The large scale instabilities of a coupled atmosphere-ocean system on an unbounded equatorial beta plane are investigated by solving the eigenvalue problem. Special attention is paid to the effect of the basic state inhomogeneity due to the meridional extent of the mean equatorial Upwelling. It is found that if the upwelling region near the equator is meridionally wide, an eastward-propagating mode becomes unstable, while if it is narrow, the second symmetric Rossby mode becomes unstable. In the narrow case, when the atmosphere-ocean coupling is strong, both modes can be unstable. Seeming inconsistencies involving the characteristics of anomaly propagation in some previously published results of atmosphere-ocean coupled models of the ENSO phenomenon may arise from this sensitivity to the meridional width of the basic state upwelling.
Pattern formation of convective activities over the Earth has been investigated by using numerical models. Several numerical experiments have been conducted by using 12-level T21 and T42 global spectral models at the University of Tokyo. In order to focus on a distribution of convective activity over the tropics, the Earth was assumed to be entirely covered by an ocean with a globally uniform sea surface temperature ("Jovian-type" Aqua-planet experiment). A simple radiational process was assumed, where horizontally uniform cooling was assumed to compensate for convective heating and maintain the globally averaged temperature. Time integration was conducted from an initial condition where temperature and moisture fields are assumed to be uniform. As moisture fields are assumed to be relatively dry, the model atmosphere tends to be moistened during time integration. The following interesting results have been obtained: 1. When the evaporation rate is large and the moisture supply from the ocean is intense, convective activities tend to be distributed all over the Earth. 2. Then, convective activities tended to be suppressed on the equator. 3. As the evaporation rate decreased, convective activities in the extra-tropics tended to be suppressed and a band structure with respect to the equator (double ITCZ structure) becomes noticeable in the T42-run. On the contrary, a single ITCZ structure is simulated in the T21-run. This is because disturbances off the equator cannot be resolved in the T21-run. 4. With respect to the propagation of convective activity, westward propagation is noted during the period of large evaporation and eastward propagation is noted after the evaporation decreases below a certain level. 5. Two time-scales were found in this experiment. One scale is about 50 days, where the zonally averaged fields are adjusted. This is due to the establishment of a symmetric circulation. The other scale is about 200 days, where organization of convective activities is realized. This is related to a decrease of evaporation and precipitation. Finally, additional numerical experiments by using another parametarization scheme for convection and radiational processes have been conducted and it is concluded that the results obtained in this study are not particularly sensitive to these two schemes.
A thin upglide cloud (400km north of the surface warm front) and the deeper upglide cloud (200km north of the warm front) associated with the same warm frontal system were investigated. The microphysical structure and precipitation mechanisms associated with these clouds were studied using data collected with a hydrometeor videosonde (HVVIS) and a Doppler radar as well as routine rawinsonde and surface measurements operated by the Japan Meteorological Agency. The HVYIS observations showed the following microphysical structures which were common to both the clouds. Columnar type crystals predominated throughout the whole cloud layer, and some capped columns and hexagonal plates were also observed below the -17°C level. These crystals were hardly rimed nor aggregated. Only in the melting layer of the deep upglide cloud were wet aggregates of these crystals found. No supercooled cloud droplets were found in the clouds, as expected from the lack of rime on the crystals. However, a low concentration of drizzle drops coexisted with snow crystals in the shallow layer (2 km deep) just above the 0°Clevel. These results demonstrate that the dominant mechanism of precipitaition formation in these clouds was the depositional growth of snow crystals above the warm-frontal surface. In the thinner cloud, snow crystals rapidly evaporated in a dry layer (R.H. ∼30 %) just below the warm-frontal surface before they melted. In the deep upglide cloud, the increase of the precipitation rate due to the collection of cloud droplets was less than 10% of the precipitation rate at the surface. Between the 0°C and -10°C levels, low concentrations of drizzle drops were observed and the air was almost water-saturated. In this region, snow crystals grew rapidly by vapor deposition and formed 80% of the total precipitation mass reaching the ground. Such a humid condition was produced by strong updrafts (with a maximum value of r∼30cm/s) associated with strong southerly flow over the frontal surface.
