Abstract
摘要：降雨量測與預報是重要科學與民生議題。決定降雨的水循環過程包含雲輻射、大氣海陸交換過程與環流的互動。在季風擾動與熱帶氣旋降雨最活躍的東亞暖季，此過程尤其重要。此外根據近三十年來衛星觀測及更長期常規觀測分析結果，伴隨全球暖化強降雨頻率似有增加趨勢。本計劃的總目標在識別並了解東亞與臨近海域內，熱帶雲-降雨與伴隨的水循環過程隨氣候暖化的改變。為達成這個目標，我們提出兩個觀測分析的子計劃以及兩個模擬實驗的子計劃，以及降水趨勢變化的假說：氣候暖化導致強降水與強上升運動發生頻率的增加（亦即降水效率更高），驅動大範圍下沉運動；受到輻射冷卻節制的下沉運動進一步影響較弱的降水。
觀測分析計劃將分析近三十年觀測資料（新一代再分析資料，地面與衛星降雨）。子計劃一將識別全球、東亞與臨近海域內的大尺度水循環特徵，做為本計劃的基礎。子計劃二將分析近幾十年來不同氣候區域內（如海面與陸面）雲-降雨頻譜分佈的趨勢變化。模擬實驗將採取區域氣候模式（RCM）模擬（子計劃三）與水循環過程模式（HCM）模擬（子計劃四）兩個途徑。子計劃三將與中研院周佳合作，分析他進行的兩類組RCM 西北太平洋臺風季節模擬實驗：1979-2008 事後預報類組以及暖化氣候類組。此RCM 模擬場的基態均被spectral nudging 方式調整至與控制場一致。子計劃四將採用新一代具備雲模式物理過程的WRF 區域模式，進行兩類組實驗。第一類組為2004 及2008 兩組六月模擬，每組針對該年六月內，每天同一時間為初始場，積分36 小時， 共 30 個實驗。另一類組為當代氣候與暖化氣候兩組模擬。每組針對其六七八月氣候，進行90 個36 小時實驗。這兩個子計劃模擬結果將與衛星資料比較（模擬量轉至衛星觀測量），並與子計劃二觀測分析結果比較，評估模式水循環過程及
其趨勢變化的模擬。綜合觀測與模擬分析的結果，識別各基本變數（溫度，水氣，流場），雲-降雨-上升流的頻譜分佈等隨氣候暖化彼此一致的趨勢變化，本計劃預期將對瞭解與模擬東亞暖季水循環、及其隨全球暖化的變化有突破性的進展。
Abstract: Measuring and predicting rainfall is an important problem both scientifically and practically. The hydrologic processes determining rainfall involve cloud-radiative and surface-atmosphere exchange processes that are influenced by, and feedbacks onto, circulation. Such interactions are particularly important during the warm season in East Asia where monsoon surges and tropical cyclones are most active events producing heavy rain. Satellite observations since the 1990’s and
longer records of conventional observations suggest an increasing trend in frequency of heavier rain and a possible risk of increasing extreme weather events (like intense tropical cyclones) with global warming. The overarching goal of the project is to identify and understand changes of tropical clouds/rain and associated hydrologic processes in East Asia and neighboring warm oceans in response to climate warming. To accomplish the goal, we propose two observational analyses
(Task1-2) and two modeling analyses (Task 3-4) with a working hypothesis that a warming climate
leads to more frequent heavy rain events (i.e. higher precipitation efficiency for heavier rain) with strong updrafts that drive the compensating subsidence, through broad-scale overturning circulation. The broad-scale subsidence regulated by radiative cooling controls the intermediate and light raining events.
The observational analyses are based on the new generation reanalysis data, satellite and surface rainfall measurements for the recent 30 years. Task 1 is to identify the broad scale features of hydrologic cycle as the climate background for the proposed study, and Task 2 is to perform a trend analysis of cloud/rainfall distribution in different climate regimes (like land vs ocean) to
identify rainfall spectral shifts in recent decades. For modeling analysis, we adopt a regional limate modeling (RCM) approach in Task 3, and a hydrologic process modeling (HPM) approach in Task
4. Task 3 is to collaborate with Dr. Chia Chou who will produce two sets of RCM experiments: a set
of hindcast experiments over the western North Pacific in 1979-2008 and a set of projected climate
change experiments. The RCM simulations are constrained by observed or simulated broad scale variability through a spectral nudging. In Task 4, we will use a high-resolution version of the Weather Research and Forecasting Model with convective-radiative and atmosphere-surface exchanges processes like those in cloud system resolving models to perform two types of experiments. The first type consists of two sets of 30 simulations (each of 36 hours, initialized every 24 h) for the month of June in 2008 (cold climate) and 2004 (warm climate). The second type consists of two sets of 90 simulations for June, July and August of current climate and a projected warm climate. The simulated hydrologic cycles will be evaluated against satellite measurements through a “model to satellite approach”, and the observed trends in cloud/rain identified in Task 2. The overall results will be synthesized to reveal consistent changes in basic variables (temperature, humidity, circulation), cloud/rainfall/updraft spectral distributions in key climate regions, and
corresponding water budget with climate warming. The proposed research is expected to make a breakthrough in understanding and modeling the hydrologic cycle in East Asian climate and its changes with global warming.
Keyword(s)
雲與降
雨
水循環過程
東亞季風
氣候暖化
Clouds and rainfall
Hydrologic process
Asian monsoon
climate change
