Observations of tropical convection and cloud clusters with two X-band Doppler radars were carried out on Manus Island, Papua New Guinea for two and a half months from 12 November 1992 during the Intensive Observation Period (IOP) of the TOGA-COARE. Several different types of clouds were observed by Doppler radars during the period, such as squall lines, convective clouds and stratiform clouds associated with cloud clusters, isolated convective clouds over the island in the day time, etc. Outlines of the observations on Manus Island and brief reviews of the results of these observations are described. Preliminary summaries of the analyses on the observations are: 1) A warm rain process dominates at the beginning stage of radar echo development and the maximum radar reflectivity is recognized at this stage, 2) The maximum echo top height is observed within 3 hours from the first echo, 3) The maximum echo area within the radar range was analyzed a few to several hours later than the time of maximum echo top height corresponding to the size of cloud cluster, 4) In the long-lasting stratiform echo, updrafts independent from those below the melting layer were identified above the melting layer. It is concluded that further studies utilizing these observational data may reveal the structure and the evolution mechanism of tropical cloud clusters.
To investigate the precipitation formation process in the two major types of weather systems (“Pacific origin” and “storm track” types) in the western Canadian Arctic region, an X ‐band vertical pointing Doppler radar, microwave radiometer were installed at Inuvik, Northwest Territories, Canada, during the midwinter of 1995/1996. Precise precipitation observations specially for ice crystals with replicator, microscope, and close‐up photos were also conducted at the same place. Precipitation formation mechanisms were quite different between these two major weather systems. For the Pacific‐origin‐type disturbance, warm and moist air was advected from the Pacific Ocean between 1.5 and 3.5 km msl. Convective echo patterns appeared by radar, and a large amount of precipitable water vapor and liquid water path were estimated by a microwave radiometer. As the air temperature was below the freezing point, the liquid water was identified as being in the supercooled state. Densely rimed dendrites and graupel particles were observed predominantly on the ground. Collision and coalescence processes of supercooled cloud droplets were dominant. On the contrary, for the storm track disturbance the moisture came from the Arctic Ocean, and strong winds were observed on the ground. The air mass was colder than −20°C throughout the layer. Stratiform echo patterns were observed by radar, a smaller amount of precipitable water vapor, and only a small liquid water path were observed by a microwave radiometer. It was suggested that the condensation growth was predominant. Snow crystal shapes of plates, columns, and bullet rosettes were observed predominantly on the ground.
In order to examine the vertical structures of the atmospheric electrical potential gradient and the behavior of charges on precipitation particles during snowfalls in the lower atmospheric layer, tethered balloon observations were carried out with several observation stages secured to its tether. Simultaneously, data observed on the ground and on the roof of the nearby building were analyzed. The main results obtained are as follows: i) The mirror image relation between the polarities of the electric potential gradient and precipitation charges was always observed near the ground, however, this relation was not always observed aloft. ii) Charges on snow particles were weaker aloft and stronger near the ground surface. Thus it is suggested that snow particles were electrified rapidly near the ground surface. iii) When the height of the cloud base was low and the highest observation stage of the tethered balloon was in the cloud or near the cloud base, snow particles were electrified weakly but positively at the highest stage. They were not directly correlated to the polarity of the local electric potential gradient. On the other hand, snow particles were electrified negatively near the ground and the electric potential gradient was positive on the ground. Therefore it is surmised that the snow particles acquired negative charges during their fall and the mirror image relation was present near the ground. iv) The atmospheric electrical potential gradient on the ground was modified by the ions which were emitted by the corona discharge. v) The most suitable electrification mechanism seems to be the Wilson's selective ion capture process below the cloud base. The behavior of atmospheric electrical elements was in good agreement with those of simple numerical experiments reported by Asuma and Kikuchi (1987).
This study is concerned with the characteristics of acidity of snowfalls brought by the northwesterly winter monsoons, low pressure systems and other disturbances around Sapporo, Hokkaido, Japan. Samplings of daily snowfalls were carried out from January 1 to March 31, 1991 in Sapporo. Falling snow crystals, whole depth of snow cover, depth of snow layers formed by each snowfall, and snowmelt water existing snow cover were measured through a period of accumulation and melting of snow cover. Snowfalls brought by the northwesterly winter monsoon winds had relatively lower pH values and higher electrical conductivity than those brought by low pressure systems. The acidity of the whole depth of snow cover did not vary greatly before the snowmelt period. The acidity of each snow layer, however, indicated characteristic variations closely connected with metamorphism of the accumulated snow cover. During the observation period, the acidity of daily snowmelt water which had run off from snow cover showed the greatest ion concentrations in the first stage of the snowmelt season. Concentrations of chemical constituents of snow cover had drastically decreased and acidity had approached neutrality at the peak of the snowmelt season. The cause of this seems to be the large quantity of chemical constituents which run off snow cover for a short time during the first stage of the snowmelt season.
To investigate a mirror image relation between polarities of electric field and charge on precipitation particles during a snowfall, a simultaneous observation system of vertical structures of the electric field and charge on precipitation particles was developed.The system consists of a tethered balloon, several observation stages fixed on the tether and a microcomputer on the ground.It was possible to measure simultaneously the vertical components of the atmospheric electric field and the maximum charges on individual precipitation particles at 5 second intervals at every 50 or 100 m altitudes above the ground surface.Several measurements were carried out in March 1985 and some interesting results were obtained.A mirror image relation was actually observed on the ground but it was not so clear aloft.
Atmospheric electrical elements during a snowfall near a building were examined by means of two dimensional numerical experiments which took into account a cylindrical building 10 m in radius and 22 m in height.Wilson's selective ion capture process and the supplement of ions due to the corona discharge from the ground surface are considered in these experiments.As the electric field on the roof of the building is intensified, a corona discharge is liable to occur there and a large number of ions are emitted from the roof into the atmosphere.Although the ions move upward owing to the electric force, they cannot reach higher altitude and a high concentration of ions appear below a 40 m level.On the other hand, snow particles are rapidly electrified below the 60 m level.Therefore, ions emitted from the roof of the building are captured by snow particles and the charges on snow particles are released to the ground surface.These calculated results qualitatively agree with the observation results described in a previous paper (Asuma et al., 1988).
