The Study on Macro-microphysical Characteristics of Stratiform Clouds over North China

Hebei Province lies in the north of China, with continental monsoon climate; the average annual precipitation is 541mm. It is one of the least water resources areas in China; the average water resources amount only accounts for 1/10 of the national average amount and 1/40 of the world average amount. In order to solve the problem of water shortage, aircraft rain enhancement operation and cloud physics research have been conducted for 21 years since 1990. The aircraft is An-26 (made in pre-Soviet) with airborne PMS (PMS Inc., USA). Weather radar, satellite, ground-based microwave radiometer, lighting detection and positioning system (LDPS) and cloud simulation models are used unremitting in the field operation and research.

Since 2004, the more advanced equipped aircraft is using with airborne PMS (DMT Inc., USA).The purpose of this study is introducing the main research results of Hebei Province in China from1990 to 2004.

In this paper, the macro characteristics, concept model, microphysical characteristics of suitable seeded clouds, liquid water content (LWC), ice crystal concentration, cloud droplet concentration and size in stratiform clouds were analyzed and discussed.

According to the study of precipitation weather of stratiform clouds, stratiform cloud is the main object in aircraft cloud seeding. There are 4 kinds of weather systems suitable for aircraft rain enhancement operation: Westerly Trough, Southwest Vortex, Cold Front and Returning Flow weather system. Based on above works, the cross section of cloud structure concept models was draw. According to 62 flights cases of observation, the macro characters results shows that 28 flights are 3000~4000m in cloud depth, 22 flights are more than 4000m.. 9 flights are 0 ~ -4°C in cloud top temperature, 26 flights are -4°C~-10°C in cloud top temperature, 24 flights are-10°C~-20°Cin cloud top temperature, 2 flights are below-20°C. When average surface rain rate is less than1mm, the corresponding average cloud depth is 3300m; when surface rain rate is 1~2.5mm, average cloud depth is 4510m, when surface rain rate is more than 2.5mm, the average cloud depth is 5050m. Compared with average surface rain rate, most of the cloud top temperatures are-4~-20°C when rain occurs. When cloud top temperature is more than -10°C, the precipitation with average rain rate more than 1mm occurs.

Observed by PMS FSSP probe, the average LWC is very small in seeding level of stratiform clouds over Hebei Province, it is only 0.04 g/m3, the max LWC is 0.45g/ m3. Further analysis shows that more than 67% of flights observed the supercooled water region with LWC more then 0.1g/m3..In seeding levels, the LWC is inhomogeneous.

Observed by PMS 2D-C probe, the average ice crystal concentration is 15.6L-1 in stratiform clouds over Hebei Province. It has the similar result with Jilin Province (14.6L-1). The ice concentration varies greatly with weather system, 88% of flights observed that the average ice crystal concentration was less than 30L-1.. The results show that average cloud droplet concentration is 62 cm-3 in 4000-6500m MSL. It is 200 cm-3 in lower level. 93% of cloud droplet diameters are 4-12µm, and the average is 9µm. The droplet spectrum is narrow; it shows that the cloud droplets have the continental characteristics in Hebei Province of north China.

This study was jointly supported by the social commonweal research program (GYHY(QX)2007-6-36) and the national science & technology pillar program (2006BAC12B00-01-01-02) funded by the Ministry of Science and Technology of China.