The structure of the rainbands and eyewall of Typhoon 8514, which landed at the central region of Japan on 30 August 1985 was observed by two ground-based Doppler radars. The main purpose of the present study is to describe a general view of the structure of the typhoon using data of the dual-Doppler radar. Although the typhoon was a small and weak typhoon, it retained the characteristics of tropical cyclones: it was accompanied by spiral rainbands (an outer rainband and an inner rainband) and had a warm core in its upper part. The outer rainband was a spiral band located about 150km from the center of the typhoon. This rainband consisted of continuous stratiform clouds and scattered convective clouds. A radar "bright band" was observed in this rainband. Wind perturbation induced by cooling-by-melting was observed just below the bright band. An updraft of 1.5 ms-1 was produced, mainly owing to the convergence of a southeasterly flow on the inner edge of the outer rainband. This updraft maintained the outer rainband. The inner rainband was a convective spiral band located ∼60km from the typhoon center. The distribution of reflectivity and vertical velocity of this rainband indicated that an old echo cell existed in the inner part, while young echo cells existed in the outer part of this rainband. An inflow (the airflow toward the typhoon center) was observed in the lower layers of the inner rainband. This inflow reached the inner edge of the young cells, and produced an updraft there. When the depth of this inflow layer became thinner, the inner rainband decayed. This indicates that the inflow had an important role to play in maintaining the inner rainband. Because the flow into the eyewall of the typhoon in the lower layers was weak, the radius of maximum wind (RMW) was located at the outer side of the axis of reflectivity maxima. The eyewall decayed when it moved to the inner area (nearer the typhoon center) of the RMW, where downdraft was predominant.
A new method for the derivation of the horizontal distribution of total ozone amounts from the brightness temperature data obtained by the HIRS/2 sensor on board the NOAA satellites has been developed. This method is based on the regression method developed by Lienesch (1988), who added a transmittance term of the ozone layer as a predictor of regression, but the present method also includes the second-order terms of the brightness temperatures of Channels 1, 2, 3, 8 and 9 of the HIRS/2 sensor and the transmittance of ozone layer into the regression calculation. The total ozone data obtained by TOMS were used as the true values in generating the regression coefficients. The transmittance for the slantwise-looking condition was converted into that for the nadir-looking condition using the angle correction method introduced by Muller and Cayla (1983). The angle correction was also made for the brightness temperature at Channel 9 using the corrected transmittance. Horizontal distributions of total ozone amounts were retrieved by this method with a standard deviation of residual errors of ∼4% for the wide latitudinal region from 15° to 60°, including Japan where total ozone varies largely with latitude. Inclusion of the second-order terms into the regression improves the accuracy of retrieval, especially in the low-latitude regions. This regression. method will offer quite valuable data on total ozone because of the high spatial and time resolution, the availability of night-time data and independence of retrieved values from the existence of stratospheric aerosol.
Distribution of BAs in Eurasia during the Meiyu/Baiu season and some possible influences of BAs on the behavior of the MB frontal zone were examined through statistical analyses. Then detailed analyses were made for a case in late June 1982. BAs appear with higher frequency in the regions of 31-60°E, 91-110°E and 131-150°E in the latitudes of 50-70°N. The maximum frequency is frond in the region of 131-150°E. The appearance frequency of BAs in region of 50-70°N/81-120°E (Region B) showed a positive significant correlation with the persistency of the Baiu period in Japan. On the other hand, the appearance frequency of BAs in the region of 50-70°N/121-160°E (Region C) showed a positive correlation with both the persistency of the Meiyu period and the rainfall amount during that period in Central China. A statistical analysis suggests that the existence of cold air in the lower layer in 32.5-35°N/112.5-117.5°E (corresponding to the area just to the north of the Meiyu front), largely associated with a BA in Region C, is an important factor in activating the Meiyu front in Central China, and in enhancing the lowlevel southerly wind component in South China. A case study for June 1982 showed that the cold air intrusion toward Central China in the situation of Type C occurs mainly in the lower layer, and is accompanied by the development of a baroclinic instability wave around 50°N/100-110°E. The formation of the Type C situation is a favorable condition for cold air coming southward to the Meiyu frontal zone. On the other hand, there was a Rossby wave-like (height) anomaly pattern propagating from the Okhotsk Sea to the tropics through Japan shown by the presence of a BA in Region C in June. Although forcing processes have not been clarified yet, a case study in late June 1982 suggests that the wave was amplified around the Lake Baikal to the Okhotsk Sea area. The propagation might result in a negative anomaly of geopotential height at 500 mb level to the east of Japan, a southward shift of the Baiu front and weakening (or southward shift of) the sub-tropical high in nearby regions. The present paper shows the different effects of the Type C stiuation on climatological features of the Meiyu front from those of the Baiu front.
