Southerly Monsoon Research Articles

Evolution and Structure of the Mesoscale Convection and Its Environment: A Case Study during the Early Onset of the Southeast Asian Summer Monsoon

Abstract The evolution and structure of mesoscale convection in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast Asian monsoon (SEAM). For the case of 15 May 1998, interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection. In the prefrontal region, the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast-to-southwest-oriented convection. In the postfrontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly postfrontal flow gener...

An ENSO-like oscillation system

This paper presents the following data-based hypothesis on the mechanism of the EI Nino-Southern Oscillation (ENSO) cycle: during the warm phase of ENSO, there are weterly wind anomalies over the equatorial west-central Pacific and the sea surface temperature (SST) in the equatorial east-central Pacific is higher. The latter can strengthen Hadley circulation, and the west component of the Hadley circulation anomaly may cause southerly monsoon circulation. When the monsoon is a southerly anomaly, SST in the South China Sea (SCS) becomes higher. When the SCS gets warmer, the atmospheric convection will move to it, and then easterly anomaly wind will occur over the western Pacific. A cycle is completed. This hypothesis was used to construct an analogical oscillator model that produces ENSO-like oscillations. The negative feedback mechanism of the hypothesis is SST anomaly in the SCS. This negative feedback mechanism is quite different from others proposed before, is not within the Pacific and does not depend on ocean waves and their boundary reflection.

Chemical composition of aerosol and snow in the high Himalaya during the summer monsoon season

Aerosol and surface snow samples were collected from Hidden Valley in the Dhaulagiri region of western Nepal during the summer monsoon of 1994. Temporal variations of major ion (Na + NH 4 +, K +, Mg 2+, Ca 2+, Cl −, NO 3 − and SO 4 2− concentrations in the aerosol samples are clearly related to the influx of monsoon air masses. Snow was enriched in NH 4 +, and NO 3 −, while Na + Cl − ratios were lower in the snow compared to the aerosol. A large part of this is explained by the difference in the air masses represented by aerosol and snow chemistry. Snow chemistry in general represented stronger southerly monsoon circulation, which resulted in precipitation events in Hidden Valley, whereas aerosol chemistry represented weaker monsoon or local circulation as the sampling was not conducted during rainy and foggy weather. Enrichment off NH 4 + and NO 3 − in snow is attributed to their biogenic and agricultural sources from villages to the south and east of Hidden Valley. In addition, scavenging of HNO 3 present in the air could also have contributed to the enrichment of NO 3 − in the snow. A lower Na + Cl − atio in snow is attributed to scavenging of HCl present locally and/or due to less fractionation of monsoon air masses during more intense circulation and shorter travel time. The observed differences in the chemistry of the two media due to the influence of monsoon versus local air masses supports the concept of using glaciochemical records from that region to interpret monsoon variations in the past. Although the aerosol samples show excess cations, our data suggest the presence of acidic gases in the air locally. The overall major soluble ion concentrations of the aerosol are comparable or lower than those measured at several other remote tropospheric sites. Our results further support the concept that high elevation mountain sites in the Himalayas can be used to investigate the composition and the evolution of the remote continental troposphere.

Seasonal variation in atmospheric aerosols concentration covering northern Kyushu, Japan and Seoul, Korea

Atmospheric aerosols were collected from October 1990 to February 1992 in northern Kyushu (Tsushima and Ogori), Japan and Seoul, Korea, simultaneously using identical sampling system and chemical analysis methods. Observed data were analyzed with meteorological data to clarify the seasonal variations in aerosol concentration and composition. Dominant ion components of aerosol were SO 4 2− in anion and NH 4 + in cation. The concentrations of ion components were higher in winter than those in summer. Equivalent concentration ratio of particulate nitrate and sulfate ( NO 3 − SO 4 2− ) showed high value in Ogori having a peak in winter season. Equivalent concentration ratio of Ca 2+ and sulfate ( Ca 2+ SO 4 2− ) is highest in Seoul and shows no seasonal variation. On the contrary, Tsushima and Ogori showed low value in summer season. In summer, this area is affected by the atmospheric high-pressure system which developed at the southern Kyushu sea area and southerly monsoon wind predominate. As the result of this, supply of Ca 2+ from soil should be diminished in summer. Equivalent concentration ratio of particulate Cl − and Na + ( Cl − Na + ) in August was half compared with the ratio observed in February. Episode analysis of June 1991 and February 1992 indicated the transportation of pollutants from the outside of Japan. Air masses from the Chinese continent have been concluded to bring the greatest amount of anthropogenic pollutants to Korea and Japan.

Dual Doppler Radar Investigation of the Tropical Convective Boundary Layer

Radar observations of the convective boundary layer wore made during the “COPT81” experiment in May–June 1981 over western Africa. The lower atmosphere was characterized by the interaction between the southerly monsoon and the African easterly jet above it. The mean wind shear between both Bows is found to organize convective elements along preferred directions and to transfer energy to the perturbed field (velocity variances, momentum fluxes). However, the convective organization is also the cause of increasing horizontal variance and in some cases of a negative shear stress production. The global behavior of the convective boundary layer in the four observed situations appeared similar to those observed in middle latitudes, but the strong wind shear is an important feature of the layer.

Mesoscale Analyses of the Sichuan Flood Catastrophe, 11–15 July 1981

Abstract During the period 11–15 July 1981, heavy rainfall occurred over the Sichuan basin in China, resulting in severe floods that took a large toll in human life and property damage. Synoptic analyses indicate that early in this period the southerly monsoon flow was particularly strong near the basin because of a favorable positioning of the Pacific subtropical high and the Indian monsoon depression. The passage of a deep midlatitude trough from the Lake Baikal region brought colder, drier air from Siberia into southwest China. The Siberian air stream met the monsoon current over the eastern plateau and the Sichuan basin, creating a region of large-scale, low-level convergence. Mesoscale analyses show that the flood was directly related to the extreme development of a long-lived mesoscale vortex [called south (SW) vortex by Chinese meteorologists] over the basin as it merged with another mesoscale vortex generated over the Tibetan Plateau. Mesoscale heat and moisture budgets suggest that the latent hea...

A synoptic study of the variations in radon activity over the Arabian Sea and the Bay of Bengal Region during the Monsoon-77 period

Measurements of Radon activity carried-out during the Monsoon-77 Observational Programme over the Arabian Sea—Bay of Bengal Region are presented and discussed along with the earlier ISMEX-73 data. Levels of radon within the monsoon region show considerable variations in the range of one to fifteen picocuries per cubic meter of air. The levels in the Arabian Sea showed a marked increase at latitudes near-about 20°N, reaching values of eight to ten picocuries per cubic meter, thereby suggesting increased input of continental air. At lower latitudes, the levels are a few picocuries per cubic meter which is characteristics of maritime air from equatorial regions. Continuous measurements at stationary positions in the Arabian sea at widely separated locations show similar variations in radon levels. These variations seem to be connected with monsoon disturbances and their westward movements which reduce radon levels due to the strengthening of the southerly monsoon current having low radon content. In the Bay of Bengal, there is some evidence of lower radon values during “break” monsoon conditions. This could be due to the prevailing pressure pattern during the “breaks” with isobars running in north-south direction resulting in the movement of equatorial maritime air mass directly to the Bay without going over the land mass of India. During strong monsoon conditions, the air mass enters the Bay after travelling over India and consequently has higher randon levels. Near the coast, randon values are high, as is to be expected.