o examine the impacts of increased vertical resolution in the stratosphere of a numerical weather prediction model, 30-day forecast experiments starting from 14 initial conditions for the Northern Hemisphere winter were carried out using a control model and a high-resolution model. It was found that forecast skill for the stratosphere was increased by increasing the vertical resolution. In particular, the climatological Aleutian high was much better simulated. However, zonal mean wind was not improved as a whole due to an easterly bias of the polar night jet of the high-resolution model. Although no clear impacts were seen in the forecast skill of daily 500 hPa height, the prediction of l0-day mean 500 hPa height was improved after around Day 15. In particular, low-frequency modes on the Pacific side, such as the Western Pacific pattern and Eastern Pacific pattern, were better predicted. Systematic errors in the troposphere were not much changed. The forecast for the zonal wavenumber-1 component of the tropospheric circulation was not improved in spite of the improvement of the same wave component in the stratosphere. The reduction of prediction error of the tropospheric wavenumber-2 component after around Day 15 was followed by that of the corresponding stratospheric component with a lag of 1-2 days. These results suggest that the stratospheric impact on the troposphere in terms of forecast skill results from local interactions between the troposphere and the stratosphere. They also indicate that higher vertical resolution in the stratosphere is necessary for better forecasting of tropospheric low-frequency variabilities.
A new objective scheme has been developed for interpreting weather from direct output of numerical weather prediction models. The scheme produces seven categories of weather types : clear, fair, high cloud Overcast, cloudy, rain, snow and rain/snow mix, from predicted cloud amounts, one-hour accumulated precipitation amount, surface temperature and relative humidity. The scheme is applied to the JMA's operational mesoscale model and is verified against observed weather. The accuracy of the scheme is comparable with a 6-hour persistence forecast. This indicates a sufficient capability of the scheme for objective weather interpretation. Besides, weather distribution charts derived by the scheme have advantages in representing mesoscale disturbances compared to the traditional output of numerical models such as sea level pressure or 500 hPa height field. The precipitation-type forecast also demonstrated a good performance.
Interdecadal variations of precipitation over the tropical Pacific and Indian Oceans and their relationships with SST are investigated by using surface and satellite observation data. The analyses of surface precipitation indicated that the precipitation in the tropical central-eastern Pacific was increased during the period from the mid 1970s to the 1980s in accordance with the increase of SST in these regions. On the other hand, the precipitation in the tropical western Pacific was rather decreased during this period probably due to changes of tropical east-west circulations affected by the enhanced convection in the central-eastern Pacific. The precipitation in the South Indian Ocean was increased since the mid 1970s corresponding to the increase of SST over the Indian Ocean, but it was nearly constant in spite of gradual increase of SST before that period. Monthly mean high-cloud amount data derived from the GMS-IR observations during 16 years from April 1978 to February 1994 are analyzed to obtain long-term trend of the convective activity in the tropical western Pacific. The computation of the linear trend of the high-cloud amount for the 16 years reveals that decreasing trends dominate over most of the tropical western Pacific, but increasing trends can be seen in the equatorial central Pacific around the dateline. These results suggest that El Nino-like environmental conditions such as enhanced convection in the central Pacific, but suppressed convection in the western Pacific prevail over the tropical Pacific during recent decades. These findings are consistent with previous studies demonstrating that there occurred decadal scale changes in atmosphere-ocean systems in the tropical Pacific during the 1970s-1980s period. The results in this study support the scenario suggested by Nitta and Yamada (1989) that large atmospheric anomaly circulations in the North Pacific Ocean appeared in recent decades might be generated by the enhanced convective activity in the tropical central-eastern Pacific due to the increase of the tropical SST.
The mixed distribution, a mixture of the discrete mass at the origin and the continuous distribution, conditional on rain, is adopted as a rain rate distribution. The kth moment of rain rate is linearly dependent on the probability that rain rate exceeds a fixed threshold level. The paper provides an asymptotic (large-sample) property of the maximum likelihood (ML) estimator of the proportionality constant, called slope. It numerically shows that the ML estimators of the slopes for the first and second moments are mutually asymptotically almost independent for the GATE I parameter if the thresholds are chosen to be optimal in the sense of minimizing the normalized asymptotic variances. The property provides a reason why the optimal thresholds for estimating the first and second moments of area rain rate should be used for estimating the variance by the double threshold method. The paper also proposes a method of choosing thresholds simultaneously for the estimation of the first and second moments of area rain rate.
