Tropical Rain Belt Research Articles

サヘルの降水傾向に関連した熱帯降水帯、大気循環、海水面温度

サヘルの降水傾向に関連した熱帯降水帯、大気循環、海水面温度

Rainfall over oceans: Remote sensing from satellite microwave radiometers

Microwave radiometer brightness temperature (Tb) measurements obtained from satellites over the oceans in dual polarization, at frequencies ranging from 6.6 to 85 GHz, reveal information about the rain and precipitation sized ice. These multifrequency measurements are composited from observations made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I). TheTb measurements at 37 GHz, having a field of view (fov) of about 30 km, show relatively strong emissions due to rain, reaching values as large as 260 K over the tropical and mid-latitude rainbelts. Only marginal effects due to scattering by ice above the rain clouds are revealed. At frequencies below 37 GHz, where the fov is much larger than 30 km and the extinction is weaker,Tb is significantly smaller than 260 K. Additional information content about rain, at these low frequencies, is not appreciable. On the other hand, at 85 GHz (fov ≅15 km), where the extinction is very strong, the sea surface below the clouds is often masked and scattering due to ice above the rain clouds is vividly noticed. However, these high frequency measurements do not yield direct information about rain below the clouds.

Potential effect of climate change on terrestrial biota

There are several important changes in Phanerozoic climates that may have influenced terrestrial biota. Prior to the time of the terrestrial invasion in the early Paleozoic, an unvegetated landscape would have been associated with a larger seasonal cycle and decreased precipitation. Albedo and transpiration changes associated with the first appearance of plants should have changed this pattern, reducing the seasonal cycle and increasing precipitation. Enhanced soil weathering associated with vegetation should also have lowered atmospheric CO2 levels. During Pangaea time the giant landmass should again have caused very large seasonal cycles and decreased precipitation. Climate models invariably simulate below-freezing temperatures in winter on the supercontinent, a result at variance with inferred “equable” climates of the Jurassic. The large landmass should also have had very large Milankovitch variations. Seasonal warming during precession half-cycles of 10,000 years duration may have been associated with intermittent occupation of high-latitude sites that could bias interpretation of warmth in high latitudes. During the later Mesozoic (140–80 Ma) CO2 levels may have been several times higher than present. High CO2 levels should have affected the cycles of temperature and precipitation on land and also affected plants through the CO2 fertilization pathway. Subsequent decreased CO2 levels and continued breakup of landmasses may have influenced location of tropical rainbelts and perhaps the distribution of tropical rainforests. Numerous studies indicate that these changes, combined with expanded ice cover in the later Cenozoic, resulted in increasingly drier conditions in higher latitude land areas. The situation in the tropics is more uncertain, because there are conflicting signals from the marine realm as to the Cenozoic history of tropical sea-surface temperatures (SSTs). If early Cenozoic SSTs were in fact low (as indicated by some δ18O records), then tropical temperature and moisture levels may not have been high enough to sustain tropical rainforests. This summary highlights some of the excellent opportunities for interactions between paleontologists and paleoclimatologists in assessing the effects of climate change on terrestrial biota.

A comparative study of the observations of high clouds and simulations by an atmospheric general circulation model

The importance of clouds in the upper troposphere (cirrus) for the sensitivity of the Earth's climate e.g., requires that these clouds be modeled accurately in general circulation model (GCM) studies of the atmosphere. Bearing in mind the lack of unambiguous quantitative information on the geographical distribution and properties of high clouds, the simulated distribution of upper tropospheric clouds in a spectral GCM is compared with several satellite-derived data-sets that pertain to high clouds only, for both winter and summer seasons. In the model, clouds are assumed to occupy an entire gridbox whenever the relative humidity exceeds 99%: otherwise the grid box is assumed to be free of cloud. Despite the simplicity of the cloud prediction scheme, the geographical distribution of the maxima in the model's upper tropospheric cloud cover coincides approximately with the regions of the observed maxima in the high cloud amount and their frequency of occurrence (e.g., intertropical convergence zone and the monsoon areas). These areas exhibit a minimum in the outgoing longwave radiation (OLR; Nimbus-7) and are also coincident with regions of heavy precipitation. The model, with its relatively simple cloud formation scheme, appears to capture the principal large-scale features of the tropical convective processes that are evident in the satellite and precipitation datasets, wherein the intense, upward motion is accompanied by condensation and the spreading of thick upper tropospheric layers of high relative humidity and cloudiness in the vicinity of the tropical rainbelt regions.

