The influence of blocking systems on the propagation of planetary Rossby waves is investigated over the South Pacific. Two episodes of blocked flow observed during the winter of 1995 in the Southern Hemisphere were selected for this purpose. Rossby wave activity is analyzed with the aid of a barotropic model linearized around zonally and meridionally varying basic states obtained from the National Centers for Environmental Prediction Center (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis Project data. Basic state flows corresponding to the blocking events, and to the seven-day period immediately prior to (after) their onset (decay) were used. The forcing, represented by an anomalous divergence pattern, is placed in two different regions—tropics and subtropics. Numerical results are discussed based on the linear theory for Rossby wave propagation. Fields of stationary wavenumber (Ks) for barotropic Rossby waves are computed for the basic state flows. Wave activity fluxes are calculated for the perturbation fields. The spatial distribution of Ks showed the jet streams as potentially efficient waveguides, as indicated by the agreement between the position of the jet stream axes and the relative maxima of Ks. A tendency for increasing values of Ks in the high latitudes (south of 65°S) was obtained along the poleward flank of the longitudinal zone where the blocking systems were positioned. This suggests a possible enhancement on Rossby wave activity along sub-polar regions during these blocking events. In contrast, for the periods preceding and subsequent to the blocking systems, values of Ks at high latitudes did not show a tendency to increase, characterizing a normal situation in which local maximum values of Ks are confined to the mid- and subtropical latitudes. The numerical results suggest that the planetary Rossby wave propagation can be influenced by this alteration in the latitudinal distribution of higher values of Ks during the life cycle of the blocking systems. A transition, from subtropical to sub-polar, of the main sector of wave propagation was obtained between the pre-blocking and blocking situations, and with the subtropical Rossby wave activity regaining importance after the decay of the blocking events. Moreover, for situations preceding the blocking systems, a region of maximum values on wave activity flux was obtained on the longitudinal zone where the blocking patterns would develop subsequently, indicating a possible participation of stationary Rossby wave propagation on the onset of the blocking episodes.
The characteristics of UV-B (λ= 290-315 nm) irradiance at Syowa Station (69°00’S, 39°35’E) in Antarctica were investigated by the numerical simulations with the multiple scattering radiative transfer model for the atmosphere-snow system, and the analyses of measured UV-B irradiance in 1996. By the numerical simulations a study has been made on the sensitivity of UV-B irradiance to the snow grain size, surface condition, cloud cover, Antarctic aerosol, solar zenith angle and total ozone amount. On the snow surface it was found that the effect of multiple reflection between atmosphere (or cloud) and snow surface of high albedo is important for the characteristics of UV-B irradiance. By the analyses of measured UV-B irradiance it was found that the occurrence of the maximum UV-B irradiance was not in December at maximum solar elevation, but in November when the low total ozone amount was observed. This shows that the time in a year reaching the minimum total ozone amount is important for determining the annual trend of UV-B irradiance. It was also found that the values of UV-B irradiance would be enhanced by the occurrence of cloud for the cloud amount of 9/10-10¯/10 due to the effect of multiple reflection between cloud layers and snow surface. As a whole, theoretically calculated values of spectral and spectrally integrated UV-B irradiance agree well with the measured ones, however, at the low solar elevation or low energy level of UV-B irradiance, the measured values of UV-B irradiance were larger than the calculated ones due to the less sensitivity of the instrument to the weak irradiance.