Significant relationships between atmospheric circulation anomalies in the Northern Hemisphere, in terms of 5-day mean 500 hPa geopotential height (Z500) and sea level pressure (SLP), and nonseasonal variations of 5-day rainfall (R5) over eastern China, where the influence of typical Asian monsoon prevails, are statistically examined by canonical correlation analysis (CCA) with time lags of 0 to a half year (36 Pentads). At a confidence level of 99. 99%, the CCA yields eight significant pairs of patterns that describe simultaneous responses of the precipitation field to anomalies in the middle troposphere. For the SLP-R5 fields, seven pairs of simultaneous canonical correlation patterns are found at the same confidence level. Most of these CCA pairs of patterns are in good agreement with well-known characteristic large scale patterns of circulation responsible for main rainstorms over China. From the physical point of view, these statistically significant correlation pairs of patterns are very reasonable. Possible synoptic processes associated with such relationships are figured out. In particular, the relationships represented by the first four robust concurrent CCA modes of the Z500-R5 fields, have been discussed in great detail and confirmed by a composite analysis. Also, significant (99.99%) canonical correlation pairs of patterns with lags up to 3 pentads for the Z500-R5 fields or the SLP-R5 fields are detected, presenting some antecedent circulation modes associated with certain rainstorms over China. Interestingly, some CCA pairs of patterns at different time-lags display the same or quite similar rainfall distributions over China, therefore exhibiting the evolutionary process of circulation systems responsible for the corresponding rainfall pattern. Those patterns are thus of crucial importance for long-range forecasting of Asian monsoon activity and hence of precipitation over China. Moreover, it is worth pointing out that some CCA patterns of sea level pressure (SLP) field for different time lags are significantly correlated with a rainfall distribution that is also robustly related with certain midtropospheric circulation regimes. These are indicative of the associated vertical structure of the circulation systems in question.
Temperature and horizontal wind fluctuations with periods shorter than 3 days in the equatorial lower stratosphere are examined by using data from routine rawinsonde observations at Singapore (1N, 104E) during 1978-1993. Internal wave-like structures having a period of about 2 days and a short vertical wavelength of 5 km are occasionally observed both in temperature and wind fluctuations. The result of power spectral analysis indicates that short-period fluctuations have significant energy, separated from Kelvin waves and mixed Rossby-gravity waves. Long-term variations of the spectral characteristics are investigated in relation to the quasi-biennial oscillation (QBO). Zonal wind and temperature variances due to the short-period fluctuations are generally large and particularly enhanced in the QBO phase when the direction of mean wind changes from easterly to westerly. Co-spectra of temperature and zonal wind fluctuations at short periods are significantly larger than the quadrature spectra, and the sign of co-spectra changes according to the phases of the QBO. These results indicate strong connection of the short-period disturbances with the QBO.
The effects of the direction of the convective-cell arrangement on the efficiency of “broken-line formation" of a squall line has been studied through numerical experiments. As fundamental experiments, small thermals were established along a line at a constant interval, and their evolutions were examined by changing the line direction relative to the vertical shear vector. The results showed that the broken-line formation occurred most efficiently when the line had a certain angle with the shear vector. The analysis suggested that the key factor controlling this efficiency is the vertical shear component normal to the line. When the component is too large, the downdrafts from pre-existing cells disturb the formation of the band-shaped updraft of a squall line. When the component is too small, a squall line cannot form easily because the low-level convergence is too weak. Broken-line formation most efficiently occurs when the component is in the middle range. Similar results were found when the shear contained veering or when the cell interval was small. As a more realistic case, we put initial thermals at random locations. In this case, squall lines formed frequently in the direction along which broken-line formation occurred efficiently in the fundamental experiment. These results suggest that there is some favorable orientation for an efficient broken-line formation, and that squall lines have a tendency to appear along that direction.
The horizontal structures of circulations induced by mobile global heating are investigated by using the linearized shallow water equations on the sphere. The parameters involved in this system are the Lamb parameter (i. e., the parameter representing the effect of planetary rotation), the velocity of heat source and the damping rate (the coefficient of Rayleigh friction and Newtonian cooling). Numerical solutions for a wide range of these parameters are obtained by using the eigenmodes of shallow water equations (Hough functions). The circulations obtained are classified into the following four types. Type I is a direct circulation between the day and night sides. This type appears when the motion of heat source is slow and the damping rate is large. Type II is a “Gill pattern"-like circulation ; its horizontal flow pattern is similar to the “Gill pattern" obtained by Gill (1980), but it extends to the extratropics. This type appears when both the motion of heat source and the damping rate are small. Type III is a zonally uniform circulation. This type appears when the motion of the heat source is considerable. Type IV is a circulation characterized by an inertio-gravity wave. This type appears when the resonance of an inertio-gravity wave with mobile heating occurs. This resonance is conspicuous when the damping rate is small. In the regime diagram (in the parameter space), the region of Type IV exists between those of Types II and III.
