Winter MONEX Research Articles

Precipitation Observed over the South China Sea by the Nimbus-7 Scanning Multichannel Microwave Radiometer during Winter MONEX

Mesoscale cloud clusters near the northwestern coast of Borneo were observed by the Scanning Multichannel Microwave Radiometer (SMMR) on three occasions during the Winter Monsoon Experiment in December 1978. A nondimensional form of the SMMR 37 GHz polarization difference is introduced and used to identify regions of precipitation, and these are compared with visible and infrared imagery from the GMS-1 geostationary satellite. For two of the three cloud cluster cases, quantitative comparisons are made between nearly simultaneous SMMR observations and reflectivity observations made by the MIT WR-73 digital weather radar at Bintulu. Though limited in scope, these represent the first known direct comparisons between digital radar-derived rain parameters and satellite passive microwave observations of near-equatorial precipitation. SMMR 37 GHz observations are found to be much better indicators of fractional coverage of each SMMR footprint by rain than of average rain rate within the footprint. Total area coverage by precipitation is estimated for all three clusters using this result.

Airmass transformation over the South China Sea during a winter MONEX cold surge

AbstractSeveral studies have linked a wintertime cycle of deep, moist near‐equatorial convection to surges of cold air from Asia. According to an early hypothesis two successive streams of surge air reach north Borneo and offshore waters of the South China Sea. The first, which follows a long over‐water trajectory, is warm by the time it arrives off Borneo. The second, which follows a short over‐water trajectory, still is cool when it arrives off Borneo. More recent hypotheses suggest only the second stage of the surge involves an airstream from Asia and drying rather than cooling is the predominant downstream thermodynamic effect on this airstream.By means of trajectory calculations and an airmass transformation (AMT) model we test these ideas for a moderate cold surge from December 1978. In particular, we contrast the roles, in the transformation process, of surface fluxes of latent and sensible heat and of boundary layer entrainment. At no time through the course of the surge did we find boundary layer trajectories of the second (short path) type. Nevertheless, late in the course of the case study surge dry air did appear at the surface off the south‐east coast of Vietnam. Neither sea surface temperature nor initial airmass structure could account for the local nature of the drying. The explanation most consistent both with observations and AMT model results is entrainment of air that had been dried by local subsidence at and above the top of the marine boundary layer.

Airmass transformation over the South China Sea during a winter MONEX cold surge

Several studies have linked a wintertime cycle of deep, moist near-equatorial convection to surges of cold air from Asia. According to an early hypothesis two successive streams of surge air reach north Borneo and offshore waters of the South China Sea. The first, which follows a long over-water trajectory, is warm by the time it arrives off Borneo. The second, which follows a short over-water trajectory, still is cool when it arrives off Borneo. More recent hypotheses suggest only the second stage of the surge involves an airstream from Asia and drying rather than cooling is the predominant downstream thermodynamic effect on this airstream. By means of trajectory calculations and an airmass transformation (AMT) model we test these ideas for a moderate cold surge from December 1978. In particular, we contrast the roles, in the transformation process, of surface fluxes of latent and sensible heat and of boundary layer entrainment. At no time through the course of the surge did we find boundary layer trajectories of the second (short path) type. Nevertheless, late in the course of the case study surge dry air did appear at the surface off the south-east coast of Vietnam. Neither sea surface temperature nor initial airmass structure could account for the local nature of the drying. The explanation most consistent both with observations and AMT model results is entrainment of air that had been dried by local subsidence at and above the top of the marine boundary layer.

Modification of the Boundary Layer over the South China Sea during a Winter MONEX Cold Surge Event

Abstract Aircraft dropwindsonde and conventional sounding data have been used to document the boundary layer structure over the South China Sea during the period of a 3-day moderate cold surge that occurred during the December 1918 Winter Monsoon Experiment (Winter MONEX). Throughout the cold surge event, equatorward deepening of the cumulus layer is observed over the northern South China Sea (to the south of a coastal cloud-free region) as cold air streaming of the coast encounter substantial surface, sensible and latent heat fluxes. Following the onset of the cold surge, the cumulus-layer depth increases over the central South China Sea. However, this depth decreases over the northern South China Sea at a distance of 200–300 km from the China coast. The depth and strength of the cumulus (or subsidence) inversion are greatest new the China coast (∼600–700 m, 4°–6°C) and weaken toward the central South China Sea (∼200 m, 1°C), whew they eventually become undetectable. Possible explanations for the observe...