To understand the role of the ocean roughness length, and find its sensitivity to the simulation of a regional climate model for the 1994 east Asian summer monsoon case, three experiments were performed providing different ocean roughness lengths. In order to analyze the role of the roughness length only, all other variables were fixed during the 92 summer days of the simulation period, except that different ocean roughness lengths in magnitude. The control experiment (CTRL) using a fixed value of 4 × 10-4m roughness length over the ocean did not properly simulate the extreme anomalous summer of 1994. The simulated results of the roughness length experiment using 1:2 × 10-3m (ZOEXP), and 4 × 10-3m (ZOEXP2) showed that the increased ocean roughness length could reduce the strength of southerly to southwesterly monsoon flows, and consequently counter the excessive northward movement of the summer monsoon fronts. ZOEXP reduced to some extent the model biases producing more anomalous features closer to the 1994 extreme summer climate than in CTRL. The most sensitive impacts resulted from the increased ocean roughness length took place in the area of northern China, Manchuria and east central China where summertime traveling cyclonic systems and typhoons associated with the summer monsoon fronts were active. In those regions, the ground temperature over the land areas was also considerably changed, that corresponded well to the changes of the surface heat fluxes. One of the important results in this study is that the simulated precipitation pattern in ZOEXP was closer to that of the Global Precipitation Climatology Program analysis data than in CTRL. The increased moisture supply near the surface along with the decreased low-level southwesterly flows due to the changes of ocean roughness length contributed to not only the organized monsoon fronts but also the appropriate movement of the monsoon fronts with the increased monsoon rain. The increased ocean roughness length altered the phase changes of precipitation in southern Korea and Japan, and the precipitation amount over central and southern China. However, in general, the magnitudes of difference fields between ZOEXP and CTRL did not overcome those of CTRL from the NCEP/NCAR reanalysis data. In the regional climate modeling not only sea surface temperature but also ocean roughness length should be reasonably provided.
We discuss the effect of the soil moisture heterogeneity on the spatially averaged fluxes in the Tibetan Plateau. Using the observational data obtained during the GAME/Tibet Pre-phase Observing Period (POP) in 1997 and the Intensive Observing Period (IOP) in 1998, we show the seasonal variability of the soil moisture distribution. The water and energy flow processes in the permafrost region are studied. Based on the observational results, the 1-D heat and water flow model is applied to the Tibetan Plateau. This model demonstrates the seasonal variability of soil moisture distribution. To examine the effect of the soil moisture distribution, spatially averaged evaporation is calculated by this model in two different ways: one by using the spatially averaged soil moisture and the other by considering the soil moisture heterogeneity. We compare with two kinds of the spatially averaged evaporation. The comparison indicates that it is important for the estimation of spatially averaged fluxes to consider the effect of soil moisture heterogeneity.
Precipitation efficiency in the tropical deep convective regime is analyzed based on the hourly data from a 2-D cloud resolving simulation. The cloud resolving model is forced by the large-scale vertical velocity, zonal wind and large-scale horizontal advections derived from TOGA COARE for a 20-day period. Large-scale precipitation efficiency (LSPE) is defined as a ratio of surface rain rate to the sum of surface evaporation and moisture convergence, and cloud-microphysics precipitation efficiency (CMPE) is de-fined as a ratio of surface rain rate to the sum of condensation and deposition rates of supersaturated vapor. Moisture budget shows that the local atmospheric moisture gains (loses) when the LSPE is less (more) than 100%. The LSPE could be larger than 100% for strong convection. This suggests that in order to avoid moisture bias, the cumulus parameterization schemes used in general circulation models should allow the precipitation rate to be larger than the sum of surface evaporation and moisture convergence in strong convective region. Statistical analysis shows that the LSPE is about 80% of the CMPE when it is less than 60%, and the CMPE is a constant of about 70% when the LSPE is more than 60%. The CMPE increases with increasing mass-weighted mean temperature and increasing surface rain rate. This suggests that precipitation is more efficient for warm environment and strong convection.
