Using results from numerical experiments, budgets of divergent kinetic energy (kD) and nondivergent kinetic energy (kND), are examined for the subsynoptic-scale low-level jet (LLJ) event that occurred during 1-2 June 1987 (TAMEX IOP 5) to diagnose the energy conversion process leading to the LLJ development. Nondivergent kinetic energy (kND) dominates the total kinetic energy. The intensification of the LLJ, and the subsynoptic-scale upper-level jet (ULJ) is due to the increase in kND. Divergent kinetic energy (kD) is rather small. In the area encompassing the LLJ, the most important energy source of kND is kD generated from the potential energy (PE) through the cross-contour divergent winds, and then converted into kND via a conversion process through the transverse secondary circulation across the jet-front system. In the upper troposphere, the cross-contour flow across the ULJ is dominated by the nondivergent winds. The major source for the increase of kND associated with the intensification of ULJ is the conversion from the potential energy to kND via cross-contour nondivergent winds. This conversion rate is independent of the strength of the secondary circulation. The upper-level divergence ahead of the upperlevel trough is mainly contributed by the along-stream agesotrophic winds along the ULJ axis, and it provides the upper-level support needed for the LLJ development in the lower troposphere. This mechanism is at variance with the CISK (Condtional Instability of the Second Kind) process in the traditional view. Without the feedback effects from convective heating (NOLH), low-level frontal cyclone fails to fully develop. The secondary circulation is weaker with a much slower energy conversion process from PE to kD, and then to kND in the lower troposphere. As a result, the simulated LLJ in the NOLH experiment is much weaker. Nevertheless, the intensity of the simulated ULJ without latent heating is about the same as that with latent heating. This is because the conversion from the potential energy to kND via crosscontour nondivergence winds, is independent of the strength of the transverse secondary circulation across the jet-front system.
This paper explores the relationship between regional circulation anomalies and the interannual variability of surface temperature in Southern South America, east of the Andes (SSA). Two sets of monthly surface temperature for the 1963-1990 period were used; one was taken from the NCEP/NCAR reanalysis, and the other was built from the synoptic network records. Although these sets present some differences between them, the conclusions found are independent of the data set. Monthly and seasonal indices were developed to explore, through linear correlation, the connection between interannual temperature variability in SSA and some features of regional circulation. The intensification (reduction) of the west component of the wind at the upper troposphere over the subtropical latitudes of South America, and the northward (southward) shift of the maximum westerly wind over SSA tend to be associated with cold (warm) anomalies of surface temperature during practically every month throughout the year. At low levels, the enhancement (reduction) of the zonal geopotential gradient between the South Atlantic high and the Chaco low, representative of the northern component of the low-level flow, is generally associated with warm (cold) surface temperature anomalies. In SSA, monthly precipitation is positively correlated with the northern component of the geostrophic flow. Therefore the advection of temperature offsets the possible influence of precipitation, caused by the associated evaporation and cloudiness, on surface temperature. Hence, there are positive correlations between precipitation and temperature except for central Argentina in summer, and for southern Brazil in certain months. The fact that increased (decreased) precipitation accompanies enhanced (less) warm advection, contributes to moderate the interannual variability of surface temperature. The partial balance between the effects of warm advection and precipitation on surface temperature variability explains why, though ENSO phases have a considerable impact on precipitation and some influence on the regional circulation, their signals in the surface temperature are relatively modest. The exception is in JJA (0) in western SSA, for the most part a region with very scant precipitation during the austral winter, where the surface temperature anomalies are positive in El Ninño, and negative in La Ninña. The maximum anomalies coincide approximately with an enhancement (reduction) of the northern component of the flow, and of the warm advection at low levels in El Ninñ o (La Ninña) events.
A new scheme that can define three rainy seasons, and the hydrological summer monsoon in Korea, has been proposed, examined, and verified to be effective. In the scheme, the Available Water Resources Index (AWRI) that is the accumulated precipitation value in which daily reduction of water, and the duration of accumulation are taken into account quantitatively. With this scheme, the onset and ending dates of three rainy seasons are defined by such singularities as the smallest, the biggest, the minimum, and the maximum of the AWRI in a year. The intensities of these rainy seasons are defined by the value of the maximum AWRI, flood index, and drought index. The first rainy season (Bom-Changma) starts in early April, when the minimum value of the AWRI in a year appears, and when the mean southerly wind at 925 hPa level become stronger then that at the 500 hPa level, and ends at May 15. The second rainy season (Changma) starts in late June and ends at 16-20 July. The third rainy season (Kaul-Changma) starts at mid August, and ends at 3-5 September that has the maximum value of the AWRI in a year, and that is the last date of mean southerly wind at the 925 hPa level. Finally, the hydrological summer monsoon is proposed to be defined as the period of increasing water resources that is from the minimum to the maximum of the AWRI, from the onset of Bom-Changma till the end of Kaul-Changma. From a turning point of meridional wind system to the end of mean southerly wind in the low level atmosphere, and that is concurrent with the summer monsoon.
To investigate the impacts of anthropogenic global warming on tropical cyclone (TC) activity, climate simulations were conducted under the present, and CO2-warmed conditions, using the National Center for Atmospheric Research Community Climate Model version 2. The CO2-warmed condition includes doubled atmospheric CO2 concentration, and about 1°C of tropical sea surface temperature (SST) warming. Simulated TCs were objectively selected from twice daily instantaneous outputs during an eight-year time integration period of each simulation. The changes associated with global warming were examined in terms of the frequency of occurrence, and mean intensity of TCs. The frequency of global TC occurrence remains unchanged in response to the CO2-induced warming. Although the hydrologic cycle is generally enhanced in the warmed climate, increased precipitation does not necessarily make a great impact on TC activity. This unchanged global TC frequency seems to coincide with almost neutral variations in the zonally-averaged moist instability in the tropics. However, there is some uncertainty in the model regarding the treatment of physical processes that control moisture distributions in the middle to lower troposphere. On the regional scale, the CO2-induced changes in TC occurrence were generally not statistically significant. TC intensities were enhanced over warmed SST regions in the western Pacific, which contribute to the significantly increased mean intensity of global TCs.