Elevation angle dependence in tropical regions

AbstractThe variation of path attenuation with elevation angle has been generally considered to approximate a cosecant law, certainly between elevation angles of about 10° and 50°. Such a theory relies on the assumption that the attenuation layer is statiform for most of the time, and is therefore probably reasonable for temperate regions. In tropical areas however the incidence of convective rain is much higher than in temperate regions, so that during rainy periods, particularly at times of high rain rate, the assumption that the attenuating layer is stratiform is no longer valid. The cosecant law does not therefore hold in such conditions.An experiment has been carried out in the tropical rain belt of northern Australia over several years to investigate the elevation‐angle dependency. Four radiometers were co‐sited, with the same azimuth. Three of these, with a half power beamwidth of 2.2°. were elevated to 15°, 45° and 85°. The fourth. with a beamwidth of 0·6° was elevated to 45° so as to allow an investigation into aperture effects. The antennae were all orientated for horizontal polarization. Rain‐gauges were co‐sited with the radiometers and placed along the look path.The results obtained clearly indicate that, in tropical regions. the cosecant law is inappropriate for predicting the elevation angle effects of path attenuation. particularly at high rain rates. The presence of rain cells with rates exceeding 300 mm/h ensures that the assumption of a stratiform attenuating layer is invalid.

熱帯アフリカにおける降水帯の長期変動とそれがサヘルと北カラハリの降水量変動に及ぼす影響

熱帯アフリカにおける降水帯の長期変動とそれがサヘルと北カラハリの降水量変動に及ぼす影響

Branches of the summer Asian lower-level jet stream and its influence on the rain belt in china

Based on the analysis of the Asian lower-level jet stream, this paper indicates that having shifted to the Bay of Bengal, this large-scale lower-level jet(LLJ) develops into two branches: the northern branch (NB) which is a strong southwest flow moving into the inland of China along the southeast side of the Qinghai-Xizang Plateau and then moving eastward to Japan along the north side of the subtropical high, and the southern branch (SB) which continues to be a west flow and travels into the West Pacific across the Indo-China Peninsula along the south side of the monsoon trough. Above the two branches are two synoptic-scale transient tubular monsoon circulation systems, the northern branch being a subtropical monsoon stream tube (SMST) and the southern branch a tropical monsoon stream tube (TMST). Their ascending branches, corresponding to a subtropical monsoon rain belt and a tropical monsoon rain belt respectively, bear considerable influence on the weather over China.

Differences between the Spring and Autumn Circulation of the Northern Hemisphere

The phase of the annual cycle in the mean temperature of the Northern Hemisphere troposphere lags the conventional meteorological seasons (centered on April and October) by about two weeds. Hence, the conventional spring season is colder and generally more winter-like than the autumn season. When the transition seasons are redefined so as to bring them more closely into line with the annual cycle in tropospheric mean temperature, a distinctive pattern of differences emerges, which is dominated by a zonally symmetric component. These differences are investigated, making use of three different climatological datasets. Spring is characterized by higher pressure over the polar regions, and lower pressures in the vicinity of 40°N at all levels in the troposphere and lower stratosphere. The amplitude of this difference pattern increases with height. Consistent with the pressure pattern, the belt of strongest zonally averaged westerlies is located farther equatorward, by 5–10° latitude, during spring than during autumn. These differences are observed during virtually every year. It is hypothesized that the difference in the position of the jet stream is responsible for some of the marked climatic contrasts between the two seasons observed over parts of the hemisphere. Differences between the climatological mean stationary wave patterns in the two seasons are documented and discussed. Differences in the phase tilts of the waves in the horizontal plane are consistent with the hypothesis that the, orographic forcing shifts equatorward during spring and poleward during autumn with the strongest zonally averaged zonal winds. The subtropical jetstreams are almost twice as strong during spring as during autumn, and outgoing longwave radiation data indicate that the tropical rain belt shifts northward from spring to autumn.

A Numerical Study of Tropical Large-Scale Air-Sea Interaction

Abstract In a series of numerical experiments using a simplified, domain-averaged, coupled air-sea model, some aspects of the low-latitude large-scale interaction between the atmosphere and the ocean are investigated. Experiments are designed with a view toward elucidating basic mechanisms involved in the coupled processes. In this paper, we focus on the response of coupled system to 1) steady forcing, 2) seasonal variations and 3) large perturbations and continuous short-period random forcings. Results indicated that the sea surface temperature (SST) distribution, which is strongly controlled by the oceanic upwelling, is the primary factor in determining the location and transition of the tropical rainbelt. The strongest convective activities in the ITCZ, however, depend mainly on the moisture supply from horizontal convergence and the static stability of lower atmosphere, and do not necessarily coincide with the occurrence of maximum SST. It is also demonstrated that the positive feedback processes betw...

