy dissipation rates produced by the two parameterizations are compared. When a similar gravity wave source spectrum is used, specifically the source spectrum recommended by Hines, momentum deposition generally tends to occur lower in the atmosphere for the Warner and McIntyre parameterization than for the Hines Doppler-spread parameterization. In order to obtain similar wave-induced forces, and dissipation rates in the mesosphere from the two parameterizations, it has been found that the Warner and McIntyre parameterization requires the source spectrum to be scaled so that the net momentum flux in the lower stratosphere is an order of magnitude higher than the Hines Doppler-spread parameterization.
Simultaneous observations with a UHF-band boundary layer radar (hereafter referred as BLR), GPS rawinsondes and a tipping-bucket-type rain gauge were conducted at Kototabang (0.20°S, 100.32°E, 865 m MSL), which is located on the mountainous region near Bukittinggi, West Sumatera Province, during 27 September-7 October 1998 (rainy season). Low-level (1-3 km) westerly wind stronger than 10 m/s was observed, and precipitation tended to occur when the low-level westerly wind became weak (2-5 October). Similar relationship was observed for two months (1 September-31 October 1998) during which only BLR and surface meteorological instruments were operated at Kototabang. NCEP/NCAR objective analysis, and GMS TBB data showed that the low-level (850 hPa) wind field, and cloud distribution, were both completely different between the Indonesian Archipelago (east of Kototabang) and the eastern Indian Ocean—including the Bay of Bengal (west of Kototabang)—uring the analysis period. Two large-scale cloud disturbances existed along the equator in the western side (80°-100°E), but precipitation at Kototabang did not correspond to these cloud disturbances. The implication is that effects of the mountain range of Sumatera blocked the large-scale cloud disturbances over the Indian Ocean. The precipitation by local-scale cloud systems prevailed at Kototabang. The convergences of local circulations, which are generally dominant under weak background winds, are considered as the major cause of local-scale cloud systems.
The increase of stratospheric aerosol caused by the Mt. Pinatubo eruption, and its effects on atmospheric temperature and chemical constituents, are studied by using a chemical-radiative coupled 1-D model. The optical parameters of the aerosols and the surface area are calculated, assuming the size distribution of the aerosols and the composition. Two kinds of numerical experiments were made in this study. In the first experiment, a simple form of vertical distribution and the time variation of the aerosols are assumed based on lidar measurement data, and satellite data analyses. The simplification is necessary for an easy and clear understanding of chemistry-radiation coupled variations in temperature and ozone. In the second experiment, lidar data are used for calculation and input into the model as a function of altitude and time. These experiments show that the temperature increased in the range of 4.7-5.8 K and the ozone concentration decreased by 13-14% at the maximum perturbation phase in the center of the aerosol layer at 20 km. The maximum total ozone decrease was 18-20 DU (5.0-5.5%). The total ozone decrease, however, was 13-15 DU (3.5-4.0%) when the effect of the temperature variation on chemistry was ignored. Sedimentation of the aerosols within the first 1-2 years after the Pinatubo eruption explains the small difference in the results of these two experiments. The experiments also show that the ground surface temperature rapidly dropped by 0.1 K-0.35 K in the first 6 months, depending on the solar radiation absorption efficiency of the aerosols, followed by a recovery of more than a half in magnitude in the next 6 months. A cooling of less than 0.1 K then continued for the next 3-4 years, until the stratospheric ozone loss disappeared. The mechanisms of this surface temperature variation in the 1-D model are studied examining the results of the first experiment. The effects of a hydrolysis reaction of BrONO2 on ozone loss are also examined. The results suggest that the hydrolysis reaction more than doubled the ozone loss. All these results from the 1-D model show substantially larger positive temperature perturbations, and a somewhat longer duration of chemical perturbations than observed.
In this study, characteristics of barotropic-baroclinic conversions of kinetic energy C(Ks,Km) are examined for atmospheric blocking, using the NCEP/NCAR reanalysis data. The energetics analysis was based on the formula derived originally by Wiin-Nielsen (1962). As a result of case studies and composite analysis, we found that there were two maxima of C(Ks,Km) located along the western and eastern flanks of the blocking ridge, indicating that the baroclinic kinetic energy is converted to the barotropic kinetic energy in those regions. On the other hand, for a nonblocking case, where a ridge had amplified rapidly but not evolved into the blocking, the interactions C(Ks,Km) indicate only one maximum along the western flank of the ridge. Around the blocking region, the nondivergent part of C(Ks,Km) was a large contributor, which is associated with temperature advection. As a meridional flow is further amplified, the enhanced temperature advection associated with the meridional flow induces larger conversion of C(Ks,Km). Because a blocking flow is characterized by an amplified meridional flow, especially for the barotropic component, the barotropic-baroclinic interactions C(Ks,Km) appear to play an important role for the formation of a blocking.
