FGGE level-IIIb data produced by the European Centre for Medium Range Weather Forecasts (ECMWF) for the DJF 1978-79 is used to analyse the kinetic energy budget over the global tropical belt. A detailed energetics of the mean and transient fluctuations, in particular, the generation, the exchange between the mean flow and the transients and the dissipation of kinetic energy are estimated for the tropics and compared with the earlier studies. It is found that about 60% of the kinetic energy generated in the tropics is dissipated by the frictional mechanisms and remaining 40% is transported to the middle latitudes of both the hemispheres. The subtropical westerly jet stream of the winter hemisphere plays a dominant role in the process of kinetic energy flux transport. It is observed that the inter-hemispheric exchange of kinetic energy is negligible. Further, the study confirms the fact that the tropical circulation is mainly maintained by the mean flow while in the extra tropics (beyond 30° latitude) the transients are quite important.
Ten 5-day forecasts have been performed to compare the error characteristics of the global spectral models of the Japan Meteorological Agency (JMA) and the Australian Bureau of Meteorology Research Center (BMRC). Operational global analyses for July 1983 obtained from the European Center for Medium Range Weather Forecasts (ECMWF) are used as initial conditions for both models. Differences in the hydrological cycle in the two models are found to be evident. Strong precipitation and evaporation in the BMRC model integrations are associated with the parameterization of cumulus convection and also the initializing of the model moisture field; strong heating due to cumulus convection results in a positive bias of the upper troposphere in the BMRC model. The hydrological cycle of the JMA model is less intense than that of the BMRC model and more in accord with the climatic estimates. Details in the application of the connective parameterization based on Kuo's scheme and the radiative parameterization appear to explain some of these differences between the two models. Sensible heat flux from the surface is considered to be too small in both models. The diagnosis of cloud amount in the JMA model has introduced some systematic error in cooling rates in the upper troposphere in the arctic region. This problem is not evident in the simpler scheme used in the BMRC model where climatological cloud amounts are prescribed. The characteristics of the errors in the wind field prediction are similar in the two models with the major exception being the excessive upper tropical easterlies in the BMRC model. The JMA model includes a parameterization of gravity wave drag which is not included in this version of the BMRC model; this appears to be relevant to the generally more intense zonal flow in the winter hemisphere.
The variations of vertical and horizontal velocity of early snowflakes, which are composed of two to six crystals, were analysed by a stereo-photogrammetric method. The following main results were obtained: 1) The standard deviation of variation in vertical velocity is usually smaller than 2cm s-1 (3 to 5% of the mean vertical velocity). On the other hand, the standard deviation of variation in horizontal velocity is considerably larger (1 to 8cm s-1) and increases with the mean horizontal velocity. These results are independent of the main type of component snow crystals. Accordingly, it seems likely that the variation of horizontal velocity plays an important role in the aggregation process of snowflakes. 2) The mean horizontal velocity can be estimated from the non-dimensional amplitude of the spiral or rotational falling motion of snowflakes. 3) The non-dimensional amplitude is inversely related to the non-dimensional frequency of spiral or rotational motion. 4) The non-dimensional frequency can be estimated from the Reynolds number with respect to the mean vertical velocity of snowflakes.
By the use of a vertically and angularly detailed, plane-parallel microwave radiative transfer model, we have conducted a series of numerical experiments in conjunction with a cloud model simulation to investigate the impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation at several frequencies across the EHF-SHF spectrum. Our overall objective is to explore the detailed physics of using multi-channel, passive-microwave retrieval techniques for the estimation of precipitation from space-based platforms. This paper is a continuation of a previous sensitivity study, which we have published in two-parts (Mugnai and Smith, 1988; Smith and Mugnai, 1988). The impact of large ice particles on passive microwave brightness temperatures over an evolving model rain cloud are examined at 10 separate frequencies in the EHF/SHF spectrum. Three separate cloud model designs are considered for both hard ice and low density ice freezing modes. The results emphasize how the range of frequencies between 10.7 and 231GHz differentially respond to the various ice models and freezing modes. It is shown how frequency-dependent, vertically distributed generalized emission/scattering weighting functions, which we have introduced to vertically resolve the contributions by individual cloud and precipitation layers to the brightness temperatures, can be used to identify the specific layers responsible for regulating the magnitude of top-of-atmosphere brightness termperatures. The weighting function is also coupled to a fractional contribution by scattering function which will exhibit the relative magnitude of the scattering source within the generalized weighting function itself. This enables a thorough understanding of how brightness temperatures are modulated by hydrometeors in individual layers. Additional salient results of the study are the identification of the importance of incorporating a mixed layer at intermediate and higher frequencies (37GHz and higher), and quantifying the influence of freezing mode at 85GHz. It is also shown that the vertical scale of the cloud column, for a fixed equivalent liquid water path, is a major determining factor in generating brightness temperatures consistent with actual observations. This result is based on a case study of a severe thunderstorm monitored with a 2-channel airborne microwave radiometer during the 1986COHMEX experiment in northern Alabama.
