Diurnal variations of cloudiness over East Asia and the western Pacific are investigated for the warm season of 1987, based on GMS-3 IR data at every three hours. The domain chosen for the present analysis is between latitude 50°N and 20°S and longitude 90°E and 160°E. The main results obtained are:(1)Diurnal variations of cloudiness result from superposition of large diurnal-cycle and small semidiurnal-cycle variations. Amplitude and phase of the diurnal-cycle variation are much different over land and ocean. Semidiurnal-cycle variations over land and ocean are almost in phase with maxima at 0300∼0500 LT and 1500∼1700 LT, and have similar amounts of amplitude.(2)A systematic phase delay of diurnal-cycle variation appears to the east of the Tibetan Plateau. A maximum of cloudiness appears near dusk over the Tibetan Plateau and at midnight in the Sichuan Basin. Phase speed of the variations is almost the same as in the eastward phase propagation of the diurnal-cycle variation of precipitation frequency east of the Rocky Mountains in the United States. Eastward movement of cloud clusters generated over the Tibetan Plateau, as well as locally induced convections, should be taken into account to understand the behavior of diurnal variation of cloudiness to the east of the Plateau.(3)The eastward phase delay of the diurnal-cycle variation to the east of the Tibetan Plateau is obscured during mid-summer. This may be caused by locally induced active convections intensified near dusk to the east of the Plateau, and seasonal disappearance of upper westerly flow passing over the Plateau.
We have examined the shallow water flow initialized by the pseudo-secant method proposed by Rasch(1985). We develop his scheme for a regional model in the plane geometry, and investigate the solution to a mountain forcing in the shallow water equations. We find that this scheme gives the unphysical solution with fore-aft symmetry, which means the ingoing steady gravity wave from upwind to the mountain, in the super-critical flow, if the diffusion term is too small. In case of low mountain, comparing numerical solutions with the analytical one, we conclude that the above mentioned solution was given improperly by the wrong radiation condition for the gravity waves from the mountain. A new scheme is proposed so that the flow fields past mountain are retained. In the new scheme, to specify the radiation condition we insert a small imaginary, the value of which is determined by the resolution in the model wave number space, into the effective angular frequency. Moreover, we take into account the large contribution from the modes with small effective angular frequency.
The sensitivity of the stratospheric circulation simulated by an atmospheric general circulation model(UCLA AGCM)to modifications in the parameterization of several key physical processes is investigated. The focus is on the northern hemisphere winter circulation simulated by the troposphere-stratosphere version of the model. The parameterization of ozone mixing ratio is first investigated by comparing multi-year simulations with distributions of ozone mixing ratio that are either prescribed according to an observed climatology, or predicted using a scheme with a simple photochemistry formulation. The prescribed ozone mixing ratios produce a considerably more realistic circulation, although the results are degraded in some regions of the stratosphere. This leads to a closer look into the radiation parameterization. Several simulations for the northern winter season are performed with adjustments to obtain more realistic radiative cooling rates mainly in the upper stratosphere. It is found that the simulation of the stratospheric polar night jet is very sensitive to these adjustments. Additional experiments performed by adding Rayleigh friction and Newtonian cooling reveal a similar sensitivity. The results of these experiments emphasize a need for caution when ad-hoc adjustments are applied in the model. They also confirm that the performance of an AGCM in the stratosphere can depend on several different model aspects, and that this dependence is not always straightforward. In addition, the authors argue that their results support the notion that parameterizations of orographic gravity wave drag that neglect or underestimate the drag in the troposphere may overestimate the drag in the stratosphere.
Using the multiple linear regression method and the standard back-propagation neural network, tropical cyclone intensity prediction over the western North Pacific at 12, 24, 36, 48, 60, and 72 h intervals is attempted. The data contain a 31-year sample of western North Pacific tropical cyclones from 1960 to 1990 and eight climatology and persistence predictors are considered. The percent of variance explained by the neural network model is consistently larger than that explained by the regression model at all time intervals with an average difference of 12%. The average intensity prediction errors from the neural network model are 10-16% smaller, except at 12 h where the errors are nearly equal, than those from the regression model. This study clearly shows potential of the neural network in the prediction of tropical cyclone intensity.
