Showing papers in "Journal of the Meteorological Society of Japan in 1994"

Trends and decadal-scale fluctuations of surface air temperature and precipitation over China and Mongolia during the recent 40 year period (1951-1990)

Abstract: 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

Short-period disturbances in the equatorial lower stratosphere

Abstract: 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 (ABO). Zonal wind and temperature variances due to the short-period fluctuations are generally large and particularly enhanced in the ABO 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 ABO. These results indicate strong connection of the short-period disturbances with the ABO

Large-Scale Circulations Associated with the East Asian Summer Monsoon and the Mei-Yu over South China and Taiwan

Abstract: 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 1). (2) The northward advance of the area of deep convection, ITCZ, and subtropical ridge at the post-MeiYu 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

The Seasonal Change of the Water Budget in the Congo River Basin

Abstract: The seasonal change of the water budget in the Congo river basin is investigated by using hydrometeorological data averaged over long-term periods. Vapor flux convergence is calculated using the global objective analysis data of the ECMWF from 1985 to 1988. Precipitation and river discharge data mainly cover the periods 1920-1960 and 1932-1959, respectively. Evapotranspiration is estimated as precipitation minus vapor flux convergence on the monthly basis. The atmospheric water balance terms are related to the Normalized Difference Vegetation Index (NDVI) derived from the NOAA/AVHRR averaged from 1985 to 1987. On the monthly basis, the NDVI and evapotranspiration are in phase with the seasonal change of precipitation in the evergreen forest region, which mainly covers the northern part of the basin. In contrast, the NDVI and evapotranspiration lag precipitation by one month in the southern deciduous forest region covering the southern part of the basin. As for the entire basin, the lag-relationship between the NDVI/evapotranspiration and precipitation is similar to that for the southern deciduous forest region. In the dry season of the southern deciduous forest region, evapotranspiration exceeds precipitation in the entire basin, causing a decrease of the basin storage to its minimum value. In addition, from the viewpoint of the seasonal change of precipitation and evapotranspiration, it is concluded that the feature of the seasonal change of the water budget in the entire basin mainly reflects the characteristics of the southern deciduous forest region.

A diagnostic analysis of the wave train propagating from high-latitudes to low-latitudes in early summer

Abstract: By using the stationary wave activity flux of Plumb (1985), the present study carried out a diagnostic analysis of the phenomenon in which a Rossby wave propagating from high to low latitudes affects the Baiu front around Japan in early summer. According to one composite and one case (during 20-29 June of 1982) analysis, there is a large horizontal southeastward component of the flux occasionally stretching from the Okhotsk Sea, via east of Japan (or Japan) to the sub-tropical region (about 25-27.5N/155E), when the blocking anticyclone around the Okhotsk Sea (the Okhotsk blocking anticyclone) establishes itself. On the other hand, a wide and strong ascending flux covering the area from East Siberia to the Okhotsk Sea (exactly where there is a wide and large divergence of the flux) was found during 20-29 June, 1982. These suggest that this area is one of the important forcing-source-areas for the quasistationary wave propagation, although a complete forcing mechanism of the propagation has not been found yet.

An evaluation of climatological data in the Indian Ocean area

Abstract: This paper presents evidence that both land-records and marine products currently used to assess the interannual and decadal variability of the monsoon system are subjected to important systematic errors due to non-climatic factors. The main inhomogeneities in the NCAR's World Monthly Surface Climatology during the 1900-1984 period affect all Sea Level Pressure (SLP) time series over the Indian subcontinent. These discontinuities are mainly linked to the non-consideration of changes in the time of observation between sample periods when the data have been compiled at NCAR. The main example of such discontinuity is observed in 1961 with the change from the World Weather Records collection to the Monthly Climatic Data for the World series. Systematic biases are also revealed for Sea Surface Temperature (SST), Air Temperature (ART) and SLP ship-measurements compiled over the Indian Ocean during 1900-1986. The main spurious jumps occur around 1932 for SLP, 1940 for both SST and ART, and 1954 for SST sampled in the east Arabian Sea and the Bay of Bengal. In addition, an artificial trend contaminates SST reports during 1954-1976. Though no attempts have been made to determine the exact causes responsible for these inhomogeneities, there is little doubt about their origins because all these dates and trends are in phase with important changes in the composition of “source-decks” merged into the marine dataset, and do not agree with fluctuations of corrected SLP and temperature measurements along the coasts of the Indian subcontinent. Finally, substantial biases in resolution of the annual cycle can be expected for all the parameters because the sampling over the Indian Ocean has a strong seasonal dependence after 1950. Many inconsistencies in the observational picture of interannual SST variations over the Indian Ocean are thought to be linked to such data problems. A comparison between land and marine trends has however suggested the existence of some significant decadal-scale fluctuations in the Indian region during the 1900-1986 period. These true climatic happenings include prominently a warming temperature trend and a general SLP decrease over both land and ocean during 1900-1939, and a sudden warming in the Indian Ocean after 1976. This recent warming does not affect the interior of the Indian subcontinent and is in phase with persistent positive SLP anomalies for the whole Indian sector. This corroborates the evidence of a climatic change in the Indian Ocean after 1976 (Nitta and Yamada, 1989).

Modelling the Evaporation from Bare Soil with a Formula for Vaporization in the Soil Pores

Abstract: A multi-layer soil model is constructed taking into consideration the diffusion of water vapor and vaporization in the soil pores. The resistance to vaporization in the pores is introduced as a function of the volumetric soil water content. The soil layer is treated as a 'porous canopy' and the vapor density profile in the soil is calculated by an equation for vapor diffusion. The model simulates the evaporation rate, the soil temperature, and the soil water content profile rather well when compared to measurements from field observations and pan experiments. Recent numerical models usually assume that the water vapor found in the soil pores is in equilibrium with the soil water at each depth, with the equilibrium humidity expressed by thermodynamical theory (Philip,1957). However, the present model calculations reveal that the relative humidity in the pores close to the ground surface is different from that at equilibrium, since the water vapor is continuously transported to the atmosphere during the drying stage.

Kelvin and mixed Rossby-gravity waves appearing in the GFDL SKYHI general circulation model and the FGGE dataset : implications for their generation mechanism and role in the QBO

Abstract: 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, wavenumberone, 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 compount 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

Interdecadal variations of precipitation over the tropical Pacific and Indian Oceans

Abstract: 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

A Rotor Circulation near the Baiu Front Observed by the MU Radar

Abstract: A rotor circulation was directly observed near the Baiu front in the lower troposphere by three-dimensional Doppler measurements with the MU radar at Shigaraki, Japan (35°N, 136°E). The temporal and vertical scale of the rotor were ∼50 min and ∼2 km, and the stratification observed by radiosondes was statically stable. The synoptic meteorological analysis suggests that the rotor existed just below and between Baiu-frontal banded precipitation clouds which were organized in a mesc-α-scale cyclone. Precipitation echoes observed simultaneously by C/Ku-band radars were quite weak in the downdraft in front of the rotor, and became significant and tall up to ∼9 km altitude at the back of the rotor circulation. The rotor was identified with a meso-β-scale depression observed by the routine meteorological network, which had a horizontal scale of ∼40 km in the zonal direction and 150-200 km in the meridional direction, and moved from west to east at ∼50 km/h. Based on brief discussions, we conclude that the rotor circulation was locally developed from an orographic disturbance by shear instability which was occasionally induced in a weak statically-stable layer maintained by (conditional) symmetric instability.