A land breeze is frequently observed along the coastal region of the Ishikari Plain during the winter season. A land-breeze front is formed offshore between colder over-land air and warmer over-sea air. In this study, we mainly used a single Doppler radar to examine the structure of the land breeze and its effect on snow clouds. The vertical profile of wind velocity was divided into three layers: the landbreeze layer (-300m in depth), the shear layer and the prevailing wind layer. The frontal surface was steep near the front, but almost horizontal at a distance from the front. Kelvin-Helmholtz instability waves were observed along the interface of the land breeze and the northwesterly monsoon wind. The land breeze strongly influenced the modification of snow-clouds. A rapid intensification of snow-cloud echo, which was often observed at the land-breeze front, might have been caused by a strong low-level convergence at the front. Behind the front, however, the echo rapidly weakened and dissipated, since the air mass of the north-westerly monsoon wind lost vapor and its lower part became stable when mixed with the land breeze. Consequently, convection was suppressed. Since the intensification and dissipation of echo recurred, the intense echo region seemed to be stagnant at the front, while each cell moved at a speed of the prevailing wind; this resulted in the localization of heavy precipitation at the front. The time-averaged distribution of echo intensity shows the concentration of precipitation in the coastal region where the front was located. The analysis of the AMeDAS data also shows that precipitation was localized along the coastal region when there was a land breeze.
The variability of the tropical atmosphere consists of a hierarchy of time-scales. In order to seethe extent to which there exists clear scale-separation among different levels of the hierarchy, several analyses based on fracta geometry theory are performed for the NOAA OLR data. The power spectra conform quite roughly to the inverse-power form, which implies no scaleseparation or "scaling" . This tendency is further examined by a fractal analysis of the time series, which measures the change of the magnitude of variability as a period of averaging is changed. Remarkably, the extent of "scaling" changes locally along the equator, due to the locality of the region with a particular dominant time-scale: the intraseasonal variability, the annual cycle, and the El Nino/Southern Oscillation. This locality is quantified by the self-affinity scale, which measures the extent of "scaling". In particular, the self-affinity scale can be identified as the characteristic time-scale of the super clusters in the region where the intraseasonal variability is conspicuous. The probability that a convectively active period continues longer than a certain time decays exponentially with a characteristic decay time-scale of 3-5 days, which can be identified as the time-scale of cloud clusters.
The intraseasonal change of the activities of the short-term tropical disturbances are investigated in terms of their amplitude modulation. It has been revealed that the amplitude modulation in the tropics bears a periodicity around 30 days. It was also found that the enhanced phase of the amplitude modulation is often accompanied by the clustered genesis and/or development of tropical cyclones. The space-time analysis of the amplitude modulation has shown that it consists of two characteristic modes. One is the eastward-propagating mode with tonal wavenumber 1. Another mode shows a westward propagation and zonal wavenumber 4 to 6 as a characteristic longitudinal scale. Both modes appear equally significant in leading to enhanced disturbance activity. The large-scale circulation changes associated with the intraseasonal amplitude modulation are also investigated referring to the modulation over the tropical western Pacific. The composite analysis has revealed that the lower tropospheric westerly and the upper-level easterly are enhanced at the time of the enhanced activity of the tropical disturbances. The horizontal distribution of the circulation anomalies indicates that an intrusion of the low-level westerlies into the western Pacific takes place at this stage from the equatorial Indian Ocean through the South China Sea. This intrusion is also accompanied by enhanced cross-equatorial southerlies over the Indonesian region. In the upper levels, the intensification of the mid-Pacific trough is observed at the same stage.
Turbulent transport processes for carbon dioxide and water vapor in unstable conditions are studied using data observed just above plant canopies in a paddy field. The analysis of joint probability distributions of w-wc and w-wq confirmed that, just above plant canopies, downdrafts were remarkably efficient for vertical transport of carbon dioxide and water vapor in neutral conditions. Furthermore, it was shown that updrafts became more efficient than downdrafts for vertical transport of carbon dioxide and water vapor in very unstable conditions. The stability dependence of carbon dioxide and water vapor transport was similar to that of sensible heat transport.