The verification of the rainfalls predicted by the 10 km-resolution non-hydrostatic mesoscale model of the Meteorological Research Institute(MRI-NHM)was performed during the 1996 Baiu season. The verification was also compared with the results of the 10 km-resolution hydrostatic Regional Spectral Model(RSM) of the Japan Meteorological Agency(JMA). The warm rain scheme that explicitly predicts cloud water and rainwater is employed in MRI-NHM, while two parameterized convective schemes are used in conjunction with large scale condensation in RSM. The MRI-NHM greatly overestimated the maximum of precipitation intensity and the precipitation area of heavy rainfall(≥20 mm h-1), slightly underestimating the area of weak rainfall(∼1 mm h-1). The statistical scores showed that the MRI-NHM more accurately predicted the rainfall over 10 mm h-1 than the RSM, while the prediction of MRI-NHM is not as good as that of RSM for less than 5 mm h-1 rainfall. The MRI-NHM could hit almost half of the rainfalls over 20 mm h-1. The rainfall was more accurately predicted in the southern region of Kyushu than in the northern region. This is resulted from the difference between the heavy rainfall formation mechanism in the northern and southern regions of Kyushu.
The general problem of retrieving the initial conditions in coupled atmosphere-ocean models by the adjoint data assimilation method was formulated. For a simple coupled equatorial model, where the atmosphere and the ocean were each represented by a linear shallow water model, retrieval of three oceanic initial conditions(the sea level height[SLH]and the two horizontal current components)was tested with identical twin experiments. Wind and SLH data, generated from a 90-day unstable local-growth simulation of a warm event, were assimilated to test the effects of(i)data type and sparsity, (ii)initial guess, and(iii)noisy data on retrieving the oceanic initial conditions. SLH data were found to be more efficient in retrieving the oceanic initial conditions than the wind data, and the initial SLH field was more accurately retrieved than the initial currents. The retrieval of the initial current fields was sensitive to the temporal density of data, especially with wind data, where once a day would be the minimum density needed. As the initial guess of the oceanic state could contain errors in magnitude and phase(i.e.location)of the warm event anomaly, data assimilation was found to readily correct the error in the magnitude of the initial guess, but not the large phase error. Assimilation of noisy data showed that the retrieval of the initial conditions was far more sensitive to noise in the SLH data, than in the wind data.
The evolution of the summer monsoon over Asia during June 1994 is simulated using a regional climate model based on the Penn State-NCAR mesoscale model MM5. The National Center for Environment Prediction(NCDP)analysis are used as the initial and lateral boundary conditions, and the integration extends up to one month. The model is capable of reproducing the monthly mean features of the monsoon circulation and total rainfall in June 1994. The changes of the large scale circulation during the evolution of summer monsoon are also well simulated, which include:(1)the northward shift of the upper westerlies over the Tibetan Plateau and the development of upper easterlies south of the Plateau;(2)the extension of low level southwesterly monsoon flow from South China Sea to eastern China and the sequential jump of the rain belt from south to north;(3)the northward shift of the mid-tropospheric western Pacific subtropical high(WPSH) at 500 hPa, and the establishment of the upper tropospheric high pressure over the Tibetan Plateau at 200 hPa. In order to examine the impact of convective heating on the evolution of the monsoon, an experiment without convective heating is conducted. The results show that the convective heating is crucial for:(a)the development of the low-level southwesterly monsoon;(b)the northward shift of the upper-level westerlies over the Tibetan Plateau; and, (c)the position and intensity of the western Pacific subtropical high.
Recent studies based on aircraft observations made over ocean by radar and multi-channel passive microwave radiometer, each with a field of view(fov)of a few kilometers, indicate significant problems in relating the radar derived rain rate with the brightness temperature measurements of the microwave radiometer. We arrive at a similar conclusion from extensive observations of rain rate made by ship-borne radars and rain rate deduced from observations of the satellite-borne, multi-channel Special Sensor Microwave Imager(SSM/I) radiometer, which has a much larger fov(∼30 km). The principal reason for these problems is that the signal due to rain drops contained in the radiometer measurements is non-linearly mixed with that of other hydrometeors that could be present in the radiometer fov. These other hydrometeors include liquid droplets, and dry and melting ice and snow particles of different densities, sizes, and shapes in clouds. Observations made by SSM/I are not adequate to uncouple the rain signal satisfactorily from such a non-linear mix of signals on the scale of the radiometer footprint. These problems are complicated further by meteorological conditions, which can significantly alter the amount and spatial distributiion of these other hydrometeors. For these reasons an empirical method is developed to estimate area-average rain rate in a mesoscale region of about 300×300 km2, based on SSM/I data. One parameter of this empirical method fR relates to the fractional rain area in a mesoscale region, while the second parameter, χ, reflects in a weak way the scattering and emission properties of the hydrometeors in that region. This method requires tuning with the help of radar data. Over the TOGA-COARE area, the rain rates retrieved from this method can reproduce the radar observed rain rates with a correlation coefficient of about 0.85. Furthermore, monthly total rainfall estimated from this method for the TOGA-COARE area has an error of about 13 %. This rain retrieval method is applicable to the Tropical Rainfall Measuring Mission(TRMM), in which multi-channel dual-polarization microwave radiometer observations over a 760 km wide swath are overlapped with those of radar, which has a swath of 220 km.
