In this investigation, we study the monsoon variability using an ensemble of long integration (34 years) of the JMA model at T42 resolution. It was noted that a major part of the interannual variability of the Indian monsoon is by internal dynamics. Leading modes of atmospheric variability are examined using a multivariate EOF analysis. EOF-1 (obtained from a model-run with observed SST) represents the model’s response to SST variations associated with ENSO, and it has a dominant periodicity of around 4-5 years. ENSO related variability is mostly confined to the equatorial region. Over the Indian land region, the loadings of this EOF are not large. Loading associated with EOF-2 are large over the Indian region. Large-scale SST variation in the equatorial region is not seen in the spatial pattern of this mode. The rainfall pattern is characterised by positive anomalies over the Indian region, and over the central Pacific north of the equator. Rainfall anomalies of opposite sign are noticed in the warm pool regions of the western Pacific and the south-central Pacific Ocean. This EOF has a dominant periodicity on the quasi-biennial time scale (with spectral peaks at about 30 months and another at about 15 months). Spatial pattern of the EOF-1 from a climatological SST run resembles that of the EOF-2 obtained from the observed SST run and it is a natural mode of the atmospheric system. Similar analyses have been made using monthly anomalies of June to September (monsoon months). Interannual variability of rainfall has been examined using filtered rainfall anomalies during the monsoon season retaining the variability on the time scales of 15-30 months, and 36-84 months. Large rainfall variability in the climatological SST run occurs on the time scales of the QBO and ENSO as in the observed SST run. Over the Indian region, magnitudes of interannual variability in these two time scales obtained from the climatological SST run is comparable to those obtained from the observed SST run.
A preliminary study is given on the bulk transfer coefficients and surface fluxes over the Tibetan Plateau, on the basis of data from four sets of AWS in Tibet from July 1993 to March 1999 according to P.R. China-Japan Cooperation Research Project on Asian Monsoon Mechanism. The surface roughness lengths are decided by the logarithmic wind law under neutral conditions using the least square method, the value at each station is scattered and seasonal mean is obtained. The bulk transfer coefficients for momentum and heat are estimated using the flux-profile relationships, means of the bulk transfer coefficients for momentum and heat are 4.44×10-3∼4.75×10-3 and 6.02×10-3∼6.40×10-3, respectively. The surface fluxes of momentum, sensible heat and latent heat are computed by bulk transfer formulations, sum of sensible and latent heat at each station is about 100 W/m2. In addition, diurnal and seasonal variations of the bulk transfer coefficients and surface fluxes as well as the relations among bulk transfer coefficients, bulk Richardson number and surface wind speed are discussed in detail using a composite method. A possible relationship between surface fluxes and monsoon over the Tibetan Plateau is suggested, i.e., the surface heat transfer on the Eastern Tibetan Plateau is dominated by the sensible heat flux and latent heat flux jointly in the pre-monsoon and latent heat flux mainly during the monsoon.
A new version of a tropical cyclone model is presented as an extension of Yamasaki’s (1986) model that intended to resolve mesoscale organized convection by a grid and treat the cumulus-scale motion as the subgrid-scale. Two important imporvements are made in this study. One is a prognostic treatment of cumulus-scale cloud water, which was treated diagnostically in the old version. The other is that the ratio of the cumulus-scale ascending area is not assumed to be sufficiently small compared to unity. Results from numerical experiments indicate that the features of tropical cyclones (including rainwater distribution) simulated by the new and old versions are essentially similar, but the cloud water distribution is simulated more realistically by the new version, as evident from the model formulation. Clarification of the impact of the finite area ratio and an improvement of its assumption remain to be studied.
Radio sounding and in-situ observations at land-based (26 January-11 February 1998) and ship stations (4-11 February 1998) were performed over the southwestern Sea of Okhotsk covered with sea ice to estimate turbulent heat flux and investigate characteristics of thermodynamic air mass transformation processes during cold-air outbreaks. Surface heat flux estimations were carried out using two indirect methods; an atmospheric heat budget analysis using the radio sounding data at three stations and a bulk method utilizing meteorological and sea ice thickness data obtained by the ship observations. The estimated total upward turbulent heat flux over the analyzed area was approximately 100 Wm-2 even within the intense cold-air outbreak period. The height of the mixing layer associated with this small amount of the heat flux is only about 1 km, which implies that sea ice acts as an insulating material between the ocean and atmosphere and thus significantly reduces the turbulent heat fluxes. Nevertheless, the upward turbulent heat flux from the ice surface was found to be non-negligible during cold-air outbreaks and it is suggested that the sea ice grows through the sea or ice surface cooling.