Diagnostic analysis is made to investigate features of the moisture circulation over East Asian during the El Niño episode in northern winter, spring and autumn. It is found that in all these seasons, the anomalies of precipitation in China, atmospheric precipitable water, water vapor transport and moisture divergence over East Asia in the El Niño mature phase, differ from those in the rest of the phases. In the El Niño mature phase, positive precipitation anomalies occur in the southern part of China. More northeastward water vapor transport appears around the southeastern coast of East Asia, where moisture converges, and precipitable water is above normal, which are consistent with the precipitation anomalies. The physical process through which El Ninñ affects the East Asian climate, is also identified. Differing from the rest of the phases in the El Niño episode, the mature phase is characterized by strong convective cooling anomalies in the atmosphere, in the area (0-15°N, 110°E-150°E) over the western tropical Pacific. As a Rossby wave response of the tropical atmosphere to the cooling anomalies, an anomalous low-level anticyclone forms to the north of the maritime continent. This anticyclonic anomaly not only transports more water vapor to the area around the southeastern coast of East Asia, but also strengthens the western Pacific subtropical high, and shifts it to the south of the mainland China, which are favorable for more precipitation in the southern part of China.
Extratropical transformation of Typhoon Vicki (9807) is diagnosed in relation to its environment, particularly with respect to interactions with upper tropospheric disturbances, using the gridded dataset of global analysis (GANAL) produced by the Japan Meteorological Agency. TY Vicki made landfall on Japan with its maximum intensity on 22 September 1998, causing damage with strong winds. Vicki lost the diabatically generated potential vorticity (PV) anomaly at 00UTC 23 September 1998, when it was supposed to complete the extratropical transformation. It still had a considerable diabatic PV anomaly during passage over Japan, which is consistent with the cyclone intensity. At 06UTC 22 September, Vicki was located in the lower-baroclinic zone, and strong horizontal divergence directly above the cyclone center was dissipated in the upper troposphere, which suggest that was already beginning extratropical transformation. Regarding the upper tropospheric environment, Vicki was moving northward to the right entrance of the jet streak, and a trough, i.e., mesoscale PV anomaly, approached to the west of Vicki. To the north of the cyclone, mid-tropospheric warm frontogenesis and the associated northward out-flow were induced. To the west of the cyclone, the upper-tropospheric frontogenesis associated with the jet entrance and the trough contributed to upper-tropospheric outflow from the cyclone and midtropospheric dry-air inflow. These contributed to organizing a wide rainfall region in the northern semicircle of the cyclone and to limiting the rainfall region in the southwest of the cyclone center. It is also suggested that interactions between upper- and mid-tropospheric outflow, and the midlatitude jet streak to the north of the cyclone contributed to a deepened layer of horizontal divergence that delayed the weakening of the cyclone during the start of the extratropical transformation.
The influence of the global warming on tropical cyclones has been examined using a high resolution AGCM. Two ten-year integrations were performed with the JMA global model at T106 horizontal resolution. For the control experiment, the observed SST for the period 1979-1988 is prescribed, while for the doubling CO2 (2 × CO2) experiment, SST anomaly due to the global warming estimated from a coupled model transient CO2 experiment (Tokioka et al. 1995) is added to the SST used in the control experiment. The results of experiments show that a significant reduction in the frequency of tropical cyclones is possible in response to the greenhouse gas-induced global warming. The most significant decrease is indicated over the North Pacific. On the other hand, a considerable increase in tropical cyclone frequency is indicated for the North Atlantic. As for the maximum intensity of tropical cyclones, no significant change has been noted. It has been found that the regional change in tropical cyclone frequency is closely related to the distribution of the SST anomaly, and the change in convective activity associated with it. The results of the experiment indicate that the change in tropical cyclogenesis is strongly controlled by dynamical factors associated with the change in SST distribution, rather than the thermodynamical factors associated with the change in absolute value of local SST. On the other hand, for the decrease in the global total number of tropical cyclones on doubling CO2, a weakening of tropical circulation associated with the stabilization of the atmosphere (the increase in dry static stability), seems to be responsible. It is found that the rate of increase in the tropical precipitation due to the global warming is much less than the rate of increase in the atmospheric moisture. With this little increase in precipitation (convective heating), a considerable increase in the dry static stability of the atmosphere leads to a weakening of the tropical circulation.