The three-dimensional response of a slowly-rotating atmosphere to mobile heating is investigated by using a multi-layer primitive equation model on the sphere. The velocity of planetary rotation and that of solar heating are fixed at the values of Venus. The global circulation is assumed to be induced only by the solar radiation absorbed by the ground. The global circulation induced in this way can produce an atmospheric stability by the upward heat transport. In order to examine this nonlinear effect, the model is numerically integrated from a state of neutral stratification. It is found that the direct circulation between the day and night sides predominates always for standard parameter values. The temperature maximum point on the equator is deviated from the subsolar point by 60°. The horizontal temperature contrast between the day and night sides is very small(0.16K in the lowest layer). The atmospheric stratification produced by the global circulation is also too weak even in the night side. In the experiments with one solar day assumed to be 1/2 or 1/4 of Venus' solar day(117 days), the zonally uniform meridional circulation appears in place of the direct circulation.
The variability of the hydrological cycle for arid/semi-arid regions is important, because desertification is occurring in these regions. Even in the arid/semi-arid regions in the interior of the Eurasian Continent, heavy precipitation sometimes occurs. However, the relationship between water vapor transport and precipitation has not been clarified yet. In this study, water vapor transport and flux divergence in the arid interior region of the Eurasian Continent were investigated using the objective re-analysis data provided by the European Centre for Medium range Weather Forecast(ECMWF) for a five-year period(1980-1984). Through the analysis of the vertically integrated water vapor transporting mean summer fields, it is clarified that Mongolia and the northern part of China receive water vapor from the northwest. One of the water sources for these regions is located over, and to the west of Western Siberia. In the lower troposphere, most of the water vapor is transported to the Taklimakan Desert from the northwest along the eastern periphery of the Tianshan Mountains in the mean summer state. The daily summer water vapor flux fields around the Taklimakan Desert for a five-year period were analyzed in relation to precipitation there. The Taklimakan Desert is one of the most arid regions in the Eurasian Inner Continent. Here, the daily mean water vapor flux patterns are classified using cluster analysis. The 460 maps prepared during the investigation are first classified into eight general patterns. Precipitation and atmospheric circulation patterns compositted by these clusters are then compared. Over 90 % of the total cases resemble the summer mean water vapor flux pattern, and northwesterly moisture flows prevail. We found that the southerly water vapor flows, which pass over the Tibetan Plateau and along the eastern periphery of the Plateau in the lower level, are related to heavy precipitation over the Taklimakan Desert. The simultaneous existence of a southwestward extending through located to the north of this region, and the ridge located in Central Asia, is peculiar to the atmospheric circulation pattern of these cases. Although such situations appeared in up to 10 % of the total cases, they tend to occur mostly in the wet years(1981, 1984), and account for about half of the precipitation in those wet years.
Using an atmospheric general circulation model(JMA89), basinwide water balances of 27 major rivers were estimated and compared with actual observations. Two different deep cumulus convection schemes were used in the 10-year integrations from 1979 to 1988 that were forced by the observed global sea surface temperature boundary conditions. Results show that the Prognostic Arakawa-Shubert(PAS) scheme can simulate more realistic global distributions of annual mean precipitation than the Kuo scheme. In both cases, simulated annual mean precipitation, and simulated runoff coincides with the observations fairly well. The differences in precipitation or runoff between model simulations and observations are caused by the land surface hydrology and other parts of the model, and these two factors cannot be distinguished from each other. To check the reliability of the parameterizations in land surface hydrology in a simple biosphere(SiB) model, simulated runoff ratio(runoff/precipitation)is compared with the observations for each river basin. There are considerable difference between simulations and observations for runoff ratios. Differences in runoff ratios(observations minus simulations)for each river basin are significantly negative because when compared to the observations simulated runoff has been considerably overestimated. Simulated runoff has been greatly overestimated in most river basins where the observed runoff is less than 200 mm/year and the runoff ratio is less than 0.2. It is demonstrated that the parameterizations of land surface hydrology in the SiB model tend to overestimate the runoff or to underestimate the evapotranspiration in dry regions.