In the Taipei basin in winter, surface ozone concentration usually displays a diurnal pattern with double peaks, one in the early morning, the other in early afternoon. The occurrence probability of double-peak days in a particular winter can be as high as 45%. In this study, a field experiment is carried out to test a hypothesis proposed by the authors in a previous paper to explain the formation of the double peaks. The field experiment shows that the double peak days tend to appear after the passage of a coldfront over northern Taiwan. At that time the Taipei basin is characterized by a strong northeasterly monsoon, low temperature, and overcasting sky with occasional light rain. An upstream hilltop ozone station shows that a background ozone of about 30ppbv is carried by the monsoon into the Taipei basin. Effective transport processes replenish the near surface ozone that is otherwise titrated out by vehicle-emitted nitrogen oxide (NO). The diurnal ozone double peaks occur at times when the NO concentration is at its minimum which are the result of the combined effect of diurnal traffic pattern and transport processes.
The present authors have developed a phased array Doppler sodar and participated with it in the International Sodar Intercomparison Experiment (ISIE) at Boulder Atmospheric Observatory (BAO) in 1988. During the period of ISIE, a significant low-level jet (LLJ) was observed in the morning of September 18, 1988. The turbulent structure of an LLJ was analyzed using sodar observations as well as tower observations, and is summarized as follows; (1) Low frequency fluctuations (<0.002 Hz) increase in the time period of LLJ appearance. (2) The turbulent momentum flux in this frequency range shows divergence from the jet core in the developing stage and convergence toward the jet core in the decelerating stage. (3) This shows that turbulence acts as the resisting force to the development of an LLJ.
Space-time variations of atmospheric ozone in the equatorial middle atmosphere are studied for the 8 year period 1979 to 1986, making use of the ozone mixing ratio data derived from the solar backscatter ultraviolet (SBUV) instrument on board the Nimbus-7. Analysis is made with respect to the long-term variation of the stratospheric zonal mean wind and temperature in the tropics. From a simple photochemical consideration, it is expected that ozone variations in the upper stratosphere are associated with temperature disturbances having a characteristic time-scale of longer than a few days. Clear evidence is presented for the appearance of equatorially trapped "ozone Kelvin waves" above the 10 mb level having a zonal wavenumber one component and an eastward migration period of about 7 days. Statistical results for the SBUV ozone data over 8 years shows how Kelvin wave amplitudes are closely related to the semiannual oscillation (SAO) of the zonal mean wind around the stratopause level. Further discussions are made on the effect of the quasi-biennial oscillation (QBO) of the tropical lower stratosphere on the vertical propagation of fast Kelvin waves, which in turn results in the QBO modulation of the mean field in the upper stratosphere and lower mesosphere.
Instantaneous images of the smoke released to the unstable surface boundary layer from a chimney are simulated numerically. The characteristic patterns of instantaneous images are formed by introducing spatial correlation of velocity. Trajectories correlated spatially are obtained by use of random variables correlated spatially in a Markov chain equation. In order to generate the field of random variables correlated spatially, a random number is given for each region divided by a length corresponding to the scale of turbulence. Empirical profiles of the mean and fluctuation of wind velocity are used in the Markov chain. The scale of turbulence is estimated from these profiles. Numerical simulations of the diffusion released from a source of 60 m height were carried out. Under certain atmospheric conditions, some samples of image are formed. The results with irregular patterns seem to be realistic. The spatial correlation of velocity was detected from the results of simulation, which is smaller than that of the given random variable.
The origin of an air-sea coupled disturbance in the Anderson-McCreary model (Anderson and McCreary, 1985) with external land heating west of the Pacific is investigated in detail. It is demonstrated that nonlinearities in the Anderson-McCreary model dramatically change the state predicted by the linear theory. The repetitious generation of the coupled disturbance has nothing to do with the linear U1 mode of Hirst (1988) as well as off-equatorial Rossby waves. It is perfectly determined by the amplitude of the external heating. The mass budget analysis demonstrates that the change of zonal wind direction in the western Pacific, which is due to relative importance between the external land heating and the heating associated with the previous coupled disturbance, modulates the oceanic heat content relevant to the origin of the following coupled disturbance. This mechanism gives the oscillation between two stable equilibria (La Nina and El Nino), which is very different from those described in Battisti (1988), Schopf and Suarez (1987) and Zebiak and Cane (1987). Despite lots of model limitations the present results seem to be compatible with the recent analyses of the air-sea-land system in the western Pacific.
