Thompson's mechanism is reconsidered in a two-dimensional plane. It is shown that a mean shear flow can be generated by the positive feedback of a mean shear flow, tilting convective motion and tilting temperature distribution. In this instability, a small perturbation of mean shear flow tilts the basic temperature distribution. The tilting temperature distribution induces tilting of convective motion, which transports horizontal momentum vertically so as to reinforce the mean flow shear. In order to examine the validity of Thompson's mechanism on the sphere, a three-dimensional nonlinear numerical model is constructed. Steady solutions which represent convection between the day and night sides on the sphere, are numerically obtained for various values of parameters, and their stability is examined. It is shown that, if the planetary rotation is neglected, three-dimensional convection on the sphere is stable and a mean shear flow cannot be generated. This result suggests that Thompson's mechanism proposed for two-dimensional convection cannot be applied to three-dimensional convection on the sphere.
The climatological variation of the residual mean meridional circulation in the stratosphere, and the upper troposphere is investigated for 10 years from December 1985 to November 1995. The residual circulation is calculated from the transformed Eulerian-mean (TEM) zonal momentum and continuity equations. The method takes account of the Rayleigh friction and the mean zonal flow change. The tropical upward mass flux across the 100hPa surface is maximum in November and minimum in June, and the annual mean is 101.8×108kg/s, with a turnover time of 1.6 years for the atmosphere above 100hPa. The cause of differences in magnitude and seasonal variation of the Mass flux between previous studies, and the present study, is discussed. It is shown that the present results is more reasonable than previous studies. In the stratospheric mass budget, it is found that the spring and summer circulations are stronger in the southern hemisphere as compared with the same seasons in the north, and the reverse is true during fall and winter. The annual mean circulation is stronger in the northern hemisphere than in the southern hemisphere. The poleward flow is weak in the winter middle stratosphere. In addition, it is suggested that the mass exchange at 100hPa surface is mainly controlled by wave forcings in the lower stratosphere.
Chemical measurements of fog water have been made near the summit of Mt. Norikura (altitude 2770m) in central Japan, during the summer and autumn from 1994 to 1996. Highly acidic fogs (pH<4) were observed both in the summer and autumn. The SO2-4 was the most dominant anion in fog water. The [SO2-4]/[NO-3] equivalent ratio was much less than that measured in the 1960s. The results may reflect the difference between the characteristics of air pollution in Japan during the 1960s, and that of the 1990s. The ratio of [SO2-4]/[NO-3] in the fog water was significantly higher during the summer than in autumn. In addition, the [Cl-]/[Na+] ratio was much higher than that of seawater in the summer. The non-sea-salt Cl- also seems to contribute to the fog water acidification. The ratio of the summertime [NH+4]/[SO2-4] was also higher than that in the autumn. A relatively high pH (pH>6) of fog water was observed, despite the high concentration of SO2-4 during July 1994. The hydrogen peroxide (H2O2) concentration during the summer varied over a range of 3-180μM, with an average concentration of 60μM. Relatively high concentrations of H2O2 (60-70μM) were observed, even in the autumn.
In order to estimate the annual surface shortwave forcing by clouds+aerosols and aerosols, the shortwave flux from pyrheliometer and pyranometer measurements, atmospheric profiles from the radiosonde measurements, and aerosol optical properties retrieved from sky radiometer measurements were integrated with high-accuracy transfer model calculations. Clear-sky flux was defined from transfer calculations for a pure Rayleigh-scattering atmosphere, with measured temperature and humidity profiles by radiosonde observations. Monthly variation of the clear-sky flux due to the temperature and water vapor variation was 10-30Wm-2. Cloud+aerosol forcing was defined by the difference between the observed flux and the clear-sky flux (positive downward). The annual mean values of the cloud+aerosol surface shortwave forcing was estimated as -81Wm-2, which corresponds to about 24% of the insolation. The aerosol-sky flux is defined with the transfer calculation using the aerosol optical depth retrieved from the sky radiometer measurements. Aerosol forcing was obtained from the differences between the clear-sky flux and the aerosol-sky flux. The mean direct aerosol forcing for 1996, except for March and April, was estimated as -18Wm-2, about 6% of the insolation. We also performed a sensitivity study of the aerosol-sky flux by varying the weight fraction of soot in aerosols. Among the selected soot fraction, the best estimates were obtained as 10% for January, February and July, 20% for October through December, 5% for May, June and August, and 0% for September. These values are close to the measured seasonal variations of soot fraction in previous studies. Surface flux calculation with the retrieved aerosol size distributions performed no better than those with the LOWTRAN 7 urban model size distribution, especially in the summer months when the water vapor column amount was large. The necessity of further examination of retrieval methods of aerosol optical properties, using sky radiometer measurements, was suggested.
The 1997 Indian Summer Monsoon is investigated in the context of interannual variations. It is evidenced that the 1997/98 El Niño, one of the strongest in history, suppressed the large-scale monsoon circulations. On the contrary, however, the Indian summer monsoon rainfall was not much reduced. In fact, it slightly exceeded the climatological seasonal average. An inquiry into circulation and precipitation data indicates that it was the intraseasonal variability of the monsoon system that brought the above-normal rainfall over India. Furthermore, it is shown that the 1997 El Niño not only suppressed the large-scale Asian monsoon circulations, but also produced a convectively unstable area off the east coast of Somalia through the modifications in sea surface and lower tropospheric conditions. Anomalous convection was triggered, amplified over this area, then intruded northward to the Indian subcontinent as it propagated eastward. A moisture budget analysis using 41 years (1958-98) of the NCEP/NCAR reanalysis data set confirmed that the transient part of moisture transport anomaly in the 1997 summer was larger than the stationary one, contributing positively to the small positive rainfall anomalies over India. The 1997 summer appears to be one of the singular cases in the context of the general positive correlation between the monsoon circulation and precipitation indices over the recent 41 years.
