The convective available potential energy (CAPE) based on monthly mean sounding has been shown to be relevant to deep convection in the tropics. The variation of CAPE with SST has been found to be similar to the variation of the frequency of deep convection at one station each in the tropical Atlantic and W. Pacific oceans. This suggests a strong link between the frequency of tropical convection and CAPE. It has been shown that CAPE so derived can be interpreted as the work potential of the atmosphere above the boundary layer with ascent in the convective region and subsidence in the surrounding cloud-free region.
Two numerical methods are presented for calculating rigorous upper bounds on the finite-amplitude growth of barotropic instabilities to zonal jets on a rotating sphere. One of the methods is based on Shepherd (1988)'s analytic method, which uses the conservation law of domain-averaged pseudomomentum density. A variational minimization problem is solved numerically with a quasi-Newton method after discretization. The other is the authors' original method to solve a minimization problem under the constraints of the conservation laws of all Casimir invariants and total absolute angular momentum. The convex simplex method, which has been used in operations research, is applied to solve a quadratic programming problem. The two methods are applied to estimate the upper bounds for several profiles of the initial unstable jet and the bounds are compared with the results of non-linear time integrations from the unstable jet with a high-resolution model (Ishioka and Yoden, 1994). The two bounds are found to be almost completely identical to each other. Evidence from high-resolution numerical experiments is that the bounds overestimate the actual wave-enstrophy achieved in numerical experiments by a factor of 1.2 to 2.3.
An analysis of the 1000-day dataset from a perpetual January integration of the Berlin Troposphere-Stratosphere-Mesosphere General Circulation Model is performed to clarify the basic nature of the internal variability in the complicated non-linear system and to understand a possible role of such variability in the intraseasonal and interannual variability of the real atmosphere. During the 1000 days, seven stratospheric sudden warming episodes occur without any clear periodicity; a power spectrum of zonal-mean zonal wind [U] is nearly red-noise. Frequency distribution functions of [U] and zonal-mean temperature [T] in the polar stratosphere are largely different from a Gaussian distribution. Based on [U] and the amplitude of zonal wavenumber 1 in geopotential height Z1 in the polar stratosphere, the 1000-day data are classified into three categories (C), (M) and (W) as follows: (C) strong [U] and small Z1; (M) moderate [U] and Z1; (W) weak [U] and large Z1, where the capitals C, M and W stand for a cold, moderate and warm polar stratosphere, respectively. Generally Category (C) persists much longer than Categories (M) and (W). Composite meridional sections of [U], [T] and Z1 show opposite extremes between (C) and (W) in the stratosphere and mesosphere. In Category (W), [T] in the polar region is much higher than the 1000-day average in the stratosphere but lower in the mesosphere, a common feature of sudden warming events. Composites in wave quantities such as the Eliassen-Palm (EP) flux and its convergence, as well as the transformed Eulerian mean meridional circulation, show a dynamically consistent picture of the time variation of the non-local effect of the wave driving in the lower stratosphere polar region; in Category (W) stronger downward motion is observed with larger convergence of the EP flux above. Composites of the EP flux and composite maps of the geopotential height field in the troposphere show some evidence of a vertical link between the troposphere and the stratosphere. In Category (W), baroclinic wave activity in the troposphere is a little weaker, while planetary waves show a pattern of wave train in high latitudes.
Response of a resting spherical atmosphere to transient localized heating in the tropics is studied theoretically with linearized primitive equations. The method of separation of variables is used to solve the problem, and time-integrations of the full nonlinear equations are also done to assess the linearity of the response. The linearity of the response is good for some realistic values of the heating. The dominant responses are equatorially-trapped vertically propagating waves whose vertical scale matches that of the heating and global normal (or free) modes. In the middle atmosphere, the equatorially trapped waves respond effectively if the angular frequency is the order of 1O×[damping rate]. If the frequency is greater than this order, the response is suppressed in a stochastic sense; while if the frequency is less than this order, it is suppressed by the damping. Spatial pattern of the response is obtained for a realization of idealized stochastic heating with a Gaussian form in space and time. For the heating, of which the time scale is a few days or longer, horizontal cross sections of the response show the "Gill pattern" at the beginning and then the response disperses zonally in low latitudes. For short-lived heating, on the other hand, the gravity wave response expands concentrically at the beginning and then the response spreads zonally in low latitudes. Energy and momentum spectra to various kinds of wave are calculated for the stochastic heating. As the time scale of the heating events decreases, gravity-wave responses increase relatively to Rossby-wave responses. As the zonal scale of the heating events decreases, on the other hand, Rossby-wave responses slightly increase relatively to the gravity-wave responses. Heating just on the equator is less effective to excite Rossby waves than that off the equator. Energy and momentum of these vertically propagating waves are of comparable orders to those of the real atmosphere if the heating has an appropriate spectrum with a realistic amount comparable to the total latent heat release in the tropics; and so does the energy of global normal modes. Wave energy propagation into the middle atmosphere has to be taken into account even for the calculation of the transient response in the troposphere if the dominant frequency is larger than the damping rate.