A transportable 3-GHz wind profiler has been developed for convenient observations of wind and precipitation. It consists of a phased array antenna with a diameter of 1 m, a transmitter with peak output power of 500 W, a receiver, a radar controller, a DSP (Digital Signal Processor) unit, and a workstation. The whole system is loaded onto a small truck for easy transportability. To verify the validity of wind velocity data obtained by the 3-GHz wind profiler, we conducted simultaneous observations with the MU (Middle and Upper atmosphere) radar at the Shigaraki MU Observatory on January 15, 1998. It showed that the 3-GHz wind profiler has sufficient reliability for wind velocity observations. We investigated the ability of observational height coverage of the 3-GHz wind profiler using height profiles of wind velocities obtained at the Shigaraki MU Observatory during May 25, 1998—November 2, 2000. It showed that the pulse compression is very effective, and the 3-GHz wind profiler has good performance to provide wind velocities in the lower troposphere, especially in precipitation. We also present preliminary results of observation of the front on May 4, 1999 to show the performance of the 3-GHz wind profiler.
Regional and interannual variations of the rainy season over Indonesia are investigated using daily rainfall data during 1961-90. Pentad-mean rainfall data, with a relatively better continuity have been obtained for 46 stations, and the annual and semi-annual cycles of rainfall at these stations have been objectively analyzed by harmonic analysis. The onset of southern-hemispheric spring/summer (SON/ DJF) rainy season starts from the Indian Ocean side of Jawa in the middle September, and propagates northward (in Jawa) and eastward (to Nusa Tenggara in middle December). Another route of rainy sea-son propagation from Irian Jaya to Nusa Tenggara is observed. The withdrawal of the rainy season starts from western Nusa Tenggara in March, and goes eastward (to eastern Nusa Tenggara), and westward (to Jawa), until late May. Rainy season onset comes later (earlier) in El Niño (La Niña) years than the average at most stations (particularly in the south-eastern part of Jawa). Correlations between rainfall amounts at those stations, and the southern oscillation index in SON, are significantly high. However, the rainfall amount throughout a rainy season is not dependent upon the length of the rainy season (between onset and withdrawal) in many areas (except southern Sulawesi). The dominant time scale of interannual variations of rainy season onset during 1961-90 is 2-3 years, which looks somewhat shorter than that during 1910-41.
Ten years (1986-1995) of ECMWF analyses and NOAA outgoing longwave radiation (OLR) data were used to investigate intraseasonal tropical-extratropical interactions on timescales of 10-25 days in the western Pacific region during Northern Hemisphere (NH) summer. Composite analyses were performed separately for the early summer (June-July) and mid-summer (August). The analyses were based on the OLR index in the base region (the South China Sea), and help describe the seasonal characteristics of circulation associated with tropical convection. Localized Eliassen-Palm (E-P) fluxes were used to diagnose eddy–mean flow interactions. Composite circulation patterns associated with enhanced convection on a 10-25-day timescale for June-July were significantly different from those for August. June-July composites show a significant Rossby wave train in both the lower and upper troposphere, extending from the convective region into the North Pacific along the mean westerly flow. In the lower troposphere, subtropical circulation anomalies as part of this wavetrain propagate southwestward along the subtropical duct. Tropical-extratropical linkage is less pronounced during August. Circulation structures in August in both the lower and upper troposphere are different from those of June-July; Rossby waves do not emanate from the South China Sea and low-level subtropical circulation anomalies show no southwestward phase propagation. Energy conversion between eddy and seasonal mean flow for each season was computed to investigate the role of the time-mean flow in maintaining the 10-25-day transient eddies. In the lower troposphere, barotropic processes dominate the conversion from the time-mean flow to eddies in the exit region of tropical monsoon westerlies for both June-July and August. In the subtropics, baroclinic processes sustain the eddies. Moist baroclinic processes associated with eddy horizontal latent heat transport are important in increasing the potential energy available to eddies, especially during June-July. In the upper troposphere, barotropic conversion of kinetic energy from eddies into the time-mean flow is most notable in the East Asian subtropics to the north of the South China Sea. This suggests that increased eddy activity due to tropical convection is an important source of energy for the mean flow.