Diurnal variations of convective activities in the tropical western Pacific are examined by using 3-hourly geostationary meteorological satellite data acquired over a 9-year period (1980-1989). Large diurnal variations of convection exist over continents, large islands and their adjacent sea regions such as the Indochina Peninsula, the Tibetan Plateau, North Australia and the maritime continent regions. Large diurnal variations are also found over the Bay of Bengal and the South China Sea. Although the diurnal variation is less pronounced over the open oceanic regions east of 150°E than over the maritime continent regions, moderate amplitudes of diurnal variations are observed over the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Features of the diurnal variation vary with the seasons and it is enhanced during seasons in which mean convective activities become strong. Over continents and large islands, the convection attains its maximum intensity in late afternoon to evening, probably due to strong surface heating during daytime. Over sea areas in the vicinity of large islands, the maximum convective activity generally occurs in the morning. It is suggested that the interaction between land-sea circulations and large-scale environmental flows may produce a diurnal cycle in the offshore convection. There exist large amplitudes of diurnal variations over the head of the Bay of Bengal during the Indian summer monsoon season with the maximum amplitude in the afternoon. The diurnal cycle of convection becomes predominant during summer and fall over the South China Sea, with the maximum convective activity near local noontime. The convection over the ITCZ and the SPCZ has a maximum intensity in the morning, in general, but a secondary maximum of convection occurs in the afternoon. The Fourier analysis of the second component (semi-diurnal variation) suggests that there exist semi-diurnal variations of convection over the ITCZ and the SPCZ with maximum peaks around 3-4 LST and 15-16 LST.
The temperature trend in the lower stratosphere was negative between 1964 and 1993. There were, however, large fluctuations on a decadal scale, and the slope of the trend can be appreciably different depending on whether the time series begins or ends in an extreme of the decadal variations. These fluctuations affect the trends in quantities which are temperature dependent, such as ozone. For instance, the steep drop in ozone measured by TOMS between 1979 and 1990 north of about 30°N occurred in a period with a similarly steep drop in temperature, but over a longer period the temperature trend differed considerably from that of the TOMS observations. The change in the ozone during the latter period is therefore not likely to be representative of a long term ozone trend. The negative temperature trend in the stratosphere below 30 km peaked at 50 hPa (18-20 km near the level of highest concentration of ozone) at all latitudes in summer, and in the polar regions also in early winter. This is in contrast to model predictions of where the level of largest negative trend in the stratosphere associated with increasing levels of CO2 should be. In January a positive trend began in the arctic above 30 hPa which expanded downward to the tropopause during the following months. The thermal west wind in the area of the cyclonic vortex thus weakened from above during the period. This trend pattern resembles the development of a warming in the stratosphere in individual years. Marked variations of stratospheric geopotential heights on a 10-12 year scale were well correlated with the sunspot cycle, and the overall trend in the heights of the lower stratosphere was similar to the rise in the surface air/tropospheric temperature averaged over the hemisphere.
We investigate the formation of inter-hemispheric asymmetry in the meridional circulation induced by planetary waves only and resulting seasonal variation of the total ozone content in wintertime, by overviewing a series of short-range (one month) numerical experiments using observational monthly-averaged zonal mean wind fields as initial conditions in a spherical semi-spectral mechanistic model. It is found that planetary waves induce asymmetric circulations between the Northern Hemisphere (NH) and Southern Hemisphere (SH) and contribute to the formation of an inter-hemispherically distinct distribution of total ozone. Amplification of planetary waves in the SH winter induces a meridional circulation whose downward motion is concentrated to the mid-latitude lower stratosphere, resulting in an ozone increase there. The meridional circulation in the NH extends to the high-latitude stratosphere and contributes to the formation of an ozone maximum in the polar region throughout a winter. During late winter to early spring, the location of maximum increase in ozone content shifts poleward with the movement of the downward region. This suggests that the seasonal variation of the location of the mid-latitude ozone maximum in the SH, known as the subpolar maximum, is controlled by the variation of meridional circulation driven by planetary waves. Diagnoses based on TEM theory show that the asymmetric circulation arises from a difference in the propagation field. Planetary waves in the SH propagate equatorward through a maximum line of refractive index which makes the convergence region of EP flux in the mid-latitude stratosphere. Furthermore, it is found that the degree of asymmetry of the ozone distribution depends on the amplitude, wavenumber and phase velocity of the forced wave, and the asymmetry is most apparent when the stationary wave with wavenumber 1 is forced.
