Methods for obtaining stationary solutions of the primitive equation model and stationary and periodic solutions of the quasi-geostrophic model are proposed. The key is to construct the spectral models based on the spectral coefficient method, in which non-linear terms are calculated by the interaction coefficient method. This is required to obtain explicitly Jacobian matrices which are necessary for solving the non-linear equation system by Newton's method. Thus, solutions can be obtained for the models with a resolution which can simulate the real atmosphere. Further, a linear stability analysis for these solutions can be made. In this course, several devices for computations are necessary, particularly since the both models have diagnostic equations and the calculations of non-linear terms is complicated. Applications to various problems and future developments are also discussed.
We investigate the relationship between mixed Rossby-gravity waves (MRGWs) and convection in a general circulation model. The experiments described are performed in a general circulation model with the lower boundary set to that of an ocean surface everywhere. Several experiments are run varying the convective parameterization scheme (using either a modified Kuo scheme or a moist convective adjustment scheme) and varying the tropical sea surface temperatures (specified to be zonally symmetric in all cases), thereby changing the location of the modeled intertropical convergence zones (ITCZs). The appearance of a robust MRGW occurs when the sea surface temperature is such that two ITCZs straddle the equator. The particular sea surface temperature distribution used and the parameterization scheme for convection also affect the structure and strength of the modeled MRGW. The vertical structure of MRGWs is analyzed in the experiment in which this wave mode is the most energetic. We show that MRGWs of several different zonal length scales exist in the troposphere in association with convection; however, it is only the longer length scales which can be discerned in the upper troposphere and lower stratosphere.
Using a re-analyzed FGGE IIIb data set, we study the large-scale aspects of the East Asian monsoon of early summer 1979. Specifically, we analyze the structure of the quasi-stationary rainy band (the Baiu front) existing in East Asia and study its relationship to environmental large-scale fields for the SOP2 period (May 5 to July 5, 1979). A composite technique is employed to obtain a reliable structure of the Baiu front. In the analysis, spatially- and/or temporarily-averaged fields are used to filter out small-scale features. The location and strength of the southwesterly moisture flux correlate with those of the Baiu front. Moreover, the Baiu front is always located between the upper-level westerly jet and the lower-level anticyclonic shear line. Two contrasting features are found in the Baiu front. Near or to the south of the front axis, we find strong convergence (divergence), cyclonic (anticyclonic) circulation, and an enhanced westerly (easterly) in the lower (upper) level with weak vertical wind shear and small temperature gradient. To the north (-10 degrees) of the front, we find a strong upper-level jet, strong vertical wind shear, and large temperature gradient. We find that all fields in the Baiu front slope upward and northward, and thus fields at all levels from 1000mb to 200mb are closely related. In particular, the enhanced westerly is seen both over the axis of the front at low levels and in the upper north side of the front. Geostrophic approximation and homogeneity along the front axis are fairly good when appropriate averaging is done. A simple calculation based on a quasi-geostrophic approximation shows that the diabatic heating due to condensation can reproduce patterns and magnitudes comparable with the observed geopotential anomaly. These results suggest that large-scale features of the Baiu front can be explained consistently and qualitatively in terms of quasi-geostrophic dynamics. These furthermore suggest that the diabatic heating is a driving force to maintain the Baiu front and it causes an enhanced westerly over the front axis and an intensification of the anticyclonic shear to the south of the front.
Wave-like echoes were observed in a weak rain cloud by a Doppler radar on September 6, 1989. The wave-like echoes presented as a series of band echoes with a wavelength of 3.5km. The wave-like echoes were generated in a strong vertical shear at a height between 3 and 4km and oriented perpendicular to the shear vector between 3 and 4km. The minimum Richardson number at the shear layer is 0.14. These features are consistent with the condition of Kelvin-Helmholtz (K-H) instability. The vertical cross section of the wave-like echoes were expressed as a series of reflectivity cores and undulation of an isopleth of Doppler velocity. The undulation of Doppler velocity, which is induced by K-H instability, is assumed to cause the horizontal accumulation of precipitation particles and adiabatic heating to form the reflectivity core at the melting level.