In the South China Sea (SCS) and western North Pacific (WNP), climatological summer monsoon sets in first in the SCS in mid-May, extends into the southwestern Philippine Sea in mid-June, and jumps suddenly northeastward to around 20°N, 150°E in late-July. The processes leading to this distinct northeastward advance of the summer monsoon are investigated using NCEP/NCAR reanalysis data for the period of 1979-1995. It is found that the cloud-radiation and wind-evaporation feedback plays an important role for the seasonal change in sea surface temperature (SST) in the WNP. Analysis shows that monsoon-induced changes in cloudiness and surface wind produce contrasting changes in surface short wave radiation, and latent heat flux between the convection and pre-convection region. The resultant SST tendency difference turns around the SST gradient east of the convection region in about one month, and induces the northeastward shift of the highest SST center. Following the ocean surface warming in the pre-convection region, the convective instability and low-level moisture convergence increase. East of the convection region, the reversal of the SST gradient tends to be associated with the transition of low-level winds from easterly to westerly. The results suggest that the SST change induced by the monsoon onset facilitates the northeastward extension of convection. It is speculated that the summer monsoon advance over the WNP may result from air-sea interactions.
In order to assess and improve the short-range tropical precipitation forecasts, an adaptive use of TRMM (Tropical Rainfall Measuring Mission) rainfall via a physical initialization (PI) technique is explored. An EOF-based perturbation method is first employed to generate 25 initial ensemble members. From these initial ensemble analyses, twelve-hour forecasts are performed. Predicted 12-hour accumulated rainfall fields are then used to find sensitive (adaptive) regions, owing to initial condition uncertainties in terms of the rainfall forecast error variance. To shorten the computing time, the 14-level Florida State University Global Spectral Model at a resolution of T106 (FSUGSM T106L14) is employed in the above procedure. Next, based on a map of the rainfall forecast error variance, higher resolution T170 experiments are carried out from the adaptively initialized states. The rainfall forecast skill of the adaptive PI experiments is compared to that of the normal mode initialization (NMI), and the regular PI experiments. In the regular PI, it is performed over the whole tropics. Our results indicate that the adaptive PI of the TRMM rainfall over regions of large rainfall variances due to initial condition uncertainties, may potentially improve 3-day rainfall forecast skills over the global tropics. In addition, more notable improvements are evident over the adaptive regions, themselves.
A one-dimensional prognostic cloud model has been developed for possible use in a Cumulus Parameterization Scheme (CPS). In this model, the nonhydrostatic pressure, entrainment, cloud microphysics, lateral eddy mixing and vertical eddy mixing are included, and their effects are discussed. The inclusion of the nonhydrostatic pressure can (1) weaken vertical velocities, (2) help the cloud develop sooner, (3) help maintain a longer mature stage, (4) produce a stronger overshooting cooling, and (5) approximately double the precipitation amount. The pressure perturbation consists of buoyancy pressure and dynamic pressure, and the simulation results show that both of them are important. We have compared our simulation results with those from Ogura and Takahashi’s one-dimensional cloud model, and those from the three-dimensional Weather Research and Forecast (WRF) model. Our model, including detailed cloud microphysics, generates stronger maximum vertical velocity than Ogura and Takahashi's results. Furthermore, the results illustrate that this one-dimensional model is capable of reproducing the major features of a convective cloud that are produced by the three-dimensional model when there is no ambient wind shear.
A 'regime shift' is characterized by an abrupt transition from one quasi-steady climatic state to another, and its transition period is much shorter than the lengths of the individual epochs of each climatic state. In the present study, we investigate when regime shifts occurred and what was the difference in climatic states before and after the shifts, using the wintertime sea surface temperature (SST) field in the Northern Hemisphere. The relationship between changes in the SST field, and those in the atmospheric circulation, is also investigated. In order to detect organized patterns of the SST variations, we apply an empirical orthogonal function (EOF) analysis. As the results, the first mode is identical to El Niño/Southern Oscillation (ENSO) and so-called Pacific Decadal Oscillation (PDO), and corresponds to the Pacific/North American (PNA) pattern. The second mode, which relates to the Arctic Oscillation (AO), has a zonally elongated signal in both the North Atlantic, and North Pacific. EOF analyses to each oceanic basin are made separately, and the robustness of these modes is confirmed. In the present study, we define the regime shifts as the 'significant' and 'systematic' changes between the two quasi-steady states, continuing more than 5 years. Then, in order to identify the years when regime shifts occurred in the SST field, we carefully inspect the time series of original gridded SST data and those of the EOF modes. As a result, six regime shifts are detected in the study period from the 1910s to the 1990s: 1925/26, 1945/46, 1957/58, 1970/71, 1976/77 and 1988/89. It is ascertained that the shifts at almost all grids are completed within one year. All regime shifts having similar SST and atmospheric circulation pattern, including the changes in an intensity of the Aleutian Low (AL), and the corresponding SST changes in the central North Pacific. All regime shifts can be well described by the combination of the first and the second EOF modes. The duration between each regime shift is about 10 years, which are identical to the PDO. The simultaneous shifts in the first, and the second EOF modes, imply that the change in the AL activity associated with the PNA pattern, might have some connection with that of the AO.