Temperature and humidity fields in the low-level atmosphere around cyclones, which are assumed to differ for each individual cyclone, were objectively analyzed through a year on the hemispheric scale to detect their representative spatial patterns. Geographical and seasonal characteristics for the appearance of cyclones as related to these patterns are comparatively shown over the middle latitudes of the Northern Hemisphere. Cyclone positions at 1000 mb and surrounding potential temperature (θ) and specific humidity (q) fields at 850 mb are objectively computed using NMC data from August, 1985 to July, 1986. The distribution of the air-mass fields and the cyclone positions in the analysis year were firstly examined in relation to the former climatological maps. Then, representative spatial patterns of θ and q around cyclones with cloud areas are obtained as a function of the three rotated principal components (RPCs). The first two spatial fields of the RPCs-θ show a relatively symmetric correlative structure with southwest-northeast and southeast-northwest dipole patterns, respectively. However, the first spatial field of RPC-q shows a concentric structure with meridional asymmetry. Case studies indicate that the RPCs-θ pattern represents the actual distribution of temperature and its evolutional variations along the border between air masses, and RPCs-q represents the distribution of absolute humidity gradient, which is recognized as the margin of the sub-tropical air mass in the middle to lower latitudes in summer. The frequency distributions of cyclone appearance with representative plus and minus RPCs are composed for θ and q, and shown with seasonal mean θ and q fields over the Northern Hemisphere. The appearance of cyclones with the first two RPCs-θ patterns was determined by the relative location along the edge of the cold air mass trough, which is formed by the intrusion of a cold air mass from the Arctic region. Large-scale topography, such as straits, large plains and chained mountains, determines the path of the cold air mass from the north, and reflects the strong regional characteristics of the RPCs-θ values affecting the appearance of cyclones. Prevailing regions for critical RPCs-θ values shift longitudinally according to the season and according to the major routes of air mass intrusion. Meanwhile, cyclones with strong RPC-q patterns appear from the southwestern Tibetan Plateau to the southeastern side of Eurasia, from the northeastern Tibetan Plateau to the northwest Pacific and southern North America to Canada along the east side of Rocky Mountains, in summer. These regions overlap with the western margins of maritime Polar Frontal Zones and the Siberian-Canadian Arctic Frontal Zone.
A hypothetical mechanism is described whereby the Quasi-Biennial Oscillation (QBO) of Zonal winds in the equatorial stratosphere actively modulates the timing of El Nino-Southern Oscillation (ENSO) events. The mechanism involves the meridional redistribution of deep convective activity throughout the tropical Pacific warm pool region in response to variable wind shear processes which are linked to the opposing phases of the QBO. Hydrostatic conditions favoring deep convective activity within approximately ±7°of the equator develop in response to the easterly shear phase of the QBO. At the same time, deep convection is inhibited in the monsoon-convergence zones farther off the equator (8-18° latitude) during the east phase of the QBO. The opposite trends occur during the westerly shear phase of the QBO wherein deep equatorial convection is suppressed while off equator monsoon convection is enhanced. It is shown that during the east phase of the QBO, the Pacific regional pressure and circulation anomalies which arise in response to QBO-linked trends in convective activity are consistent with conditions leading to warm ENSO events (i.e., El Nino). If the heat content of the warm pool is sufficient, a warm event will occur. Conversely, conditions favoring the development of cold (or La Nina) events tend to occur in association with the westerly phase of the QBO. Although several aspects of the hypothetical mechanism remain tentative, extensive empirical results present a compelling argument for the QBO as an active and fundamental component of ENSO variability
We explore the relationships between monthly tropical Chinese rainfall (TCR) and June-September Indian summer monsoon rainfall (ISMR) for the 35-year period from 1954 to 1988. The largest positive correlations between the TCR and the ISMR occur during the months of May and September, with the May correlation exceeding the 99% level of significance. The TCR between June and August, the most intense period of rainfall in tropical China, shows no apparent relationship with the ISMR. Similar behavior is found in composite anomaly patterns of outgoing longwave radiation (OLR). Repeating the rainfall computations for four subdomains in tropical China supports these results. The significant correlation between the May TCR and the ISMR suggests that the May TCR may be a precursor of the ISMR, consistent with the westward migration of the onset of the Asian summer monsoon seen in climatological mean patterns of OLR.
Steady axi-symmetric flow in a rotating annulus with differential heating is computed with a highresolution full-non-linear model in two dimensions. The velocity and temperature fields are investigated for a wide range of external parameters, and their dependence on the parameters is discussed. A simple diagnostic model of the steady axi-symmetric flow is constructed to understand intuitivelyhow the velocity and temperature fields are determined for given external parameters. The model is applicable to both the conduction-dominated flow and convection-dominated flow, and explains qualitatively how the heat transfer process is determined in each flow. Moreover, the model gives several important quantities characterizing the axi-symmetric flow, such as the intensity of the meridional circulation, that of the zonal flow and the horizontal temperature difference in the interior, for a wide range of the parameters. The results in the simple model agree well with those obtained in the full-non-linear model.
The relationship between sea level at Yap Island (9°31'N, 138°08'E) and sea-water temperature along 137°E for July is investigated. Sea level at Yap Island is positively correlated with subsurface sea-water temperature in the domain of 5°N-13°N, depth 10-400 m around Yap Island. These results indicate that sea level at Yap Island is associated with the shift of the thermocline in the vertical direction. Since the shift of the thermocline is related to the accumulation of warm water in the upper layer, it follows that sea level at Yap Island can well indicate the accumulation of warm water around Yap Island. Sea level at Yap Island tends to fluctuate in phase with that in the western tropical Pacific but out of phase with that in the central equatorial Pacific. The sea level at Yap Island is a good indicator for monitoring the accumulation of warm water in the western tropical Pacific.