In this study we examine the mechanisms of the onset of the Southeast Asian monsoon (SEAM) over the Bay of Bengal and the South China Sea in terms of thermal contrast between the Tibetan Plateau and surrounding ocean based upon 5-day mean ECMWF circulation field data (1980-89) and 5-day mean GMS equivalent black body temperature (TBB) data. The early onset of the SEAM is recognizable at Pentad 28 (May 16-20) with accelerated low-level monsoon westerlies followed by second enhancement of the monsoon activities in early June. The warming over the Tibetan Plateau from spring to summer is found in the 200-500hPa thickness data on about 15-day intervals. Of importance is the observational evidence that the warming phase over the Tibetan Plateau around mid-May is concurrent with the early onset of the SEAM. Thus, the thermal contrast between the Tibetan Plateau and the adjacent ocean is likely to induce the acceleration and eastward extension of the low-level monsoon flow, causing the abrupt commencement of the SEAM including onset of the South China Sea monsoon (SCSM). This relationship between low-level wind over the key region (10°-20°N, 80°-120°E) and 200-500hPa thickness over the Tibetan Plateau is also confirmed based on the correlation analysis in the interannual variabilities. An influence for the mid-latitude atmosphere, stationary Rossby waves are generated over the South China Sea and propagate in a northeastward direction toward Japan because of the cyclonic vorticity and the tropical heat source associated with the onset of the SCSM. As a result of this wave propagation, a high pressure anomaly appears over Japan, which is consistent with a singularity of clear skies around Japan in mid-May (Kawamura and Tian, 1992).
A continuous sampling at every 0.5mm of precipitation during rain (average sampling time of 19 minutes) and analyses of ions involved in the rainwater were carried out from April 1, 1993 to March 31, 1994 in Nagakute, 16km east of Nagoya, Japan, to understand temporal changes in ion concentrations in precipitation in detail. The average pH, nitrate (NO3-), and non-seasalt sulfate (nss-SO42-) in the collected rainwaters were 4.76, 14.5μeq l-1 and 18.0μeq l-1, respectively. The average concentrations of ions obtained in this study were about half of those obtained during 1984 and 1986 by the Japan Environmental Agency, which was thought to be caused by either the longer rainy season in 1993 or the reduction of sulfur dioxide emission in recent years. The seasonal changes in ion concentrations show that they were significantly influenced by Asian dust storms and typhoons. Asian dust storms and typhoons were found to play important role in characterizing the precipitation chemistry in Japan. A rapid change in ion concentrations during several minutes was frequently found in individual precipitation accompanied by weather change. Concentrations of NO3-, NH4+, and SO42- decreased with an increase in wind speed, while those of Na+ and Cl- increased, showing the differences in the production and the deposition processes of ions. Higher concentrations of NO3- and nss-SO42- in rain samples with southern wind direction were frequently observed, showing a clear relation to the industrial facilities of ion sources located in the southern direction. A greater amount of ammonium ion (NH4+) was observed to be transported from the suburbs of Nagoya rather than from the downtown area. This measurement (short period sampling and continuous observation) showed characteristics typical of a precipitation chemistry, especially for the seasonal concentration change in major cations and anions and ion transport processes, observed in suburbs in the eastern Asia region.
The linear response of the tropical troposphere to a heat source resembling a moderately large mesoscale convective system (MCS) is modeled. The spectral representation of vertical structure, with and without rigid lids atop the atmosphere, is illustrated graphically and discussed physically. The spectrum characterizing a mean radar-observed MCS heating profile in an unbounded, realistically stratified atmosphere can be well approximated with just two spectral bands. The equations governing the wind and height amplitudes of each spectral band are linear shallow-water equations, combined with a time-dependent spatial smoothing that accounts for the finite spectral widths of the bands. This spectral band smoothing mimics the tropospheric smoothing effects of the preferential upward propagation of high horizontal wavenumber components. During the period from 0-72 hours after the MCS, the flow evolves from initially circular expanding wavelike rings, toward the planetary-wave patterns studied by Matsuno, Gill, and others. This size expansion is not truly an “up-scale” evolution: the red horizontal spectrum of scales excited by a meso-sized heat source merely undergoes phase evolution with time. Small scales do preferentially propagate upward, however. An interesting feature strongly excited by an equatorial MCS is a zonally elongated inertio-gravity motion along the equator, which cools the troposphere 48 hours after a brief heating event. This oscillation has a large zonally-symmetric component. The linear solutions developed here are superposed to obtain estimates of the wind and temperature changes forced by satellite-observed ensembles of MCSs. Low-level winds comparable to observed Australian monsoon winds spin up from rest in just 2-3 days, while some of the monsoon convective heating escapes into the equatorial waveguide as downwelling Kelvin and mixed Rossby-gravity waves. Synthetic data constructed from model fields illustrate how observing systems experience the effects of MCS heating events. Deep vertical motion on tropical rawinsonde-array scales responds rapidly to convective heating, with a response time (1-2h) small compared to the observed lifetime of MCSs (6-12h). These results indicate that rawinsonde array observations of a “quasi-equilibrium” between largescale vertical motion and convection may be due to the rapid response of the former to the latter, not vice versa as has sometimes been supposed.
