Middle-level stratiform clouds associated with cyclones were observed in spring 1985 simultaneously by a 19.35 GHz microwave radiometer and an 8.6mm-radar. The vertically integrated liquid-water content was estimated by analysing the data of the microwave radiometer together with upper-air sounding data, and the vertically integrated ice content was estimated from radar-echo intensity. Structure and precipitation formation in middle-level stratiform clouds were studied, especially their degree of glaciation which is defined by the ratio of vertically integrated ice content to the total amount of condensed water (the sum of vertically integrated liquid-water and ice). Analysed cases are divided into two types of clouds (type-A and type-B). Type-A clouds showed a low degree of glaciation, a large amount of integrated liquid-water, and a comparatively thin and weak radar-echo layer only at middle levels. The degree of glaciation in three cases was slightly less than 10% and an average and the total amount of condensed water was about 55mg/cm2 . Type-B clouds had a deep and intensive radar-echo layer, whose tops were far above middle levels. The degree of glaciation in three cases was about 65% on average and the total amount of condensed water was about 40mg/cm2. The degree of glaciation was significantly different between the two types, though total condensed-water amounts were comparable. The structure and precipitation formation processes of the two types of clouds can be summarized as follows. The type-A cloud is a one-layer middle-level cloud. Ice particles are initiated near its top where the air temperature is not low enough to allow a sufficient number of ice-nuclei to be activated. The glaciation process is not efficient though the supercooled water content is large due to strong updraft in it. On the other hand, the type-B cloud is composed of an upper-level cloud and a middle-level cloud which seem to have formed independently. Ice particles supplied by the upper-level cloud promote the glaciation in the middle-level cloud. By comparing glaciation proceses in two types of clouds we showed quantitatively that only a middle-level stratiform cloud could not produce solid precipitation efficiently, even though it contains a large amount of super-cooled water, unless an upper-level cloud supplies sufficient ice particles to it.
Maps for ship icing potential distribution over the oceans are reviewed. Special attention is paid to maps presenting forecast icing rates on ships. The NOAA Experimental Ice Accretion map with a 24-hour categorical forecast of ice growth rate and the AES approach to mapping icing intensity are evaluated. A new mathematical model for the growth of spongy saline ice on a ship's superstructure is presented in detail. This model calculates the icing rates over the entire front face of the ship's superstructure within the zone of spraying. The effect of the salinity of the moving water film and the ice sponginess are both taken into account. The model input is the ship speed and heading, the air temperature, the seawater salinity, the sea surface temperature, and the wind speed and fetch. Computer-produced maps of the icing rates on the ship are produced for the cold waters east of Canada. The first results indicate that our new ship icing model could be applied for operational purposes (hindcasting and forecasting icing rates on ships) if an automated data acquisition system was available.
The first two principal components of the North American summer temperature field are examined in reference to the antecedent oceanic and atmospheric conditions of the Northern Hemisphere, and are shown to have considerable prognostic value. The planetary scale circulation associated with the thermal field over the western tropical Pacific and the ocean-atmosphere interaction over the central North Pacific-Aleutian region appear to be important in determining the summer surface temperature pattern over North America. The examination of the cross-correlation patterns and their temporal variations indicates that the anomalous SST fields in the North Pacific in the preceding seasons are closely related to the temperature field in North America in the following summer. The negative anomalous departures in the central North Pacific SST precede the high first component of the North America summer temperature field, and the positive departures precede the low first component. For the dominance of the first component in the mid-western United States, this directly infers the predictability of the summer temperature in the region. The signal of anomalous SST departures appears early in the preceding fall, reaches its maximum in the winter and spring, and diminishes toward the summer. An El Nino mode is also detected for the North American summer temperature, indicating the impact of ENSO on the general circulation beyond the winter half of the year.
Hourly and annual variations of the mixed layer characteristics were obtained by coupling the temperature profile of routine aerological data to near-surface hourly temperature and dew-point temperature, using a practically modified enchroachment mixed-layer model. Net radiation changes sign from minus to plus one or two hours after sunrise. This time is regarded as the effective sunrise when the mixed layer starts to develop. In most months, the mixed layer develops slowly at the beginning and then increases linearly till around noon. The Bowen ratio is unity during daytime in May. A large proportion of the net radiation is consumed by the surface latent heat flux during the summer season. During the winter season, the surface latent heat flux is larger than the surface sensible heat flux in the morning, but decreases around noon. The convective velocity scale W∗ increases parabolically, and continues to increase up to early afternoon all the year. Relative humidity has a peak value at the effective sunrise. On the other hand, specific humidity has a peak one or two hours after the effective sunrise, and after the peak it decreases gradually till 14h-15h local time due to development of the mixed layer. The diurnal patterns of the specific humidity are similar during each month, but decrease rate of the specific humidity in the daytime from July to September is small. By considering the water vapor budget in the mixed layer, the specific humidity profile in the upper stable layer at the effective sunrise estimated.
Tropospheric and stratospheric air samples have been collected over Japan (33-38°N) since 1978, and analyzed for CF2Cl2, CFCl3 and N2O by a GC-ECD method. Mean volume mixing ratios in the troposphere were 390ppt for CF2Cl2, 238ppt for CFCl3, and 308ppb for N2O in November 1986. Those of CF2Cl2 and CFC13 were increasing at almost constant rates, i.e., 13.5ppt/year for CF2Cl2 and 9.5ppt/year for CFCl3 between January 1979 and November 1986. Those of N2O appear to be increasing at a rate of 2.0ppb/year between December 1982 and November 1986. Volume mixing ratios of CF2Cl2, CFCl3 and N2O in the lower stratosphere (15-28km) decreased with increasing altitude in agreement with the vertical profiles calculated from a one-dimensional photochemical-diffusive model.
