使用VVM探討陸氣交換過程對台灣地區高解析氣象模擬的影響--理想個案模擬

An idealized simulation to understand impact of land-air exchanging process on fine scale meteorological characteristics in Taiwan using VVM

林曉君(Hsiao-chun Lin)

指導教授 ： 鄭芳怡

國立中央大學/地球科學學院/大氣物理研究所/碩士(2015年)

摘要

台灣夏季大氣常處於熱力不穩定的狀態，加上盛行西南風的吹拂，帶進充沛的水氣，容易於西南部平地及山區產生午後熱對流，可能會於局部地區降下瞬時大雨導致淹水，另外在盆地及山區會有局部熱力環流(如：山谷風環流)產生，也會影響降雨的形成及分布，還有大氣汙染物的擴散及累積，對人民的生活造成衝擊，不過這些較小尺度的系統無法被中尺度氣象模式解析。如今，隨著電腦計算能力進步，數值氣象模式也走向高解析度的模擬，並可用來探討小尺度天氣系統及其對大氣及環境的影響，然而應用至台灣的困難點在於複雜地形的處理，在地形陡峭的地方容易因氣壓的急遽變化，使模式變得計算不穩定。本篇使用vector vorticity equation model (VVM)進行台灣地區高解析氣象模擬，因為VVM使用高度垂直座標，應用至崎嶇複雜地形時較為合適，但因VVM缺乏完善的地表處理過程，因此本研究主要目的為(1)將Noah地表模式(land surface model, LSM)加入VVM中以改善陸地與大氣交換過程；(2)並藉由理想化的模擬實驗探討陸氣耦合過程對台灣地區高解析氣象模擬的影響。
本研究共進行六組敏感度實驗，用以探討陸地與大氣交互作用、都市熱島效應對局部環流及邊界層發展的影響以及土壤濕度與溫度初始值對氣象場模擬的影響。目前已成功將Noah地表模式耦合進VVM，而耦合系統的理想模擬實驗結果顯示，地表熱通量會受到地表特性(如：土地利用及土壤型態)影響而呈現異質性分布，地表潛熱及可感熱通量的多寡與正負會影響地表與大氣之間的熱量與水氣的傳送大小及方向，地表於白天的加熱作用與晚間的冷卻作用也因此被突顯出來，而加強了海洋與陸地之間的溫度梯度，導致顯著的日夜向岸與離岸氣流特性；再者，高解析VVM/Noah氣象模擬顯示都市熱島效應對邊界層結構及午後內陸輻合強度的影響；然而，改變初始土壤濕度及溫度對氣象場的模擬沒有太顯著的影響，主要原因可能是spin-up時間不足。實驗結果亦顯示完善的地表模式是必要的，因為地表邊界條件會大大影響後續的熱量與水氣傳送，影響氣象場模擬。

關鍵字

Vector Vorticity equation Model ； Noah 地表模式 ； 陸氣交換過程 ； 高解析氣象模擬

並列摘要

Nowadays, with the improvement in the computing capability, high-resolution meteorological simulations can be conducted to understand the fine scale weather systems and its impact on the atmosphere and environment. However, the difficulty of applying fine-scale meteorological simulation in Taiwan lies in the treatment of the complex topography. In the region with steep terrain, the error induced by the abrupt change of pressure would cause the computational instability in numerical model. In this study, vector vorticity equation model (VVM) was applied in the area of Taiwan. Unfortunately, the VVM lacks the capability to simulate the heterogeneous land surface processes variations. Regarding the deficiency of providing the land-air exchanging processes from VVM, we first implemented the Noah land surface model (LSM) to VVM. The study objectives include: (1) to implement the Noah LSM in VVM; (2) to understand the impact of land surface exchanging processes on the fine-scale meteorological simulation in Taiwan through idealized experiments.

Six idealized experiments were conducted to understand the impact of land-air exchanging process on the thermal structures and local circulations in area of Taiwan using VVM/Noah coupling system. The current implementation of Noah LSM to VVM is successful and the simulated results show the impact of land surface heterogeneity on the meteorological characteristics through the idealized VVM simulations. The quantity and sign of surface heat fluxes represent the amount and direction of the heat and moisture transport. With the Noah LSM, the land-sea heating contrast is magnified and thus leads to the increase of temperature gradient between land and sea. Hence, onshore and offshore flows at the daytime and nighttime are apparently noticed. Further, the results also shows that urban heat island did has an impact on PBL development and convergence zone in the inland. Moreover, the change of initial soil moisture and temperature show slight influence on the meteorological simulation due to the lack of proper spin-up process. In general, the implication of the simulated results claims that an adequate land surface model is necessary because it serves to provide the lowest boundary condition to drive the meteorological modeling.

參考文獻

Kerns, B. W. J., Y.-L. Chen, and M.-Y. Chang, 2010: The Diurnal Cycle of Winds, Rain, and

Krueger, S. K., Q. Fu, K. N. Liou, and H.-N. S. Chin, 1995: Improvements of an Ice- Phase

Anthes, R. A., 1984: Enhancement of convective precipitation by mesoscale variation in

clouds and precipitation induced by land surface forcing. J. Geophys. Res., 101(D3), 7499–7518, doi:10.1029/95JD03031.

Moisture, Coastline Curvature, and Land-Breeze Circulations on Sea-Breeze-Initiated Precipitation. J. Hydrometeor, 2, 193–211.