The purpose of this study is to make an exploratory investigation of the interannual variability of winter snow cover in the Northern Hemisphere, using the satellite-derived monthly snow cover data from 1967 to 1987. The data set was supplied in digitized form by the National Oceanic and Atmospheric Administration/National Environmental Satellite Data Information Service (NOAA/NESDIS). An empirical orthogonal function (EOF) analysis is made to find the typical pattern of snow cover variations during winter. The first component of the EOF (EOF1), which represents 46.6 % of the total variance, shows concurrent snow cover patterns between the Eurasia and North America. The time series of the EOF1 is similar to that of total winter snow cover in the Northern Hemisphere. The second component of the EOF (EOF2), which represents 23.6 % of the total variance, shows a negatively correlated pattern between the eastern and western parts of the Eurasia, and also shows a negatively correlated pattern between eastern Eurasia and North America. This EOF2 shows the importance of subcontinental-scale snow variations as a climatic control to a large continent. We examined the time series of mean snow cover for the representative areas depicted in the EOF patterns to investigate the persistency of snow cover in more detail. Snow cover features for a specific years (eg. a heavy or light snow years) are likely to be sustained during December to February and disappear in March. The significant decrease of the snow cover area from February to March in a heavy snow year is prominent in eastern Eurasia. Two key regions were selected which represent continental-scale snow variation: One is the eastern part of Eurasia and the other is North America. The time series of the two key regions show an apparent 1-year lag relationship of heavy snow years; winters with extensive snow cover over Eurasia tend to be followed by relatively heavier snow cover over North America during the succeeding winters.
Annual and QBO-synchronized variations (variations synchronized with the QBO cycle of the zonal wind) of lower-stratospheric equatorial wave activity were investigated by spectrally analyzing the upper-air data at Singapore (1.4N, 104.OE) for the period January 1961 to February 1990. To measure the equatorial wave activity, the quadrature spectra between the zonal wind component and temperature integrated over the period range 7.4-32 days were used for Kelvin waves, and the power spectra of the meridional wind component integrated over the period range 3.3-5.1 days for mixed Rossby-gravity waves. Annual variations in the mixed Rossby-gravity wave activity were found at the 30, 50 and 70hPa levels with a maximum around March. The amplitude was largest at the 70hPa level. For the Kelvin wave, a weak annual variation was found at 70hPa. QBO-synchronized variations were found in the Kelvin wave activity at the 30, 50 and 70hPa levels. A QBO-synchronized variation of mixed Rossbygravity wave activity was found at 30hPa, but was unclear at the 50 and 70hPa levels. Annual variations of mixed Rossby-gravity wave activity in the 50-70hPa layer appear to be related to a rapid descent of the easterly wind regime during March-June and a slow descent in July-February during the layer 30-50hPa layer.
The meridional transport of quasi-geostrophic potential vorticity is calculated for the Northern and Southern Hemispheres. The results show that the transport of potential vorticity is negative over most of the atmosphere except over a thin layer near surface where it is positive. Exchange coefficients for the transport of potential vorticity are computed for both the hemispheres using the evaluations of the transport of quasi-geostrophic potential vorticity and calculating the meridional gradient of potential vorticity. The values of the exchange coefficients are used in a simple, two-level quasi-geostrophic numerical model for tonally averaged conditions. The model is applied to the Northern and Southern Hemispheres and the general characteristics of mean annual temperature and zonal wind variation are reproduced reasonably well.
In most general circulation models, the "maritime continent" is poorly resolved because there are many islands too small to be represented in the model. We investigate a possible effect of these small islands on the atmospheric general circulation by introducing surface drag over islands in model's ocean grid points. It is shown that the increased drag coefficient in the maritime continent is very effective in increasing the precipitation there by inducing stronger moisture flux convergence. Walker-type circulation anomaly is distinct over the entire equatorial Pacific along with the westerly anomaly in the lower troposphere over the Indian Ocean. There are also large impacts on both the Asian and the Australian monsoon not only for their strength but also for the onset/duration. These results imply the need of carefully handling surface conditions around the maritime continent where moisture flux convergence/heating feedback is strong, by considering the surface heterogeneity.