A modified scheme of OIM - a zero-guess scheme - for direct estimation of the total value of meteorological fields at grid-points, independently from dynamical assimilation models, is developed and successfully tested over the sparse-network tropical region. In its formulation, the random empirical error' (observational error + small scale noises) is shown to have a remarkable influence not only on other statistical parameters used in the scheme, but also on the weighting factors, the interpolation error, and therefore the grid-point values themselves. Effective stations for each grid-point are selected using a search algorithm designed to fit the spatial structure of the statistical properties of the fields. A simple and steadily computable expression of the overall error at a grid-point is given, and the positive effects of low-pass filtering the data on this error is demonstrated. The 5-year mean seasonal statistics obtained with these grid-point data show a significant consistency with both the observation and the existing statistics for which data from various sources were used in order to supplement the sparseness of the tropical observational network. The generalization of this new scheme in order to use data from various sources, its applicability to climatological studies in the extratropics or its possible use in NWP assimilation schemes are to be investigated, keeping in mind that realistic estimates of statistical parameters should be used for the analysis.
Force restore treatment for the prediction of the surface volumetric soil water content proposed by Deardorff (1977) was applied to the precisely measured data of evaporation from the bare surface and soil water content close to the surface. Parameters C1, C2 and wb involved in the force restore method equation were investigated for the Kanto loam at Tsukuba, Japan. The results show that C1 monotonically decreases with an increase of the surface volumetric water content ws, and this relation is similar to that proposed by Deardorff (1978) for the Adelanto loam. The parameter C2 is not related to ws. It is suggested that C2 be taken as about 2 to 4 for the Kanto loam. It is shown that taking the soil water content at the depth of 1cm around 06JST as wb is effective for the further simplification of force restore treatment. Combination of the parameterization by the force restore method with the bulk method for the turbulent flux using the concept of surface moisture availability gives a good prediction of diurnal variations of evaporation and surface soil water content.
Interannual variations of winter mean precipitation and upper-tropospheric stationary eddy and the influence of tropical sea surface temperature (SST) on their variability are investigated. The data used are obtained from a series of 15-year general circulation model (GCM) integrations with (the SST runs') and without (the control run') interannual variations of tropical Pacific SST observed during a 15-year period. By comparing the variability appearing in the SST runs to that of the control run, it is shown that the fluctuation of tropical Pacific SST accounts for a large fraction of precipitation variability in localized tropical regions, particularly over the central Pacific and Indonesian subcontinent, and the variability of upper-level stationary eddy over the subtropical central Pacific in both hemispheres. However, the amplitudes of variability in the extratropics are not influenced by the SST. The east-west seesaw between fluctuations of precipitation over the equatorial central Pacific (cp) and the tropical western Pacific (wp) and the induced anomalous Walker circulation owe their existence mainly to the SST fluctuation over cp. Both precipitation anomalies over cp and wp in the SST runs induce the circulation anomaly in the Pacific/North America (PNA) region, but the stronger response results from the precipitation anomaly over cp. Among the tropical regions, the central Pacific is the region where SST has the strongest impact on the large-scale fluctuations of tropical precipitation and midlatitude circulation over the PNA sector. In the control run, however, tropical precipitation anomalies over cp and wp do not result in any organized large-scale circulation pattern in the midlatitudes.
Three schemes for nonhydrostatic models with orography incorporated, consisting of an-elastic (AE), elastic horizontally and vertically implicit (E-HI-VI) and elastic horizontally explicit, vertically implicit (E-HE-VI), are compared in their formulation and performance. If sound waves are damped enough in the elastic models, the results of these schemes are similar, in spite of the use of the flux form of the advection term in AE. Efficient damping methods of sound waves are examined, and a linear analysis is made of their stability, where both sound and gravity waves are taken into account simultaneously. An iterative application of a direct method for solving the elliptic equation of pressure needed for the AE and some E-HI-VI schemes appears to be accurate. One application of a direct method increased the central processing unit (CPU) time by about 15%, which is comparable to the CPU time required for a small time step integration of the E-HE-VI scheme. For the E-HI-VI scheme, a partially implicit method was found to need no iteration in solving the pressure equation for the case of a low or moderate mountain, since the erroneous forcing at the boundary is practically negligible. However, for the case of a high and steep mountain, the method needs iteration to get rid of the erroneous forcing at the boundary which is no longer negligible. This method is found to be unstable for the case of a very steep mountain.
The importance of stationary and travelling planetary-scale waves for southen hemisphere blocking has been investigated. The data used are 500-mb geopotential height data from nine winter seasons. The first three zonal harmonics of these data were decomposed in a low-pass filtered quasi-stationary part, and a travelling part being the difference between the unfiltered and filtered values. The behaviour of the planetary-scale waves during periods with blocked flow is documented. It is found that unlike blocking in the northern hemisphere there is no systematic behaviour of the ultra-long waves during southern hemisphere blocking.
The relationship between the asymmetry and movement of storms is investigated using the numerical simulation of a model cyclone. Subtracting the symmetric vortex from the total flow yields the asymmetric wind. In the asymmetric wind field in the lower converging layer, there are two eddies. Looking downwind of the mean wind, the cyclonic eddy is found on the left and anticyclonic eddy on the right. The inner areal mean wind averaged within a 1.8° latitude radius in the lower convergint layer is directed to the left, while the outer areal mean wind averaged over the band from 1.8° to 9.5° latitude radii is directed to the right of the direction of movement during the developing period. Both of these mean winds during the decaying period are nearly in line with the direction of movement of the vortex.