Short-Term Variations of the Tropopause Height over the Winter MONEX Area

Abstract An observational study of short-term (∼1 week) tropical tropopause height variations has been undertaken using measurements from the December 1978 Winter Monsoon Experiment (Winter MONEX). Sounding data from three Soviet ships in the southern South China Sea and conventional rawinsonde stations in the region of the Indonesian maritime continent am used in the analysis. During a one-week period (10-17 December), a ∼1.5 km reduction in the height of the tropopause and a warning of 5-8°C in the lower stratosphere were observed over the southern South China Sea. Outside this region a systematic lowering of the tropopause was not observed. In the region of tropopause descent there was a concurrent increase in the amount of deep convective activity. Within the time period of them events, two winter monsoon cold surges (on the l0th and 15th) and a weak tropical depression (on the 14th) affected the circulation over the South China Sea. During the period of tropopause lowering. tropospheric pressure heig...

中規模積雲対流系における下部対流圏の昇温および乾燥化:積雲対流のパラメタリゼーシヨンへの示唆

Observations from the International Winter Monsoon Experiment (Winter MONEX) have been used to determine temperature changes in the lower troposphere beneath the stratiform components of tropical mesoscale convective systems. It is found that while the apparent heat source Q1 in the lower troposphere beneath the stratiform cloud systems is negative (due to hydrometeor melting and evaporation), the time tendency of dry static energy (*s/*t) is positive. This low-level warming is a result of mesoscale downdrafts within the mesoscale convective systems. The warming and drying at low levels in such systems may contribute importantly to temperature and moisture changes in large-scale tropical disturbances. The effects, if important, will need to be considered in the development of schemes for the parameterization of cumulus convection.

Rain Amounts Near and Over North Borneo during Winter MONEX

Abstract Radar and rain gauge data are used to determine the precipitation pattern in the vicinity of the north coast of Borneo during Winter MONEX. The results show that separate maxima of rain occurred offshore and inland. Satellite data for other years suggest that this rainfall pattern typifies the winter monsoon in this area.

Microphysical Structure of Winter Monsoon Cloud Clusters

Abstract Images of hydrometeors in the size range 0.1 to 1.6 mm were obtained by aircraft in precipitating clouds over the South China Sea during Winter MONEX. These observations document for the first time the particle structures and concentrations in tropical cloud clusters above the 0°C level. The data were obtained under nearly constant atmospheric conditions (altitude 7–8 km, temperature −14 to −19°C). The particle samples were stratified according to the type of precipitation in which they were observed as determined from airborne radar, and according to particle size and shape. The convective cells in the observed clusters were not unlike cells observed elsewhere. They had typical particle concentrations of 100–300 per liter, and riming was apparently the dominant growth mechanism. Ice multiplication may have been active in the convection. The stratiform regions of the clusters were microphysically quite distinct from the convective cells. “Strong” stratiform precipitation areas (radar reflectivity...

Short-Term Fluctuations in the Australian Monsoon during Winter Monex

Abstract Satellite cloud imagery and single station rainfall data are used to define large-scale cloudy and clear phases of the Australian monsoon for the Winter (W) Monex period 20 December 1978 to 31 January 1979. To examine the flow characteristics and possible forcing mechanisms of these phases, large-wale, objective wind and mean sea level pressure analyses have been produced using W Monex IIB data. Composited Row configurations for each phase show marked differences in areal mean divergence and vertical motion over the monsoon region. The other main distinguishing features are the strength of the northeas trades, the strength of the southeast trades over the Indian Ocean, the amplitude of Southern Hemisphere midlatitude upper troughs and the location of the Northern Hemisphere jet maximum. No obvious differences in the wind field are evident over the deep tropics. For each mean phase, diagnosed divergent wind analyses indicate that the ITCZ is farther south and the Southern Hemisphere Hadley cell mo...