To evaluate simulations and theories of equatorial Kelvin and mixed Rossby-gravity (MRG) waves, and to gain insight into their generation mechanism and role in the quasi-biennial oscillation, a space-time spectral analysis is performed on output data from the 40-level, three-degree latitude GFDL "SKYHI" general circulation model and on the GFDL FGGE dataset. The SKYHI and FGGE stratospheric Kelvin waves are dominated by an eastward-moving, wavenumber-one, 10-20-day period component in the lower stratosphere. These waves are accompanied by higher wavenumber-frequency components, which can be detected more clearly in the upper stratosphere than in the lower stratosphere. On the other hand, the SKYHI and FGGE MRG waves are dominated by a westward-moving, wavenumber 3-5, 4-6-day component in the lower stratosphere. These waves are dominated by lower-wavenumbers (1-2) and shorter periods (2-4 days) in the upper stratosphere. The amplitudes of the SKYHI/FGGE Kelvin and MRG waves are comparable to those estimated from observed (non-FGGE) station data, whereas the SKYHI model produces only a very weak quasi-biennial oscillation. The SKYHI precipitation data intermittently exhibit grid-size pulses of precipitation, but do not clearly exhibit spectral peaks which correspond to Kelvin and MRG waves. On the basis of the present analysis, it is proposed that Kelvin, MRG, and gravity waves result from wave-convection interactions and are intermittently triggered by random pulses of convective heating. It is speculated that the quasi-biennial oscillation is produced primarily by gravity waves and will increase in amplitude with horizontal resolution, as grid-size pulses of convective heating and small-scale gravity waves are more adequately produced in the model.
The time-space structures of long-term trends and the decadal-scale variations of seasonal temperature and precipitation over China and Mongolia are investigated by using the 5-year moving averaged data from 1951 to 1990. An empirical orthogonal function (EOF) technique is applied to seasonal temperature and precipitation. The changes of atmospheric circulation patterns with these trends, and long-term fluctuations are deduced by using 500 hPa height and surface pressure fields. The correspondence of these components with the Northern Hemispheric surface air temperature (NHT) is examined. The increasing linear trend in the annual mean temperature is remarkable, especially in the northern part of China and Mongolia, while the decreasing trend is conspicuous in the area from Sichuan to Yunnan Province. The increasing trend in the annual mean temperature is due mostly to the anomalies in winter and spring. These trends in winter temperature are directly related to the decadal-scale change of the locations of Siberian High and mid-tropospheric trough over Eurasia, and are associated with the hemispheric circulation changes. The annual precipitation, in contrast, does not show a clear linear trend over the whole of China except in the southern part of China, where a significant increasing trend is noted. Summer precipitation shows a remarkable decadal-scale fluctuation in the first two dominant EOF modes. The first EOF represents the increasing trend in the middle and northwest part of China, while the second EOF represents the oscillation between the southern part of China and the rest of the country, which is closely related to the summertime NHT.
Grid point data, visible and infrared cloud imageries, and cloud top temperatures during the period of May-June 1981-1986 were used to study the large-scale circulations in Asian monsoon region. Stream function, velocity potential, divergent part of the wind, convection index, cloud top temperature index, and moisture field were analyzed. The distribution of the half-monthly mean of these parameters is presented and discussed to reveal the characteristics of the evolution of large-scale circulations from the pre-Mei-Yu season (1-15 May) to the post-Mei-Yu season (16-30 June) in South China and Taiwan region. Also, convectively active and inactive Mei-Yu seasons and fronts were selected to study the interannual and intraseasonal variations of the large-scale circulation patterns. The results can be summarized as follows : (1) The Mei-Yu over South China and Taiwan occurred concurrently with the onset of the summer southwest monsoon over the South China Sea during the period of May 16-31 (Phase I). (2) The northward advance of the area of deep convection, ITCZ, and subtropical ridge at the post-Mei-Yu season (June 16-30) occurred concurrently with the establishment of the quasi-stationary position of Mei-Yu front over the Yangtze Valley and Japan. At the same time, a quasi-equivalent barotropic monsoon circulation system was well developed with the low-level cyclone under the upper-level anticyclone over northeastern Indian and Burma area. (3) The active Mei-Yu season was characterized by the further southward penetration of the northern (baroclinic) system and moisture flux convergence over the Mei-Yu region. The reversed situations were observed for the inactive Mei-Yu season. (4) The difference for the active and inactive Mei-Yu fronts was mainly controlled by the low-level flows. The active Mei-Yu front was accompanied by the southwest monsoonal flows originating from the Bay of Bengal and the tropical western Pacific, whereas the inactive front the southeasterly or easterly flows of the Pacific high circulation prevailed over the Mei-Yu region. Higher mixing ratio, stronger moisture flux, and flux convergence were observed over the Mei-Yu region for the active front as compared to those for the inactive one. (5) More frequent occurrence of the active fronts led to an active Mei-Yu season and the reverse was true for an inactive one.