The annual mean number of tropical cyclones in the Central North Pacific (CNP) is approximately three. Although this number is low in comparison to other basins, the interannual variability of tropical cyclones, which includes tropical storms and hurricanes, is high. For the period 1966-1997, the annual number ranges from 0 to 10 tropical cyclones. There is a large and positive correlation between CNP tropical cyclone counts and El Niño 3.4 region SST anomalies with a 95% significance level. In the El Niño Hurricane Season (ENHS), a greater number of cyclones formed in the CNP and more cyclones propagated into this area from the east. The monsoon trough, low level relative vorticity, and tropospheric vertical wind shear in the CNP undergo pronounced changes during warm and cold phases of ENSO. For instance, the 1000 hPa relative vorticity values within the CNP in an El Niño autumn composite are double the values in a corresponding La Niña composite. The El Niño autumn composite of tropospheric vertical wind shear shows a two to three times reduction equatorward of 16°N-17°N when compared to the La Niña autumn composite. The increased values of the dynamic potential term in Gray’s (1977) seasonal genesis parameter correspond well with the increased cyclone frequency in the CNP for an ENHS composite. Furthermore, a majority of initial detection points of named storms is found within a band of relatively large values of dynamic potential. This suggests that this term can be used to diagnose favorable areas for tropical cyclogenesis on a seasonal time scale.
The climatological pentad mean OLR data are partitioned into symmetric component OLR´, and asymmetric component OLR´´, with reference to the equator. Objective criteria are then introduced to define the intensity, center and areal extent of strong convections with OLR´ (OLR´´) of less than 220 (-20) Wm-2 over the equatorial (subtropical) domains, and seasonal migration of monsoons between the two hemispheres is investigated. Over the equatorial continents, such as Africa and South America, OLR´ exhibits a common feature of distinct semi-annual standing oscillations, while OLR´´ is of annual standing oscillation character that is peculiar to the monsoon system. Coexistent of two standing oscillations of different periodicity implies occurrence of a systematic cross-equatorial propagation of strong convections twice a year, northward during the spring transition, and southward in fall. The manner in which the monsoon transits from one hemisphere to another is distinctly different between the Indian Ocean and Western Pacific. This is associated with difference in the systematic equatorial basic flow which is, to a large extent, regulated by equatorial convections occurring over the maritime continent. Eastward (westward) of the maritime continent around 125°E, is a Kelvin (Rossby)-type basic flow regime; that is subject to significant annual variations. During late fall to early winter over the equatorial Western Pacific is a very pronounced Kelvin-type basic flow enhanced by northerly surges emanating out of the Siberian and North Pacific high pressure systems. The symmetric Kelvin flow acts as a bridge through which convections migrate southward from the WNPM (Western North Pacific Monsoon) to WSPM (Western South Pacific Monsoon) domain by crossing the equator in November and December. Gradual weakening of the Kelvin-type flow regime after winter inhibits northward return journey of convections from WSPM to WNPM in April and May. During spring and early summer over the eastern equatorial Indian Ocean is a prominent symmetric Rossby-type basic flow, which serves as a medium for convections to propagate northward, although not well organized, from SIOM (South Indian Ocean Monsoon) to SEAM (Southeast Asian Monsoon). The fall transition is of different character. The asymmetric convection center established in SEAM migrates southward, until reaching the equator by October. Instead of penetrating into the South Indian Ocean, the convective center suddenly changes its direction and propagates eastward along the equator, reaching near Borneo by the end of the calender year.
The relationship between the diurnal variation of convective activity and precipitable water was examined in summer seasons using water vapor radiometer, C-band radar and radiosonde data. Convective activity over the “semi-basin” exhibited a diurnal variation with dual peaks at 15-17 JST and 19-20 JST, and the SSI (Showalter Stability Index) decreased from morning to evening due to the increase of water vapor in the lower layers associated with a thermally induced local circulation. When precipitable water exhibited a pronounced diurnal variation, the evening convective activity maximum (19-20 JST) was evident and cumulonimbus clouds had a tendency to develop over a limited region south and east of Mt. Haruna. On average the convective activity maximum occurred about an hour before the precipitable water maximum, and when precipitable water reached a maximum at an early (late) time, cumulonimbus clouds also formed at an early (late) time. Furthermore, the mechanism responsible for the evening convective maximum over the “semi-basin” is discussed, based on the results of the diurnal variation of convective activity and precipitable water.