Regional Circulation Characteristics Associated with a Cold Surge Event over East Asia during Winter MONEX

Abstract A case study is presented of the cold surge over East Asia during 9–13 December 1978; using nine vertical levels of Winter MONEX data. The surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes (∼30°N) to the equator within three days. During the period studied, the lower tropospheric circulation dramatically changed in extent and intensity. With the onset of the surge event, the main divergent maximum began shifting from the South China Sea to southeast China. At the same time, the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere, and a direct vertical coupling between flows in the low and high troposphere was observed. The time-averaged meridional mass circulation between 100 and 126°E reveals a two-cell structure; the southern cell is located between the northern South China Sea and the equator, and the northern cell between midlatitudes and the northern South China Sea. Analysis of s...

Mesoscale Updraft Magnitude and Cloud-Ice Content Deduced from the Ice Budget of the Stratiform Region of a Tropical Cloud Cluster

Abstract Radar observations obtained during Winter MONEX describe the three-dimensional structure and development of the mesoscale stratiform region of a tropical cloud cluster. The magnitudes of the mesoscale updraft and cloud-ice mass content within an upper tropospheric stratiform cloud deck are deduced from a formulation of the ice budget of the stratiform region. Measurements of radar reflectivity at upper levels serve as the primary input data. Aircraft observations of ice particles are also used to guide the calculations. The mesoscale updraft magnitude and cloud-ice mass content are determined hourly over a period of 12 h. The updraft velocities were 10-23 cm s−1, and the cloud-ice contents were 0.1-0.3 g m−3. The vertical velocity estimates agree well with estimates obtained for the same cloud cluster by Johnson (1982) using the kinematic method.

Heat and Moisture Budgets of Tropical Mesoscale Anvil Clouds

Abstract An analysis of the heat and moisture budgets of tropical mesoscale anvil clouds has been carried out. Mesoscale anvils, defined as widespread (∼100 km) cloud systems extending from near the freezing level to the high troposphere, are characterized by light, stratiform precipitation. These cloud features, which are preceded by and generally merged with cumulonimbus clouds, are prevalent throughout the tropics and summertime midlatitudes and may account for an important fraction of the total tropical rainfall. Sounding data from the December 1978 field phase of the International Winter Monsoon Experiment (Winter MONEX) are used to determine heat and moisture (Q1t and Q2) budgets for a number of mesoscale anvil cloud systems. The composite heating (Q1) profile shows a warming peak in the upper troposphere near 350 hPa or 8–9 km that can be attributed to condensation and freezing in the anvil and a cooling peak in the lower troposphere near 700 hPa or 3 km due to rainfall evaporation and melting. The...

Satellite observations of the southern hemisphere monsoon during Winter MONEX

Satellite observations of the southern hemisphere monsoon during Winter MONEX

The Onset of the Australian Monsoon During Winter MONEX: Synoptic Aspects

Abstract A case study is presented of the onset of the Southern Hemisphere summer monsoon at longitudes near Australia during December 1978. The numerical analyses comprising this case study are used in conjunction with station data and operational manually derived analyses for other years to investigate the following: 1) the case or definition of monsoon onset; 2) the three-dimensional structure of the troposphere during an active monsoon situation; and 3) the flow changes preceding and during the transition from a period of suppressed to a period of enhanced cumulonimbus activity over tropical Australia. A well-defined onset occurs in six of the seven years considered. Onset, defined as a satellite-observed, large-scale increase in tropical convection, is consistent with that determined by the wind criterion of Troup (1961). In 1978 onset occurs in two stages: an increase in convergence, followed by an increase in convection. The monsoon cloudiness exists entirely in the region of low-level westerly win...