In this study we examine the large-scale atmospheric circulation associated with the spring persistent rains (SPR) over Central China in March and April based on the climatological means and we propose a physical explanation of this rainy season. Low-level southwesterlies to the south of the middle and lower reaches of the Yangtze River (southern China) are responsible for SPR. Low-level southwesterlies are identified over southern China on the climatological mean wind field in SPR, and the appearance of the southwesterlies at the end of February is consistent with the onset of SPR. The southerlies, which are limited to southern China, the Indochina Peninsula and the South China Sea, are important for moisture transport to Central China and the moisture convergence there. Seasonal evolutions of low-level temperature, geopotential height and wind fields suggest that the low-level southerlies over southern China, the Indochina Peninsula and the South China Sea in SPR are caused by the westward pressure gradient associated with the eastward temperature gradient around the region from the Indochina Peninsula to the western North Pacific to east of the Philippines. The southerlies are the geostrophic winds associated with the westward pressure gradient. The eastward temperature/westward pressure gradients are most evident in March and April, and they are a result of the time-lag in the seasonal warming between the Indochina Peninsula and the western North Pacific to east of the Philippines. In addition, the coincidences of spatial distributions and seasonal evolutions from February through May between the low-level temperature and the surface sensible heat flux (SHF) suggest that the differential heating due to SHF between the two regions is likely responsible for the east-west thermal contrast. Much higher correlations than the 99% significance level among the year to year fluctuations of SPR, the eastward temperature/westward pressure gradients over the region from the Indochina Peninsula to the western North Pacific to east of the Philippines are identified. The close relationship between SPR and the eastward temperature/westward pressure gradients on both the seasonal and interannual bases strongly suggests that the east-west thermal contrast in spring between the Indochina Peninsula and the western North Pacific to the east of the Philippines plays the primary role in SPR formation.
Estimates of any precipitation characteristics based on temporally sparse observations entail uncertainty because of the natural variability of rainfall in space and time. This study measures the sampling-related uncertainties of monthly mean reflectivity profile and surface rainfall distribution. Radar and rain gauge data collected during the 1993/94 monsoon season at Darwin, Australia, are used to show the sensitivity of monthly three-dimensional radar-echo and precipitation characteristics to the frequency of observation. The data are partitioned into convective, stratiform, and anvil components according to the horizontal and vertical structure of the echoes. The analyses of this study reveal the expected trend that the uncertainties of estimated precipitation characteristics using infrequent observations scale with rainfall amount. The results have implications for climatological studies using spaceborne observation platforms revisiting a given area intermittently. The Tropical Rainfall Measuring Mission (TRMM) satellite radar, which will revisit a given 500km by 500km region approximately twice daily, will likely encounter significant problems in estimating the vertical profile of radar reflectivity in the tropics. Monthly mean reflectivity statistics (based on observations within 150km of the Darwin radar) exhibit a sampling-related uncertainty of about 20% in both rain and snow. In addition, the radar signal of the TRMM satellite will be highly attenuated below the 0°C level, and the precipitation radar will be insensitive to reflectivity less than about 20 dBZ. Therefore, the spaceborne radar will have an obscured view of the vertical precipitation structure. Reliable reflectivity statistics based on TRMM satellite radar data may be obtained primarily within an altitude range of about 5-7.5km-an altitude range though that is important for cloud electrification because of the mixed-phase precipitation processes taking place there. The sampling uncertainty, signal attenuation, and radar sensitivity vary with precipitation type. Moreover, estimation of the convective rain fraction will be compromised by uncertainties in the echo classification as well as a choice of Z-R relation. These results imply the importance of information collected by ground validation site radars to improve upon TRMM satellite estimates of precipitation characteristics and the derived vertical profile of latent heating.