Low-concentration N2O regions in high latitudes of the lower stratosphere were observed by the Improved Limb Atmospheric Spectrometer (ILAS) for one and half months after the Arctic vortex breakdown in May 1997. A new Chemical Transport Model (CTM), referred to as the CCSR/NIES nudging CTM, has been developed, and used to simulate these low-N2O air masses. The simulation shows that one of the air masses had a horizontal scale of 1,000-1,500 km, and remained at high latitudes for 1.5-2.5 months after the polar vortex breakdown. The simulation also shows that the contrast between the N2O concentration of the low-N2O air mass and that of the ambient air was diminished when the air mass was stretched, which had a considerable influence on the duration of the air mass. From the duration, and the horizontal scale simulated by the CTM, an estimate was made of the horizontal eddy diffusivity after the polar vortex breakdown.
The surface energy flux balance and total evapotranspiration were estimated using the eddy correlation, and bandpass covariance technique over a tropical monsoon environment within the framework of GEWEX (Global Energy and Water cycle Experiment) Asian Monsoon Experiment (GAME). The aim of this present study is to obtain information on the seasonal variation of heat and water vapor exchanges between the atmosphere and terrestrial land cover (complex area) in tropical monsoon environment. The result indicated the daily integrated values of net radiation, sensible heat, latent heat and ground heat flux during the observation period from July 1998 to February 1999 were 10.76 MJ m-2, 2.32 MJ m-2, 5.18 MJ m-2 and 0.03 MJ m-2, respectively. Sensible and latent heat fluxes were the dominant energy partitioning components throughout the year. The seasonal difference in surface fluxes between wet and dry seasons was seen, and the latent heat flux was dominant in the monsoon, corresponding with the increase of specific humidity after frequent precipitation. Whereas the sensible heat flux increased as the surface temperature increased in the absence of rainfall during the dry season. However, the closure of energy balance remained unresolved as with the foregoing experimental studies. The estimated amount to evapotranspiration was 526 mm versus 641 mm of actual precipitation, and accounted for about 80% of the precipitation during this period.
In this study, a parameterization method based on satellite remote sensing and field observations is described and tested for deriving the regional land surface variables, vegetation variables and land surface heat flux densities over heterogeneous landscape. As a case study, the method is applied to the HEIFE area in northwestern China. The regional distribution maps of the NDVI, the MSAVI, vegetation coverage, leaf area index (LAI), surface reflectance, surface temperature, net radiation density, soil heat flux, sensible heat flux and latent heat flux have been determined over the HEIFE area. The derived results have been validated by using the “ground truth”. Comparisons between the results derived from the method proposed in this study, and the previous results (in which the land surface variables and surface heat fluxes were directly derived from the Surface Energy Balance Algorithm for Land—SEBAL) have also been given in this paper. The results show that the more reasonable regional distributions of land surface variables (surface reflectance, surface temperature), vegetation variables (the NDVI, the MSAVI, vegetation coverage and the LAI), net radiation, soil heat flux, sensible heat flux and latent heat flux can be obtained by using the method proposed in this study. Further improvement of this method is also discussed.
Due to the fact that conventional data is insufficient over the ocean, satellite observations are the best tool for analyzing mesoscale convective systems (MCSs), which frequently grow and develop over the ocean. In this paper, an objective potential index (OPI) is developed from satellite data. Since the OPI includes two kinds of information—the magnitude and the evolution of the air-sea interaction over the ocean, it is suitable for surveying areas where MCSs may develop. Various air-sea parameters were derived from SSM/I microwave and GMS-5 infrared channels covering an area from 15°N to 30°N, and 110°E to 125°E in May and June 1997 to 1999. These parameters were then combined to produce the OPI. The results of this paper show that an OPI value larger (smaller) than 0.5 implies a higher probability that the GMS-5 IR1 brightness temperature will be smaller (larger) than 235 K. It may serve as a good reference in pointing out areas where convective clouds may occur and develop into MCSs.
A weak-wind region, whose horizontal scale is comparable to that of an urban area, is occasionally formed ahead of the sea-breeze front, over a large coastal urban area (Yoshikado and Kondo 1989; Yoshikado 1990; Ohashi and Kida 2001). In this study, the characteristics and the formation mechanism of the weak-wind region were investigated, using a 2D mesoscale atmospheric model. The following results were obtained: The weak-wind region is created by the development of the heat-island circulation over the urban area; the heat-island circulation (i.e., the urbanward pressure-gradient-force) weakens the inlandward ambient wind during the morning hours. At that time, the sea-breeze circulation is inessential in creating this weak-wind region. The weak-wind region cannot be created under the influence of the seaward ambient wind. Subsequently, the weak-wind region is persistently formed ahead of the sea-breeze front, and gradually moves inlandward. A long-term balance between the urbanward pressure-gradient-force, and the turbulent mixing, causes such a phenomenon. Thus, interactions among the sea-breeze circulation, heat-island circulation, and inlandward ambient wind play an important role in the above processes. The spatial scale of the weak-wind region strongly depends on the urban size (width), i.e., the spatial scale of the heat-island circulation developing over the urban area, during the daytime hours.