When the winter monsoon prevails over the Japan Sea, many shallow convective snow clouds appear in the atmospheric mixed layer, and provide much snow over Japan. The behavior of the snow clouds and airflow affected by a complex terrain in the Tsugaru district of Japan, was studied based on single Dopplerradar observations. At ∼15km upstream of the Tsugaru mountains, which is a low-altitude (200-700m ASL) ridge, a weak-wind zone accompanied by upward developing snow clouds appeared parallel to the mountains. Above and in the lee of the Tsugaru mountains, the tops of the snow clouds were significantly lower towards the east accompanied with wind acceleration. In accordance with a theory of 2-D two-layer flow passing over a ridge, sonde-observed environmental parameters described a flow regime, where the flow is partially blocked and associated with a hydraulic lee jump. The observed flow pattern was consistent to this theoretical prediction, although the hydraulic lee jump could not be conclusively identified due to observational difficulties. Low-level wind convergence in the front of the weak-wind zone sustained the upward development of the snow clouds. The upward development of the snow clouds, however, did not lead to an obvious increase of snowfall intensity. A WSW to ENE elongated zone of increased snowfall appeared from the north of the Shirakami mountains and Mt. Iwaki (both high-altitude mountains with peaks exceeding 1000m ASL), to Mutsu Bay, passing through a corridor of the Tsugaru mountains. The orographic effects of the Shirakami mountains, Mt. Iwaki, and the corridor of the Tsugaru mountains were examined with respect to maintaining this elongated snowfall zone.
The sensitivity of mean monsoon and its variability simulated by Hadley Centre Climate Model to changes in convection scheme, has been examined by integrating the model with and without parameterisation of convective downdrafts. Summer monsoon seasons during 7 years 1982, 1983, 1984, 1987, 1988, 1991 and 1994 have been simulated. These years have been selected for the study as the monsoon rainfall over India showed large interannual variability in these years. Results are compared with NCEP reanalysis circulation and Xie-Arkin precipitation data. Analysis of the model output shows that some of the systematic errors in the model in the monsoon simulation are reduced with incorporation of downdrafts. The systematic error by which the monsoon low level westerlies extend deep into the west Pacific ocean is substantially reduced by the inclusion of downdrafts. The lower tropospheric monsoon flow over the north Indian ocean is strengthened with downdrafts. TEJ simulated with downdrafts is closer to NCEP reanalysis. Inclusion of downdraft improves the precipitation simulation over land areas of India, and also simulates the correct position of the monsoon trough. When downdrafts are not included there is excessive precipitation over the west equatorial Indian ocean and less precipitation over India. Surface latent heat flux increases with the downdrafts. The systematic error of the model which simulates stronger low level circulation in El Niño years than in La Niña years is reduced, though not completely eliminated. The interannual variability of simulated monsoon precipitation is more realistic with downdrafts. Spells of active and ‘break, ’ or weak monsoon situations, are more prominently simulated when downdrafts are included. Larger number of strong synoptic disturbances are simulated over India with the inclusion of downdrafts.
The annual cycle and the large-scale low-frequency variability of surface winds over the tropical Pacific simulated by an atmospheric general circulation model (AGCM) are examined. The AGCM was integrated for 10 years with the observed sea surface temperatures (SSTs) from 1979 to 1988 and the model outputs were compared with National Center for Environmental Prediction (NCEP) reanalysis. The AGCM successfully simulates the climatological features associated with the annual mean and annual cycle of the surface winds. The amplitudes of the simulated annual harmonics are nearly comparable with observations over the south Pacific convergence zone (SPCZ). On the interannual time scale, the AGCM realistically captures two warm events (1982-83 and 1986-87), and one cold event (1988). The first two empirical orthogonal functions (EOFs) and their principal components (PCs) of simulated wind anomalies reproduce the important features, which are crucial for coupled air-sea interaction, like westerly anomalies associated with warm events, eastward propagation of westerly anomalies during 1982-83 and realistic convergence pattern in the first EOF mode over tropical eastern Pacific. Also, the simulated wind anomalies do not exhibit eastward migration during 1986-87 and the strength of anomalies is much larger during 1982-83 than 1986-87. Thus, the AGCM succeeds in reproducing the distinct characteristics of different warm events. It is found that the PCs of wind anomalies are strongly correlated to the PCs of precipitation anomalies. Also, the correlation between PCs of observed and modeled precipitation anomalies is very high (0.93 for both PC1 and PC2) at zero lag. Therefore, it appears that the AGCM's capability to represent the large-scale low-frequency variability of convective heating realistically contributes to its success in simulating the large-scale low-frequency part of surface winds. The most apparent deficiencies in the AGCM simulation include the incorrect location of westerly maximum, and less westward extension of westerly anomalies in EOF1. Also, the mean south-westerlies over Peruvian coast are weaker and not extended up to 170°W, especially during January. To get more insight into the AGCM simulated winds, the response of a steady-state linear Matsuno-Gill type model to two type of forcing (AGCM simulated precipitation anomalies, and AGCM simulated precipitation anomalies+SST anomalies) is compared with AGCM simulated winds. The linear model, forced only by the AGCM precipitation anomalies, can reproduce the AGCM simulated surface winds. The results of the linear model, when it is forced by both AGCM precipitation anomalies and SST anomalies, suggest that the deficiencies of AGCM in the western Pacific are mainly related to the insufficient treatment of surface turbulent fluxes.