The stationary band-shaped torrential rain, observed over southern Kyushu on 1 August 1993, was successfully simulated by a three-dimensional anelastic non-hydrostatic model. A warm rain scheme that explicitly predicts cloud water and rainwater is employed in the model. The Japan Spectral Model (JSM) had previously failed to reproduce the stationary feature of the rainband and had predicted the precipitation area to move eastward. The failure of JSM was examined by using a 10km-resolution model. The failure was traced to the use of the moist convective adjustment scheme. Neither coarse-resolution nor absence of hydrostatic water loading led to the failure. By the moist convective adjustment scheme stabilizing unstable layers, the air was cooled and dried in the lower adjusted levels. This drying reduced the near-surface moisture and cut off the supply of highly humid air required for the maintenance of the rainband. The causes of the organization and maintenance of the simulated stationary rainband were then investigated using 2km- and 5km-resolution models. Highly humid air in the lower layer was advected by southwesterlies, condensing over the region with a large temperature contrast in the north-south direction corresponding to the Baiu frontal zone. At first, shallow convective rolls formed parallel to the low-level westerlies. As they developed, individual cells grouped into severe convective systems, resulting in a marked depression of pressure in the lower layer by more than 1.5hPa over the region of (100km)2 for two hours after the condensation. This low pressure induced the convergence of winds. An inflow of intensified cold northwesterlies increased meridional temperature contrast, forming a strong convergence line and an organized rainband. The maintenance system of this rainband had the characteristic features of the back-building type, with significant vertical wind shear in the repeated generation of convective cells upstream of the rainband. For this torrential rain event, evaporation was not very important in the enhancement and maintenance of the rainband, while the relatively large meridional temperature contrast over the Baiu frontal zone played a significant role. The maintenance and enhancement processes of the low-level jet (LLJ) in the torrential rain event were also investigated by calculating the mean horizontal momentum budget and examining the trajectories of air parcels. The pressure gradient force produced by the convection-induced low pressure accelerated the mean horizontal velocity in the LLJ core, while horizontal advection acted to decelerate the jet. It was found that the acceleration and deceleration of the jet canceled each other to maintain the jet over southern Kyushu. The inflow of southwesterly winds into the lower layer below 1km was also accelerated, by 6ms-1 over 150min after the start of integration, by the pressure gradient force. These accelerated horizontal winds were transported upward by convection, a portion of which intensified the LLJ to the north of the rainband.
Using a 31-year (1960-1990) sample of western North Pacific tropical cyclones and monthly mean sea surface temperature (SST) for each year, an empirical relationship between SST and the maximum intensity of western North Pacific storms is determined and used to calculate the relative intensity, a measure of how close a storm reaches its maximum potential intensity. The analysis method in this study follows that by DeMaria and Kaplan (1994b) and results are compared with observations over the North Atlantic and theoretical studies. Similar to previous studies, an upper bound of storm intensity for a given SST was determined. It is shown that a larger fraction of Pacific storms are observed over warm waters than Atlantic storms and the maximum potential intensity of Pacific storms tends to be stronger than that of Atlantic storms or theoretically calculated storms. The analyses of the relative intensity at the time of each storm's life-time maximum intensity indicate that the maximum intensity of Pacific storms is well below the maximum potential intensity. The average relative intensity of the total sample is 37% (47%) when the regression curve for the maximum (99th) intensity percentile is used to compute the relative intensity, implying that environmental influences appear to be more important than SST in determining the maximum intensity of Pacific storms. The average relative intensity of late-season storms tends to be, as in the Atlantic, larger than that of early-season storms, and the yearly-averaged relative intensity shows to some extent interannual variability but with little correlation either with quasi-biennial oscillation or with El Niño.
Two noticeable abrupt changes in Northern Hemispheric winter circulation have occurred in recent decades, one around 1977 and the other around 1989. In the 1970s occurrence, changes in equatorial sea surface temperatures (SSTs) occurred concurrently with those in mid-latitude atmospheric circulation. In the case of the late 1980s, however, no important variation in SSTs was seen in the equatorial regions. In order to identify the differences in the characteristics of the changes in circulation between the two cases, observed 500hPa heights were compared to simulations with an atmospheric general circulation model (GCM) that used observed SSTs as surface boundary conditions. The results of simulation suggest that the change in circulation in the late 1980s was not the direct response to the changes in SSTs, but can rather be characterized as a triggered planetary scale internal mode of variability in the winter atmosphere.
Statistical analysis was made to test the relationship between typhoon intensity and sea-surface temperature (SST). The 10-day mean values of SST on 1° latitude-longitude grid with unit of 0.1°C were used. It was shown that intensity percentile of tropical cyclones computed at every 0.5°C bin of SST varies as a function of SST. Relative intensity of a storm is defined with reference to the maximum intensity of a storm. For tropical storms locating over waters warmer than 28.5°C at an observation time, it was shown that storms of higher relative intensity have been over wamer waters for a day or two prior to the observation time than those of lower relative intensity. It was also shown that falling rate of central pressure is dependent on SST. Higher SST allows more rapid falling of central pressure.