Precipitation in the monsoon season on the Tibetan Plateau occurs as rain or snow depending on the elevation and temperature conditions. Point measurements of precipitation may not describe the accurate spatial distribution if systematic-error corrections of precipitation measurement are not applied. Precipitation data at 5 points intensively observed in the Tanggula basin (33°N, 92°E) were corrected, and the characteristics of the spatial distribution of precipitation are discussed. The largest correction was required for wind-induced loss of solid precipitation. The average ratio of corrected and measured precipitation was 1.38. An obvious increase of precipitation toward the south and higher elevations was found. Three years of precipitation data at two Chinese weather stations located north and south of the Tanggula basin were also corrected by the same methods. The ratio of corrected versus measured precipitation showed a clear annual variation, nearly constant at around 1.2 times in the monsoon season, but some months showed increase of more than 1.5 times in the non-monsoon season. Recycling of water between the atmosphere and land surface may strongly prevail in the Tibetan Plateau, which means that the corrected higher precipitation amounts directly correspond to a greater evaporation (or sensible heat source).
In Part 1(Miyoshi and Morita, 1993), we investigated how the general circulation of the atmosphere was affected by all the physical processes of H2O. In this study, we divide all the physical processes of H2O into the radiative process of H2O and the hydrological cycle, and investigate the effect of both of the processes separately by a series of GCM experiments. Results are as follows. The north-to-south temperature gradient and the zonal wind distribution below 5 km height is strongly affected by the radiative process of H2O. The strength of the meridional circulation, the magnitude of the poleward energy transport, the magnitude of the sensible and latent heat fluxes and the magnitude of the diabatic heating rate in the atmosphere also depend on the radiative process of H2O. Thus, the differences of the general circulation between with and without all the physical processes of H2O is mostly caused by the radiative process of H2O. The zonal wind distributions above 5 km height, on the other hand, are affected by not only the radiative process of H2O but also by the hydrological cycle. Numerical experiments using a vertically one-dimensional radiative-convective equilibrium model are performed. By comparing the radiative-convective equilibrium temperature with the temperature in the GCM, the relation between the poleward heat transport by the dynamics and the physical processes of H2O is also examined. In the experiments without the radiative process of H2O, the temperature near the surface in the GCM is almost the same as the radiative-convective temperature. At low latitudes, the temperature difference between the GCM and the radiative-convective model is larger in the experiments with the radiative process of H2O than in the experiments without the radiative process of H2O.
A one-dimensional radiative transfer equation for a finite cloud array is obtained. First-order scattering of a solar beam is included exactly, and cloud-cloud interaction is parameterized by the horizontally averaged radiance inside a cloud. The cquation yields the accurate solutions of solar flux reflectance for two-dimensional bar array clouds and three-dimensional regular array clouds when compared to Monte-Carlo simulations.
Field investigations of the dynamic interaction between nocturnal drainage winds and a cold-air lake were conducted in a hilly area near Fukuoka City, Japan. It was found that nocturnal drainage winds can be classified into two types according to where they occur, that is, below or above the surface of the cold-air lake. The nocturnal drainage wind which contributes to the development of the cold-air lake over a basin and flows down into the surface layer of the lake is one type of nocturnal drainage wind (NDW I). This type of flow, in general, has a thickness of more than 1O m, and a speed which oscillates with periods ranging from 30 to 60 minutes, though the cause of the periodicity has yet to be determined. Within the cold-air lake, that is, on the hillside buried in the lake, another type of drainage wind (NDW II) can be found, the thickness of which is less than about 10 m. Winds of this type also oscillate, with periods of 20 to 30 minutes and of about 40 minutes. These periods agree fairly well with those of internal gravity waves induced in the cold-air lake, suggesting that the periodic behavior of the NDW II type is associated with the wave-like movement of cold air in the lake. Although cold air is produced over the entire ground surface, nocturnal drainage winds, which are the flow of cold air down the sloping surfaces, exhibit different characteristics according to whether the slope is located above or beneath the surface of the cold-air lake.
By using data for 34 years (1961-1994), climatological features of the 1994 hot summer in Japan were examined with attention to boundary-layer processes. The thermal anomaly in the boundary layer was analyzed in terms of two parameters: screen height-850 hPa temperature difference (δT) and day-night pressure difference (Δp). It was found that the 1994 summer was characterized by a positive anomaly of δT and a negative anomaly of Δp, namely a larger temperature anomaly in the daytime boundary layer than in the free atmosphere. The anomalies of δT and Δp were found to be correlated with the precipitation and sunshine during the preceding days. This indicates that the high temperature in the 1994 summer is partly referred to the reduction of soil moisture due to persistent sunny weather. On the other hand, there was little indication of urban warming in the daytime except at some stations, although a warming trend in the nighttime was found for many stations. It is therefore unlikely that urban warming was the main cause of the high temperature in the daytime of the 1994 summer.
Aspects of the associations between summer (June-August) rainfall over China and ocean temperatures in the 'warm pool' region in the tropical western Pacific have been examined. This has been achieved with precipitation records for 160 stations and sea surface temperature data from the COADS covering the 40-year period 1951-90. The spatial distribution of the rainfall-SST correlations were found to vary considerably according to location of the SST target area. There were significant positive associations between the interannual variation of summer rainfall in the lower part of the Yangtze River valley and SST in the 10-20°N, 120-130°E Marsden square. To the north, over the Huaihe River valley, the correlations assumed opposite signs. When the SST over the Marsden square to the southeast (i.e., 0-10°N, 130-140°E) was considered, the correlations became very small and were not significant. For the period considered here, there appears to be little synchronous connection between the Southern Oscillation Index and summer station precipitation in China. Our results differ from those that have been obtained with data periods shorter than we use here. The findings suggest that the nature of the association between summer rainfall and 'warm pool' temperatures exhibit multi-decadal variability and, in particular, underwent a change in about 1970.