An observational study has been made of the recent decreasing trend of the Sahelian rainfall from the 1950's to the 1980's and its relation to the tropical African rainbelt, tropical atmospheric circulation, and sea surface temperature (SST) pattern. This investigation incorporates the trend and correlation analyses of the rainfall, rainbelt, upper-air, and SST data, along with a rotated empirical orthogonal function (R-EOF) analysis of SSTs. An analysis for the northern summer season shows that the reduction in total rainfall of the rainbelt is more responsible for the decreasing trend in the Sahelian rainfall than the equatorward retraction of the rainbelt. The decreasing trend in the rainbelt's total rainfall is related to increasing trends in 700hPa heights and 850-500hPa thicknesses throughout tropical Africa. The vertical profile of temperature for Niamey suggests that these trends in the lower troposphere may result from intensified large-scale subsidence. The trends of the tropical African upper-air are accompanied by a tropical east-west contrasting pattern for the 700hPa heights having increasing trends between the Atlantic-Africa regions and between the Indonesia-central Pacific regions, and decreasing trends over the Indian Ocean. The R-EOF analysis of SSTs indicates that the occurrence of the above east-west trend pattern is probably associated with the dominant warming trends of the Indian Ocean, while the SST anomaly pattern contrasting the tropical North and South Atlantic documented in previous studies plays a major role in the year-to-year latitudinal displacement of the rainbelt.
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).
A thermal belt or a warm zone on a hillside, developing under calm weather conditions during the night, is one of the well-known local climatic phenomena. Although many models describing its fundamental concept have been constructed, very little observational data exists to support the models directly. The difficulty in studying local climatic phenomena lies mainly in collecting data. In the present study, an infrared radiation thermometer installed on the roof of a high building was used to obtain data for analyzing the air temperature structure in complex terrain. The altitude data set (Digital National Land Information, Japan) also was utilized to examine the dependence of air temperature on topography by applying multiple regression analysis. It is confirmed by observation that a thermal belt on a hillside is produced by nocturnal drainage winds and by a cold-air lake which is formed over the basin by the drainage wind and radiative cooling. The height and intensity of a thermal belt are subject to local and temporal variations due to topographical environments and weather conditions, and can vary from hill to hill, even in situations where they are located face to face. These results show that a thermal belt is a phenomenon produced dynamically by the movement of cold air.
We document a relationship between tropical cyclones of the Indian and western Pacific oceans and the Madden-Julian oscillation (MJO). Cyclones preferentially occur during the convective phase of the oscillation, and cluster around the low-level cyclonic vorticity and divergence anomalies that appear poleward and westward of the large-scale convective anomaly along the equator. Although the absolute numbers of storms and typhoons are enhanced during the convective phase of the oscillation, the ratios of storms and typhoons formed per depression are the same in the convective phase as the dry phase of the oscillation. There exist more storms and typhoons simply by there being more depressions in the convective phase. The third result of this study is that the increase in cyclone activity during active periods of convection is not restricted to MJO activity. In fact, we find that an equal increase occurs during the convective phase of an arbitrarily chosen, completely independent band from the MJO. We conclude that the MJO does not influence tropical cyclones in a unique fashion, but it is important because of the relatively large proportion of tropical variance that is explained by it.
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.
Twice-daily time series upper-air data at Singapore (1.4N, 104.0E) during the years 1983-1993 are analyzed to examine disturbances in the period range of about 2 days. The covariance between the zonal wind component and the temperature change per day is found to be prevailingly negative in the 100-10hPa layer. This indicates upward transport of the westerly momentum. The fluxes are shown to be related strongly with the QBO cycle and the largest flux occurs in the region where the westerly régime is descending. The fluxes caused by the disturbances in the period range of about 2 days are as large as those by Kelvin waves in the period range of 7.4-32 days. The disturbances behave like Kelvin waves to the extent that the disturbances are associated with the zonal wind component and temperature but no corresponding meridional component.
Spectral features of large-scale cumulus convective systems over the tropical oceanic area are studied and classified in terms of the atmospheric equatorial waves. Three-hourly GMS infrared data for the period of 1981-1989 are utilized. Some significant features of cloud-atmosphere systems are deduced. As an average feature for the total analysis period, robust signals are identified with Kelvin waves, westward-propagating mixed Rossby-gravity waves (MRGW), n=1 Rossby waves and n=1 westward-propagating inertio-gravity waves (WIGW). Some indications of eastward-propagating n=0 gravity waves and n=2 WIGW are also presented. A common equivalent depth of 15-30m among different wave modes is indicated from the frequency-wavenumber distribution of cloud disturbances. On the other hand, an equatorial radius of deformation of ∼7° is deduced independently from the latitudinal distribution of cloud spectra which shows a good agreement with the above equivalent depth. Seasonal variation of dominant cloud disturbances is also described and discussed in relation to the mean wind profile.
The dominance of 1.5-2.5-day variance in the cloud field in the December-February season and its characteristic features are described in detail utilizing three-hourly infrared blackbody temperature data obtained by the Japanese Geostationary Meteorological Satellite. It is shown that 1.5-2.5 day-period cloud system possesses the properties of n=1 westward-propagating inertio-gravity waves. The dominant zonal wavelength is 30°-40° and the propagation speed is 20-30ms-1 relative to the 850hPa mean wind. The equivalent depth of ∼20m was indicated from the composite analysis and from the spectral analysis. Significant correlation of 1.5-2.5-day-period mode with the total cloud variance suggests that this mode always accompanies equatorial cloud activity. Eastward-propagating n=1 inertio-gravity waves were, on the other hand, not observed.
Some features of mesoscale distribution of precipitable water and atmospheric conditions surrounding its are described using split-window data from a NOAA satellite, rawinsonde data and wind data obtained by surface station and wind profiler. Mesoscale patterns of precipitable water were recognized. Two band-shaped regions of lower precipitable water almost coincided with surface divergence zones, and an area of high precipitable water was observed in the vicinity of surface a convergence zone associated with a “small-scale sea breeze”.
Balloon observation of stratospheric aerosols was carried out at Tsukuba, Japan, on 24 August 1993, about two years after the Pinatubo volcanic eruption. Stratospheric aerosol particles collected from the stratospheric aerosol layer using an impactor were analyzed by an analytical electron microscope. Crustal particles were not detected in all size ranges. The results indicated that most of the particles observed over Tsukuba were aqueous H2SO4 droplets without a solid nucleus. This suggests that these particles were formed through homogeneous nucleation processes.