Small-scale disturbances in the lower stratosphere are investigated with the aid of operational rawinsonde observations over Japan. The disturbances appear not only in wind but also in temperature fields. The disturbances show a clear seasonal variation: the intensity is large in winter and spring, in a similar manner to that of the mean zonal wind. They also have latitudinal and altitudinal dependency with the peak of intensity at 15km to 20km height around 40°N. The dominant vertical scale of the disturbances is 2 to 5km and they extend north and south with a scale of a few hundred kilometers. By applying a vertical band-pass filter, it is found that they show a wave-like form with a characteristic phase relation such that the lag of T' to u' and lag of u' to v' are both in between -180° and -90°. Supposing that they are inertia-gravity waves, the phase relation suggests that the wavenumber vector points to the direction of the north-west and downward and hence the waves transport the energy north-westward and upward relative to the background wind. This inclination of the propagation direction can be interpreted as a consequence of the selectivity due to the critical layers as well as the wave generation. From all of these results, we deduce that the generation and the characteristic structure of the disturbances are closely related to the subtropical jet at the tropopause level.
Following the previous paper (Part I, Hanawa et al., 1989), long-term wind stress vector (WSV) fields over the North Pacific are analyzed by means of the composite method with respect to the ENSO events (ENSO composite). As made in Hanawa et al. (1988), every winter during the 24 years from 1961 to 1984 was classified into one of four categorized winters, i.e., ENSO-1 year, ENSO year, ENSO+1 year and the other year winters. Among four categorized winters, WSV anomaly fields for the ENSO year winter are relatively wellorganized and those in the mid-latitudes of the western North pacific and in low-latitudes are quite similar to those for the warm winter of Part I. That is, the East Asian Winter Monsoon (Kisetsuhu) weakens and in the equatorial region, WSV anomaly fields suggest that the central to eastern Pacific is the strengthened convergence region of wind, i.e., convection region. The weakening of the East Asian Monsoon is responsible for the appearance of remarkably positive SST anomalies in the mid-latitudes of western North Pacific in the ENSO year winter described by Hanawa et al. (1988). However, although the mid-latitudes westerly shifts northward, it strengthens compared with the other three categorized winters and the warm winter of Part I. Composited sea-level pressure fields show the marked strengthening and eastward shift of the Aleu-tian Low, and positive and negative anomalies over the North Pacific appear in mid- and high-latitudes respectively, which is responsible for the strengthening of the mid-latitudes westerly. For the other three categorized winters except for the ENSO year winter, the stabilities of WSV anomalies are low, and the time series of scores of Degree of similarity introduced in Part I do not show that the composited WSV anomalies are typical for each categorized winter. That is, characteristic basin-scale WSV anomaly patterns could not be extracted using the present viewpoint. Therefore, it can be concluded that although ENSO events have remarkable effect on the atmospheric general circulation in the ENSO year winter, they are not necessarily good sampling conditions for the extraction of the characteristic pattern of mid- and high-latitude wintertime WSV fields over the North Pacific, at least in the frame of the present study.
Effects of tropospheric aerosols on the radiation budget in the lower atmosphere have been studied through aircraft measurements of aerosols and the solar and infrared radiative fluxes combined with a concurrent ground-based spectral attenuation measurement of the direct solar beam. Six cases of observations during cloudless winter midday hours with the solar zenith angles around 60°, were carefully analyzed. Of the six cases, four were made over land (Tsukuba) and two cases were conducted off the coast (Kashimanada). The vertical profiles of aerosol concentrations were associated with those of water vapor, and the aerosol size distributions were typically bi-modal. Tropospheric aerosols effectively scatter and absorb the solar radiation, greatly influencing the vertical profiles of the upward and downward solar fluxes. Over the lower troposphere, the average aerosol absorption effects were found to be at least the same order of magnitude as those due to water vapor. In dense haze layers, however, the instantaneous solar heating rate was as large as 5°C/day, and the contribution from aerosols was about three times larger than that of water vapor. Since the IR flux profiles were mainly determined by the distribution of gaseous constituents and temperature, the effects of tropospheric aerosols were not appreciably large. The lower troposphere experienced IR cooling of the order of 1°C/day, but the cooling was lessened in the surface layers when the surface temperature was much higher than the surface air temperature. Presence of the tropospheric aerosols had only a small effect on the radiation budget at the top boundary of the lower troposphere-surface system. However, the tropospheric aerosols greatly affected the distribution of solar energy in the system. Compared to an aerosol-free case, the lower troposphere experienced a substantial amount of additional solar heating due to aerosol absorption, at the expense of a comparable amount (of the order of 10W/m2 as the 24-hour mean) of the solar energy absorbed by the surface. As a consequence of the solar heating combined with the IR cooling, the lower troposphere has a substantial net heating in its lower part and a net cooling in its upper part. This, as well as the net solar heating of the surface may be responsible for the formation of the well-developed mixed layer over Tsukuba.