El Nino Expedition first cruise completed

With its arrival at Lima, Peru, the University of Hawaii's research vessel Moana Wave has completed the first cruise of the expedition to study El Nino, the warm current that generally develops concurrently with a southerly shift in the tropical rain belt shortly after the first of the year. El Nino is a major destroyer of marine life in the coastal waters of Ecuador and Peru.As predicted in September 1974 by K. Wyrtki and W.H. Quinn, El Nino conditions developed off Peru and Ecuador in February and March 1975. The R.V. Moana Wave has observed a massive transgression of low‐salinity water across the equator just east of the Galapagos Islands; a thin layer of warm water with a salinity less than 33‰ has advanced to 4°S. The area off Ecuador is also covered with warm water of low salinity (less than 34‰). Weak upwelling is still present along the coast of northern Peru, but no cool water is advected northwest from this upwelling area, as it has been in normal years.

Weather Variability, Climatic Change, and Grain Production

A cooling trend in the world's climate would have serious effects in the monsoon belts depending on whether or not the recent changes in snow and ice cover in the polar regions were responsible for the droughts in Africa and the failure of the monsoons over South Asia. The cooling and shrinking of the atmosphere at the higher latitudes is believed to have brought the subtropical anticyclones nearer to the tropical rainbelt and have caused a shifting of the monsoon belt. The regions that would be most severely affected by a continuation of the cooling trend to the year 2000 would be the higher latitudes (above 50 degrees) where spring wheat is grown and the warm band below 30 degrees latitude where rice is the principal grain crop. Weather variability is a much more important consideration in grain production than a cooling trend. Our highest yields are made when weather is near normal or slightly cooler than normal. It is when weather variables deviate greatly from normal that yields are lowest. Even if the weather does trend toward the coolness of a century ago yields will not be reduced significantly unless the weather becomes more varible.

The seasonal variation of the hydrologic cycle as simulated by a global model of the atmosphere

A numerical model of the atmosphere with a seasonal variation of insolation and sea surface temperature is time integrated for over 3 simulation years on a finite difference grid network having a nearly uniform horizontal resolution of approximately 265 km. There are 11 levels in the model from 80 m to 31 km above the ground. The model has realistic continents with smoothed topography. In addition to wind, temperature, pressure, and water vapor, the model simulates rainfall, snowfall, and evaporation at the surface. The runoff rate and the rates of change of soil moisture and snow depth are determined by a budget of liquid water, snow, and heat at the ground surface. The simulated precipitation and other hydrologic quantities are compared with those derived from observed data. In addition, the correspondence between the distribution of precipitation rate and those of other relevant quantities, such as sea level pressure and kinetic energy of transient disturbances, is examined. To obtain an overall impression of the climate simulation, a map of Koppen climate types, which is obtained from simulated temperatures and precipitation rates throughout the year, is compared with a similar map based upon observed climatic data. It is shown that the model locates the major arid regions of the earth, such as the Sahara Desert in northern Africa and the deserts of central Asia and Australia. Furthermore, the tropical rain belt and its seasonal movement are well reproduced. The model approximately simulates the changeover from dry winter to wet summer conditions in southern Asia and the seasonal reversal of the monsoon wind system over Asia. The wet zone in middle and high latitudes, such as in Canada, Europe, and western Siberia, is also simulated by the model. Examination of the distribution of runoff over continents of the model reveals that the watersheds of many important rivers of the world are correctly positioned in the simulation. In general, the quality of the simulation tends to deteriorate in the neighborhood of major mountain ranges. Furthermore, the rate of precipitation and that of runoff over continental regions in the model tropics is much larger than estimates of the actual rates of these quantities. Nevertheless, this study demonstrates that the model is capable of reproducing many of the basic features of the seasonal variation of hydrology and climate on a global scale.

The Seasonal Variation of the Tropical Circulation as Simulated by a Global Model of the Atmosphere

Abstract A mathematical model of the atmosphere with a seasonal variation of insulation and sea surface temperatures is integrated numerically with respect to time over three model years. The model has a global computational domain and a realistic distribution of mountains. It contains a highly idealized parameterization of convection, i.e., dry and moist convective adjustment. It is found that the model accurately simulates the seasonal variation of the location of the tropical rainbelt as well as that of the flow field associated with it. Over the continental regions of the model, the tropical rainbelt tends to form very close to the equator, whereas, in certain oceanic regions, it has a tendency to form away from the equator. Based upon a comparison of these results with those of another numerical experiment, it is concluded that this tendency is not due to an inherent characteristic of the rainbelt of the model to avoid the equator in oceanic regions, but rather it is due to the equatorial belt of low...