Convective snow clouds over the Sea of Japan were observed with hydrometeor videosondes (HYVIS), hydrometeor video dropsondes (HYDROS), Doppler radars and a microwave radiometer in four winter seasons from 1989 through 1992. This paper describes the evolution of microphysical structures in short-lived convective snow clouds with a cloud top temperature of -20°C ±3°C, based on a composite of separate snow clouds at various stages. At developing stages, moderate updraft (wmax=∼4 m s-1) produced a high concentration of supercooled cloud water (close to the adiabatic liquid water content) throughout a cloud. Number concentrations of ice crystals (d < 200 μm) were about 10 particles L-1 and few precipitation particles (snow and graupel ; d &ge ; 200 μm) were present. At mature stages, the maximum ice crystal concentrations sometimes exceeded 100 particles L-1 and concentrations of precipitation particles consisting of graupel and heavily rimed snow crystals were ∼10 particles L-1. These solid particles depleted a material amount of supercooled cloud water through depositional and accretional growth. By decaying stages, almost all supercooled cloud water had been depleted by snow and graupel particles. Only unrimed and lightly rimed snow crystals were still suspended in upper and middle layers. Ice crystals, especially isometric ones such as thick plates and columns, initiated in coexistence with supercooled cloud water (a likely nucleation mechanism is the freezing of cloud droplets), continue depositional and accretional growth and finally produce graupel particles. On the other hand, supercooled and frozen drops were also found in snow clouds at developing and mature stages, which suggests that the warm rain process (precipitation formation through collision and coalescence among cloud droplets) also operates and frozen drops serve as graupel embryos, although these contributions are considered to be small because of low concentration of such drops and their spatially and temporally limited distributions in snow clouds. The cloud water budgets are computed and indicate that, at the developing stage, excess vapor production due to adiabatic ascent of air parcels is predominant and cloud water contents in snow clouds become close to adiabatic values. However, once precipitation is well developed (at the mature stage), clouds (even with an updraft velocity of 1 m s-1) can not maintain a steady (equilibrium) state and precipitation particles quickly deplete the cloud water through their depositional and accretional growth.
Time change in airflow structures in isolated snow clouds over the Sea of Japan was investigated by means of dual-Doppler radar observations. Rawinsonde and dropsonde observations were also carried out to support the Doppler radar observations. Isolated snow clouds were observed in a late stage of an outbreak of winter monsoon and had a lifetime of about 1 hour. The sonde observations showed that these clouds were formed in a convectively unstable layer with a weak vertical wind shear. The Doppler radar observations demonstrated that the airflow patterns inside the clouds varied with time. In the developing stage, updrafts dominated in the clouds with slight downdrafts. In the mature stage, updraft and downdraft were comparable in area. In the decaying stage, downdraft was dominant in the clouds. The updraft and downdraft existed in the forward and rearward portion with respect to the cloud movement, respectively. The magnitude of updrafts was at most 2 ms-1 in the developing and mature stages, and had smaller values in decaying stage. A conceptual model of airflow structures is proposed based on the results of the observations.
Snow clouds were observed by vertically-pointing radar and microwave radiometer at Syowa Station in Antarctica in 1989 to measure vertically integrated ice water content (IWC) and liquid water content (LWC) in the clouds. Most of the water-rich clouds, which are defined here as those of more than 40 mg/cm2 in LWC, appeared in autumn, when the area of sea ice was at its annual minimum. On the other hand, the water-poor clouds, which are defined as those less than 10 mg/cm2 in LWC, appeared in almost all seasons, especially frequently in winter and early spring when the sea ice area was at its annual maximum. The occurrence frequency of these clouds seems to correlate with the area of sea ice rather than with air temperature. The convective activity to produce supercooled water droplets becomes suppressed during their passage above the sea ice with less supply of water vapor from the sea. There was a difference in the amount and area of snowfall among the clouds. The water-rich clouds brought much more snowfall within 50 km of the coastline than the water-poor clouds. This localization of snowfall would stem from the orographic effect caused by production of water condensate in clouds due to lifting of air along the slope of the continent. The water-poor clouds brought less condensed water after lifting.