Gravitational Character of Cold Surges during Winter MONEX

Abstract The surface pressure, temperature, dew point and wind data over the South China Sea and vicinity during Winter MONEX are examined to determine the timing of the passage of cold surges at various reporting stations. It is found that for more than half of the surge cases during Winter MONEX, a surge occurs in two stages separated by a time interval of several hours to approximately one day. The first stage is often characterized by a significant rise in pressure, while the second stage by a sharp decrease in dew point temperature. Freshening of surface winds may accompany either or both stages. The time separation between the two stages is relatively short at the upstream stations but generally long at the downstream stations. The second stage is associated with a frontal passage. On the other hand, the first stage is not clearly associated with any significant synoptic events. From its very fast propagation speed which increases with an inferred depth scale, and an increase in the local cross-isob...

Satellite observations of the southern hemisphere monsoon during Winter MONEX

Time-longitude strips of infrared GMS satellite imagery have been used to study the southern hemisphere summer monsoon during Winter MONEX. This representation reveals a sudden monsoon onset in late December characterized by a transition to spatially organized convection. Active and break regions of the monsoon show a strong coherence in time but are very longitudinally dependent. The latitudinal dependence, on the other hand, is quite weak. The dominant spatial scale over which changes in convection take place is of order 35 degrees longitude by 15 degrees latitude. These synoptic scale convective regions can move in either the eastward or westward direction. Typical phase speeds of movement range between 3 and 8 degrees longitude per day. Tropical cyclones in the region are revealed to be embedded in the large-scale monsoon circulation. DOI: 10.1111/j.1600-0870.1983.tb00196.x

Thermodynamic and Circulation Characteristics, of Winter Monsoon Tropical Mesoscale Convection

Abstract During the December 1978 field phase of the International Winter Monsoon Experiment (Winter MONEX), a regular diurnal cycle of deep convective activity occurred over the South China Sea immediately to the north of Borneo. The convection was initiated during the late night hours by convergence of the low-level northeast monsoon flow with the land breeze along Borneo's north coast. The precipitation feature that developed was characterized during its mature stage by a nearly continuous stratiform cloud, referred to here as a mesoscale anvil, of ∼ 100–500 km dimension extending from the mid-troposphere (∼5 km) to near the tropopause (∼ 17 km). Using rawinsonde data from the sounding network, thermodynamic and Circulation characteristics of the convection are documented. Mesoscale upward motion is found to within the anvils, with a MCSOMC downdraft below producing a warm, dry region over a lame area just above the boundary layer. Air flow relative to the anvil on both its leading and trailing edges i...

Mesoscale Features and Cloud Organization on 10–12 December 1978 over the South China Sea

Abstract Aircraft data from Winter MONEX have been combined with other data to study mesoscale features, and organization of cumulus clouds, on 10–12 December 1978. A moderate cold surge in the northeasterly monsoon flow, toward cloudiness in an equatorial trough off Borneo, peaked on 11 December. Clouds in the northeasterly monsoon flow were similar to those in the trades, with variations in convective regime on length scales on the order of 100 km. Marked mid-tropospheric subsidence was accompanied by low-level divergence near 20°N. During 10 December, anvil clouds near Borneo expanded; cumulus congestus and cumulonimbus formed on the periphery of this area. The approach of the low-level northeasterlies to the area of anvils was marked by a diminution of subsidence, conditional instability, and a weak field of low-level convergence, with randomly organized cumulus of increasing height. A low-level easterly jet was found in this transition zone, downstream from cloudiness over the Philippines. South of V...

Analysis of atmospheric hydrocarbons during winter MONEX

Analysis of atmospheric hydrocarbons during winter MONEX

インドシナ海域における大規模気圧変動

Surface meteorological and satellite observations during the winter MONEX observational period over the Indonesian Maritime Continent have been analyzed. The results of the analysis show the existence of large scale disturbances in the pressure field. These disturbances, which affect the entire region, have periods of approximately 20 days and amplitudes of about 1 to 2 millibars. They appear to have no appreciable movement. There is no significant correlation between the pressure disturbances and the cloudiness distribution.