The formation mechanism of the Baiu front, which appears in early summer and gives a typical rain band around Japan, was reporduced using a regional atmospheric model. The initial and boundary conditions of the model were derived from the European Center for Medium-Range Weather Forecasts (ECMWF) data. In the “realistic simulation”, temporary variable boundary conditions derived from twice-daily ECMWF data were utilized. In the “zonal mean simulation”, zonally uniform and temporally constant atmospheric fields obtained from ten-day averaged global zonal mean ECMWF data was utilized as the initial and boundary conditions. The simulated Baiu front in the zonal mean simulations, as well as that in the realistic simulations, has similar structures to the real Baiu front, i.e., the Low-Level Jet (LLJ) runs parallel to both the precipitation zone and the upper-level jet in the eastern part. Basically, the simulated Baiu front is formed by the deformation of the zonal mean field due to the Land/Sea contrast and topography. Although the distribution of the simulated rainfall over the Baiu front depends on the cumulus convective parameterizations, the fundamental structure of the Baiu front does not depend on them. A comparison between the zonal mean simulations of early and late June indicates that the Baiu front is formed at a higher latitudes in late June, when the upper-level jet is weak and located northward. The location of the Baiu front is quite sensitive to the zonal mean field. The Baiu front accompanied by the LLJ is also represented by numerical experiments without topography, which suggests that the Baiu front could be reproduced by two factors alone, the zonal mean field and the Land/Sea contrast. The orography, including the Tibetan plateau, intensifies the LLJ and the precipitation over the Baiu front. The LLJ also appears in the zonal mean simulation without a condensation process. However, the LLJ is formed along the eastern coast of the Eurasian continent and locates in the northern side of the upper-level jet eastward of Japan, which is a different feature from the zonal mean simulations with a condensation process. Accordingly, it is speculated that the condensation process acting on the atmospheric field is necessary to keep the LLJ in the southern side of the upper-level jet as in the real Baiu front.
Rawinsonde observation was performed in late January through early February in 1998 around the southwestern part of the Sea of Okhotsk to estimate the turbulent fluxes of sensible and latent heat over the ice-covered ocean during the winter monsoon. Upstream cold and stable air mass originated from the Eurasian continent was significantly modified through heat and moisture supply from warm sea surface with offshore cold-air outbreaks, which consequently formed mixed layer characterized by neutral stability over downstream areas. Associated with reduction of the top of the mixed layer height through the observational period, estimated heat fluxes also decreased gradually from 210 W m-2 to 30 W m-2. These decrease tendencies of the mixed layer height and estimated heat fluxes may reflect the increase of the insulating effect of the sea-ice cover on heat and moisture exchanges between the atmosphere and ocean.
The relationship between vertical wind shear and tropical cyclone intensity change at each time interval of 12, 24, 36, 48, 60, and 72 h in the western North Pacific is investigated using the NCEP/NCAR reanalysis data from 1983 to 1996. As expected, regression coefficients at all time intervals are positive (storm intensity is represented by the minimum surface pressure), indicating that the vertical shear weakens storm intensity. When the total sample is stratified by latitude band, it is found that the intensity of low-latitude storms is more sensitive to vertical shear than that of high-latitude storms. This is consistent with theoretical results and observations for Atlantic storms. A minimal predictor model of predicting tropical cyclone intensity change in the western North Pacific up to 72 h is presented. The model has only three predictors (potential intensification, intensity change during previous 12 hours, and vertical shear), but the explained total variance is shown to be reasonably good in comparison to other statistical models with larger numbers of predictors. The average intensity prediction errors from the three-predictor model are reduced when the multiple linear regression method is replaced by the back-propagation neural network.
Observations made by the Precipitation Radar (PR) and the Microwave Imager (TMI) radiometer on board the Tropical Rainfall Measuring Mission (TRMM) satellite help us to show the significance of the 85 GHz polarization difference, PD85, measured by TMI. Rain type, convective or stratiform, deduced from the PR allows us to infer that PD85 is generally positive in stratiform rain clouds, while PD85 can be markedly negative in deep convective rain clouds. Furthermore, PD85 increases in a gross manner as stratiform rain rate increases. On the contrary, in a crude fashion PD85 decreases as convective rain rate increases. From the observations of TMI and PR, we find that PD85 is a weak indicator of rain rate. Utilizing information from existing polarimetric radar studies, we infer that negative values of PD85 are likely associated with vertically-oriented small oblate or wet hail that are found in deep convective updrafts.
The campaign of stratospheric aerosol measurement by balloon-borne optical particle counter was conducted four times from April 1997 to March 1999 at Bandung, Indonesia (6.9°S, 107.6°E). Within a few kilometers above the tropopause low aerosol mixing ratios were observed. The layer of relatively small aerosols (size ranges from 0.15 μm to 0.25 μm and from 0.25 μm to 0.4 μm in radius) resided at high altitudes around or above 30 km, while the layer of relatively large aerosols (size range larger than 0.4 μm in radius) resided around 23 km. Peak mixing ratio of the latter layer decreased year by year, suggesting the layer to be originated from the Pinatubo eruption in 1991. The height of the peak mixing ratio of the relatively small particles was relatively low on October 4, 1998, when the stratosphere above 23 km was in the westerly shear phase of the quasi biennial oscillation.
The NCEP-NCAR 1958-97 upper-air dataset has been evaluated for evidence of equatorial zonal circulation cells around the globe. For this it is essential not only to isolate the divergent part of the wind but also to ascertain the continuity following the flow between the centers of upward and downward motion. Over the eastern to central Pacific a well developed cell persists all year round, with subsidence in the East, but with the compensating divergent westward flow concentrated in the realm of a midtropospheric jet rather than at the surface. A cell over the western Atlantic in January is characterized by subsidence in the East and divergent westerly flow at the surface. In the Indian Ocean sector during July, a small cell is recognized over the western part of the basin, contained in the upper-to mid-troposphere and with subsidence over the East African coast. In October a well developed cell spans the entire basin, with subsidence over the East African coast and eastward flow in the lower tropophere.