When fully established in mid-August, the western North Pacific summer monsoon (signified as WNPM) becomes as intense as, or even more dominant than, the Southeast Asian summer monsoon (abbreviated to SEAM). The boundary between the WNPM and SEAM appears to exist somewhere over the South China Sea where relatively dry weather persists with a climatological OLR exceeding 230 Wm<-2>, in contrast to a much smaller value of less than 190 Wm-2 at updraft portions of the WNPM and SEAM. The major downdraft leg is located over the central North Pacific where the divergent Pacific High is capped by the convergent Tropical Upper Tropospheric Trough. This implies a dominant E-W vertical overturning which extends from about 110°E (South China Sea) to 140°W (central North Pacific), with active convection occurring over the key WNPM domain (10°-20°N, 130°-150°E) of the world's highest SST (in excess of 29°C). This E-W overturning along l0°-20°N is baroclinic in the vertical, i.e., upper-level westerlies overlying low-level easterlies eastward of 150°E, while there is a superposition of upper-level easterlies above low-level westerlies westward of that longitude. Presumably, the WNPM is driven by combined effects of zonal asymmetry in SST at 10°to 20°N, and E-W (continent-ocean) heat contrasts between about 20°and 30°N. Absence of a large land mass poleward of the WNPM domain relegates the contribution due to N-S differential heating to that of secondary importance. In contrast, the SEAM can be envisioned as a distinct meridional vertical overturning, primarily driven by N-S (continent-ocean) heat contrasts. The SEAM withdraws before early October due to commencement of continental cooling. The WNPM lives much longer because of the persistence of a high SST (> 29°C) until the beginning of November.
The concept of "periodic sequences" in aperiodic motions is demonstrated in order to describe the global structure of the strange attractor by using the Lorenz system of three variables. We define each periodic sequence as a time interval when the trajectory passes in the vicinity of a periodic solution (PS) or a "pseudo-periodic solution (PPS)" with a relatively short period ; the PPS, which is characterized by almost periodic motions, bifurcates from a limit point of a PS. Hereafter, we use the abbreviation "MPP (local minimum point based on the periodicity)" to refer to either a PS or a PPS. For a wide range of bifurcation parameter values, the statistical significance of the relation between each periodic sequence and an MPP is obtained. Some MPPs are preferentially selected to generate periodic sequences in aperiodic motions. This selection does not depend on the linear stability of each MPP. The probability that each periodic sequence persists over n cycles is well expressed by exp(-n/τ), where τ is a characteristic time. For periodic sequences near each PS, the characteristic time is also determined primarily by the linear stability of the PS. These facts support the relation between each periodic sequence and an MPP. Thus, the aperiodic motion can be grasped by the transition between MPPs embedded in the strange attractor.
A global climatic model (GCM) with a simple Q-flux based ocean has been run out for a 500-year period in order to generate time series indicative of natural climatic variability, especially global mean surface temperature. Multi-decadal warming and cooling episodes with a maximum range of 0.7°C were obtained. The geographical pattern of the warming episodes was similar to that obtained in simulations of the enhanced greenhouse effect, with maximum response at high latitudes, indicating the difficulty of distinguishing between these two phenomena. The observed warming trend for 1877-1987 was compared to the standard error of trends estimated from the power spectrum of the model results. These comparisons indicate that the observed warming trend is unlikely to be explained simply by natural variability as simulated here.
An exact modon solution in a parabolically sheared zonal flow is constructed analytically in the equivalent barotropic system on a β plane. For a given parabolic shear, the propagation velocity of the modon is determined so that the potential vorticity becomes a linear function of the streamfunction. Circularity of the modon boundary is assumed. The obtained flow inside of the modon boundary has a high-over-low dipole structure in the central area, together with a low and high to the north and south thereof, respectively. The outside flow rapidly approaches the parabolically sheared zonal background flow as it recedes from the modon boundary.
A method for the identification of Asian dust-storm particles mixed internally with sea salt was developed on the basis of electron microscopic analysis by using an energy-dispersive X-ray spectrometry and dialysis (extraction) of water-soluble material. The method was applied to samples collected in Nagasaki (Japan) and Beijing (China) during the spring of 1991. Dust particles containing Na and Cl or with Na/Si weight ratios larger than 0.25 (even if Cl was not detected) were identified to be Asian dust-storm particles mixed internally with sea salt.
Long-term pressure changes in the Kanto plain in the warm season (April-September) are evaluated by using data spanning 32 years (1961-1992). It is found that daytime pressure, defined as departure from nighttime pressure, has fallen over the central and northern Kanto plain at a rate of 0.2 hPa/(30 years) on the average of the warm season, and 0.3-0.4 hPa/(30 years) on sunny days with weak synoptic pressure gradient. Correspondingly, daytime temperature is found to have risen in the central-northwestern Kanto plain at a rate of 1-1.5°C/(30 years). These facts give observational evidence of widespread warming due to extensive urbanization.