1. Introduction
The Lower Atmosphere and Precipitation Study (LAPS) is one of the projects of the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST). The CREST projects are under several strategic research themes (http://www.jst.jp/kisoken/en/). LAPS is one of the projects under a theme of "R&D of Hydrological Modeling and Water Resources System" supervised by Prof. K. Musiake, Fukushima University. The objective of LAPS is to improve the understanding on the relationship between the atmospheric boundary layer (ABL) and the precipitation system. LAPS consists of several components: field experiments, large scale data analysis, and a ABL model study. Current status of each component will be described.
2. Field experiment
The field experiments were conducted in: (1) the southwestern islands (Okinawa) in Japan, and (2) Huaihe River basin in China. The first one is partly under a strong collaboration with the National Institute of Information and Communication Technology (NICT), Japan. In Okinawa, we had precipitation observations using a dual-Doppler radar system. One of the interesting results is the effect of Okinawa main island on the precipitation system. When a precipitation band passes over the Okinawa main island, the intensity of the system weakened and then it recovered. This suggests an orographic effect on the water vapor supply from ABL. Another interesting one is that a capability of a Doppler radar to observe ABL was confirmed. We used a C-band full polarimetric Doppler radar (CRL Okinawa Bisgtatic polarimetric RAdar: COBRA) which is belonging to NICT. Intensified RHI scanning in summer season revealed ABL development. A 400 MHz windprofiler radar belonging to NICT was also operated and a good correspondence between COBRA echoes and windprofiler radar echoes were confirmed. Intensified radiosonde observation also showed that strong vertical gradient of water vapor mixing ratio which means strong gradient of the refractive index in microwave corresponds to strong echo region. While the windprofiler radar only shows vertical structures of the ABL, COBRA observes two or three dimensional structure with time variation. The China experiment is another major and important component in LAPS and is being conducted under a strong collaboration with the China Meteorological Administration (CMA). Shouxian local meteorological observatory, Anhui Province Meteorological Agency, CMA, is the main observation site, where an L-band windprofiler radar, a Doppler sodar, a water vapor profiling radiometer, and a flux observation tower were installed. Though we had schedule delay due to SARS problem in 2003 and several observational interruptions due to mal-functioning of instruments, power supply problem, etc., we have already accumulated more than two year data set. The target period is the transition period from the dry to wet season. In 2004, the onset of the wet season was not clear, but the vegetation change due to irrigation of the paddy field was revealed by the flux tower measurements and windprofiler measurements. In 2005, we had very hot and dry days but finally wet season came in July. While the annual difference is bigger than we expected, a good data for annual comparison were obtained.
3. Large scale study
To understand the seasonal migration of the wet region, large scale studies using re-analysis data and satellite data were performed. Difference of the atmosphere between south and north of Meiyu front was investigated, and a difference of vertical structure of horizontal transport of water vapor flux was revealed. That is, the horizontal transport of water vapor is deep in the south of Meiyu front. Satellite data analysis using the Tropical Rainfall Measuring Mission (TRMM) was another topic. The analysis has not yet reached to the Huaihe region, but comparisons of rain maps retrieved from the radar and microwave radiometer were performed. There exist systematic differences which are suggested to be due to variety of vertical structure of precipitation systems. The difference distinctly appeared in the peak local time of precipitation. The peak local time appears first in the rain from the radar, and the microwave radiometer follows. The infrared radiometer is the last. The precipitation system evolution is speculated to be the cause.
3. Model study
The model study is on ABL, and the method is the so-called large eddy simulation (LES). Three dimensional high spatial resolution model called CReSS is used. As the first step, dry eddies in a simple environment was successfully simulated. Now, more realistic simulation with actual environmental parameters were experimented. Incorporating the moist convection is the next target.
4. Original questions and current stage
Here, I would like to recall the original questions and evaluate the current stage. The questions and current status were: (a) There exist patchy wet areas in the wet/dry transition regions. We have nearly no progress. The water vapor channel in GMS or FY2 may give something, but no trial has been done. (b) Cloud and precipitation system transports water vapor in ABL to middle layer and moistens there. We have some indirect evidence from re-analysis data. (c) Turbulent structure in ABL determines the onset of precipitation system. We have no direct observational evidence. (d) ABL strongly affects the diurnal variation of precipitation system. The diurnal variation of ABL has been confirmed, but the effect of ABL to the diurnal variation of precipitation has yet been clear. (e) Diurnal variation of precipitation system enhances high precipitation system and transport moisture into upper layer. It is not yet clear. (f) Humid middel layer is essential for full development of precipitation system. Not yet clear from the LAPS observation. (g) Dry middle layer suppresses shallow convection but enhances deep convection. Not yet clear from the LAPS observation. type. (h) High precipitation system likely exists in the boundary area of dry and wet regions. From the global analysis of TRMM strongly suggests, but locally it is yet clear. (i) When the land is sufficiently wet, the precipitation system becomes more likely ocean From the global analysis of TRMM strongly suggests, but locally it is yet clear. (j) Above high precipitation layer is maintained by a feedback system. When the feedback system is broken, another regime appears. It is not yet clear. (k) Migration of wet region needs sufficient moist air and wet soil. Not yet clear, but the observation data suggest that the answer is “no”. (l) Precipitation system in China takes an essential role in the rain over Japan. The results from other projects suggests “yes”. Thus, we still have many questions open, and we still have long way to go. However, we have had new findings. Major findings are: (a) Atmospheric structure is different in north and south of the Meiyu front. (b) ABL development is affected by the surface condition. (c) ABL development may be affected by the environmental condition, such as, large scale subsidence. (d) A C-band Doppler radar is capable to observe three-dimensional structure of ABL. (e) Rain retrieval from space depends on the sensor. We have more results, such as, model progress, ABL structure over ocean, but we need more discussions to concrete conclusions.
5. Conclusions
The current status of LAPS were described. We have progressed but still have many questions to answer, and we need more efforts. Another important issue is the continuation of LAPS. The field observation in China is planned to continue to cover Meiyu season in 2006, and we will have data in transition season over three years. Do we need more observations? More year observation might not be so much useful, since we can expect good annual variation data over three years. If we continue the observation, other observations should be included. An important new observation could be intensive radiosonde observation at t he Shouxian site. Acknowledgements The author would like to express his thanks to all members of LAPS and strong support from CMA, JST and NICT.