The relationship among tropical convection, the weather over Japan and the atmospheric circulation in the Northern Hemisphere during winter was statistically examined with the use of monthly mean temperature data over Japan, outgoing longwave radiation (OLR), 500mb geopotential heights and the sea level pressure. It was found that the temperature over Japan is highly correlated with convective activity around the Philippines, i.e., when the convection around the Plilippines is active, the temperature over Japan is lower than normal. Conversely, when the convection is inactive, temperatures over Japan are above normal. Convective activity around the Philippines is also correlated with the Northern Hemisphere 500 mb geopotential heights, especially in the region of east Asia. When convective activity around the Philippines is strong, the 500mb heights around Japan are lower while those around Siberia are higher. Weak convection means higher 500mb heights over Japan and low heights over Siberia. Sea level pressure in the northwest Pacific east of Japan is lower while that in Siberia is higher for active convection cases. The converse is true for inactive conviction. Conclusions from these results indicate that for active convection the strength of the east Asia winter monsoon becomes stronger resulting in a cold winter over Japan. Inactive convection weakens the east Asian winter monsoon creating a warm Japanese winter. The results of composite analyses show that the global height anomaly patterns such as the Eurasian and Pacific/North American patterns can be found more clearly in the active convection cases than in the inactive cases.
In sky radiance calculations for clear and turbid atmospheres, the neglect of polarization effects produces systematic errors up to about 10%. This paper describes a technique for reducing such errors by adding a correction term to the result of scalar transfer code. A semi-empirical expression for the correction term was constructed based on the successive order theory and several parameterizations. Our expression reduces the error to about 0.6% in the case of homogeneous, optically thin or moderately thick atmospheres.
Elemental composition and features of individual aerosol particles collected on 25 January 1983 in the Antarctic upper troposphere of 7.2km altitude over Syowa Station (69°S) with use of an aircraft were examined by an electron microscope equipped with an energy-dispersive X-ray analyzer. Mineral-containing particles were dominant (74% by number) in aerosol particles of 0.1-1.6μm radius, indicating the global transport of particles from the eruption of El Chichon volcano (17°N) in April 1982. Modified sea-salt or halite particles were present in 6% of particles. Particles composed of only sulfate and/or sulfuric acid constituted in 15% and soot-like particles constituted 5%. The transport routes of the mineral-containing particles were studied with the 3-dimensional trajectory analyses on the basis of the twice-daily U.S. NMC data. Although the back trajectories exhibit that majority of the air particles spread in the whole extratropical troposphere 20 days before, there are some air parcels (about 2%) which passed above 200mb in the Antarctic region. Since the mineral particle concentration in the stratosphere in January 1983 was very high, most aerosol particles of El Chichon origin collected in the Antarctic upper troposphere would be transported downward from the Antarctic stratosphere. If the modified sea-salt (or halite) particles collected in the Antarctic upper troposphere are of marine origin, the trajectory analysis shows that the particles must come from the subtropical ocean. The present study also suggests that the main transport of aerosol particles from the stratosphere to the troposphere in summer occurs over Antarctica and the subtropical jet region. The Antarctic route is of primary importance for transport to the Antarctic upper troposphere. Moreover, it is expected that the transport from the mid-latitude troposphere to the Antarctic upper troposphere become important when the aerosol concentration in the stratosphere is lower than that in the troposphere. The results obtained in this study are also useful for the transport problems of other trace gases such as ozone and carbon dioxide.
Japan Spectral Model (JSM) was developed for operational forecast use in JMA. The model is a 19-level spectral limited-area model with a horizontal resolution of 40km. The purpose of the model is to predict meso-α-scale phenomena and fine structures of orographically induced disturbances. The model predicts quite well the mesoscale structure within a synoptic scale disturbance and the evolution of a mesoscale cloud system associated with a polar low. The model also shows sufficient ability to predict severe rainfalls in the Baiu season. Forecasts of precipitation show a good performance in spring even in the early hours of the forecast time. However, the scores for the summer season are worse than those for spring. This may be due to the cumulus parameterization scheme used in JSM. Slow spin up of the model is a serious problem in the summer season.
A remarkable windstorm associated with a squall line occurred in Kita Village near Sapporo, Hokkaido, 2125JST September 23, 1986. A detailed ground survey of the damaged area revealedthat the phenomenon was concluded to be a microburst. The estimated total length of the damagewas more than 6km and the damage scale was F2 using the system introduced by Fujita et al. (1972).Radar echoes measured at the Sapporo District Meteorological Observatory indicated a bow echo evolution in connection with the microburst when a squall line accompanied with heavy precipitation that passed over Kita village. Therefore, it was concluded that the Kita village event was one of the wet type microbursts by the synoptic situations and the radar echo patterns.