Measurement of point-discharge carrent density in the atmosphere

The effective point-discharge area of trees and raised metal points have been determined from discharge currents measured simultaneously in the elevated dischargers and grass blades isolated from the Earth. From measurements extending over two thunderstorm seasons at Ibadan in the tropical rain belt of Nigeria, a mean point-discharge current density of −(1.2 + 0.2) × 10 −9 A m −2 is obtained. Adopting the figure −1.3 × 10 −9 A m −2 as the global mean we obtain the mean charging current generated through worldwide thunderstorm activity to be 1300 A. As this is just about the current required to balance the leakage current in the fairweather areas of the Earth, the result provides evidence in support of the classical picture of atmospheric electricity; though final acceptance of the picture must await the availability of reliable data on the diurnal variation of the integrated thunderstorm area.

ANALYTIC MODEL FOR ZONAL WINDS IN THE TROPICS

We consider the zonal mean circulation on an equatorial β-plane and obtain analytic solutions for small-amplitude disturbances in response to sources of heat and momentum. The effect of dissipation is roughly approximated by using assumed constant drag and radiative damping terms. We apply the model to the tropospheric zonal winds, temperatures, and meridional circulation to give simple insights into the maintenance of the observed mean state of these parameters by the existing sources of heat and momentum. The heat source consists of a relatively sharp maximum in heating at the Equator caused by latent heat release in the tropical rainbelt against a relatively smooth background of mean radiative cooling. As a momentum source, we use the convergence of horizontal eddy momentum fluxes. Numerical results are presented for an idealized annual mean circulation. The model solutions show how the thermal and momentum sources jointly maintain the zonal wind distribution. A concomitant meridional circulation redistributes the heat and momentum, allowing the temperatures to remain in balance with the zonal wind. This is part I of a two-part study.

A New Frog of the Genus Syrrhophus from the Bolivian Plateau

A new Syrrhophus (S. laplacai, new species) is describ- ed from the Bolivian Plateau, 100 km east of La Paz, in the Unduavire- gion (3400 m), on the borders of the tropical rain belt or yungas. This genus of frog has not previously been reported from the Bolivian region. The affinities of the new species with S. montium Shreve are discussed. This latter Andean form has been recently placed by Lynch in the South American genus Niceforonia, leaving only Mexican and Central American forms in the genus Syrrhophus.

Treponematoses in Gambia, West Africa

1. (1) Surveys for treponemal disease were made in Gambia, West Africa, of villages of the main tribes scattered throughout the country. Approximately 7,500 persons were examined once during the dry season and again during the rains. Serological investigations employing S.T.S. and T.P.I. tests were undertaken in London. 2. (2) The early lesions of yaws, i.e. florid lesions on the skin, were found in one village, and a second form of non-venereal treponematosis known locally as siti was found in four out of five villages of the Fula tribe. 3. (3) The early lesions of siti occurred on mucous membranes of the mouth, and the tip of the penis of small boys before puberty. Florid lesions were found on the vulva and round the anus. Circinate lesions were found on the skin and also axillary lesions resembling yaws. Sera from the above lesions contained treponemes morphologically identical with T. pallidum, and the serological tests in these cases were positive. 4. (4) The most common late manifestation of treponemal disease in Gambia appeared to be depigmentation, particularly depigmentation of the hands. It has been shown statistically that depigmentation of the hands is probably due to treponemal disease. Other late manifestations were too infrequently seen for accurate conclusions to be drawn. 5. (5) No evidence was found of the development of lesions of the cardiovascular or central nervous systems. 6. (6) Contractures of the fingers were encountered in 3 per cent. of the female population and in 1.9 per cent. of the male population. These figures are significantly different. No statistical evidence was found to suggest that contractures were the result of treponemal infection. 7. (7) Siti appeared to be transmitted by the use of a common drinking-cup and common food-bowl, and close personal contact. 8. (8) Gambia lies at the edge of the tropical rain-belt, and the climatic conditions are unusual in that there is one short, intense rainy season and a long dry season, giving for part of the year a desert climate and for a short part of the year a humid climate. The fact that both yaws and the other form of non-venereal treponematosis were found side by side is believed to be further evidence in the hypothesis of the common aetiological agent of the treponematoses, and that the clinical manifestations are affected, among others, by climate. 9. (9) It is suggested that the term “endemic syphilis” is a misnomer and should be abolished; and that, with the exception of the Bosnian form of the disease, all the non-venereal treponematoses, including yaws, should be classed together.