Long-term trends and inter-annual and inter-decadal variabilities of the global land air temperature are analyzed by using the 110-year time series data over 8 sub-regions. The global mean temperature exhibits a generally increasing trend during the 100 year period with a rate of about 0.54°C/(100 years). However, the temperature has inter-decadal variabilities, with the largest warming for the recent 20 years, but with a cooling trend during 1940-1970. Spectral analysis of the air temperature shows that there exist three dominant time scales with period ranges of several years, 10-20 years, and longer than 30 years. 2-7-year-period variations over the tropical sub-regions are greatly affected by El Niño and Southern Oscilation (ENSO) cycles, but those over the extra-tropical sub-regions are not so affected by ENSO. The 8-25-year band-pass-filtered variations have amplitudes of about 0.1°C-0.2°C. Their phases were nearly the same over the whole sub-regions before about 1920, but became different between different sub-regions after 1920. The phases of this component of the global mean temperature agree quite well with those of the sunspot numbers in recent years after about 1960, but they were nearly out of phase before about 1920. The low-pass-filtered variations with periods longer than 30 years had a positive peak around 1930-1940 and negative peaks around 1890 and 1970. This component seems to contribute to the large warming during recent 10 years.
Forecast experiments are performed from various initial states using an operational mesoscale model of JMA in order to improve the spin-up of precipitation forecasts. The initial conditions are designed to investigate the impact of FA cycle, diabatic NNMI and moisture initialization. The diabatic effects in the NNMI are treated in two ways; diabatic heating rates calculated by the model physical processes, and those estimated from observed rain rates with an assumed parabolic heating profile. The examined case includes a mesoscale convective cloud system. The operational forecast is characterized by very slow spin-up of precipitation and position errors after late build-up of precipitation. In the forecast experiments, use of the FA cycle was effective for improving the description of the mesoscale features such as a low-level vorticity maxima in the initial state. As a result, position errors of the rain area in the late hours were reduced, while the model still showed slow spin-up of precipitation. The diabatic NNMI with the model physical processes did not introduce large changes of the divergent motion due to weak diabatic heating calculated in the NNMI. The NNMI using observed rain rates, in contrast, produced strong divergent motion and associated updrafts corresponding to the cloud system in the initial state. However, diabatic heating in the forecast is still weak, and the divergent motion in the initial state dissipated rapidly. In the case where all three methods, i. e., the FA cycle, the NNMI using observed rain rates, and the moisture initialization were combined, the model produced realistic precipitation from the first hour of the forecast, and the effects persisted for a long time. It was found that smooth transition from initialization to forecast is important for the rapid spin-up of precipitation.
An initialization method for the Japan spectral model (JSM) is developed for the use of precipitation observations from a radar-raingauge network over Japan. The initialization executed the physical initialization method to calibrate thermodynamic and dynamic variables of objective analysis data in such a way that model precipitation from the calibrated data is equal to precipitation observations. Afterwards, the non-linear normal mode initialization with the precipitation process, which was similar to that developed by Takano and Segami (1992), was conducted. Forecast experiments on a case of Baiu-frontal disturbances (June 30, 1990) show: 1) The initialization method reduces both spin-up error and position error of precipitation forecasts. 2) Reduction of position error is ascribed to the modification of relative humidity due to calibration of thermodynamic variables, while, in the model precipitation region where no precipitation was observed, this calibration adopted a primitive method which reduces relative humidity to an empirically-determined threshold value. 3) Calibration of dynamic variables contributes greatly to determining the initial p-velocity. This resulted in improvements in precipitation forecasts for the first few hours. 4) The non-linear normal mode initialization with the precipitation process itself made little change to the initial p-velocity, even in the strong model precipitation areas, while Takano and Segami (1992) pointed out that this procedure did not largely change the initial divergence field due to the weak diabatic heating. Since the non-linear normal mode initialization with the precipitation process retains circulations which are accompanied with precipitation, this procedure contributes to the reduction of slow spin-up of precipitation caused by adiabatic normal mode initialization.
The downward water vapor flux over deserts in the daytime has been observed by many scientists including Wang and Mitsuta (1990). Tsukamoto (1993) pointed out the importance of density correction to flux measurements and showed that the magnitude of the water vapor flux measurements made by Wang and Mitsuta should be reduced by about 22%, but the direction was still downward. The present authors discuss this problem from the viewpoint of the behavior of water vapor near the ground surface and arrive at the conclusion that the analysis of the problem must take into consideration water vapor advection if correct conclusions are to be drawn.
Power of the Lepage test, a distribution-free two-sample test, is studied by means of statistical simulation. The results show that the Lepage test detects such changes as long term trends, cyclic variations and step-like changes in different ways for each type of phenomenon. The Lepage test can be a very good tool for statistical researches in climate change.