An Introduction of Mesoscale Meteorology Ben Jong-Dao Jou Department of Atmospheric Sciences National Taiwan University January 2011, Taipei
Temporal and spatial scales of atmospheric phenomena Non-hydrostatic and ageostrophic 大氣現象依水平空間尺度及時間尺度之分布圖 A: 塵暴 B: 龍捲風和水龍捲 C: 積雲 D: 下爆氣流 E: 陣風鋒面 F: 中尺度氣旋 G: 雷暴 H: 海陸風 山谷風等環流及中尺度高低壓 I: 雨帶 J: 海岸鋒 K: 中尺度對流系統 L: 低層噴流 M: 乾線 N: 熱帶氣旋 ( 颱風 ) O: 高層噴流 P: 地面鋒 Q: 溫帶氣旋與反氣旋 R: 西風帶槽脊線 斜壓波
Scales of Moist Convection < 1 km ~1 km 1 km ~100 meters Cumulus Cumulus Congestus > 100 km Supercell Thunderstorm ~ 1000 km ~100 km Mesoscale Convective System (MCS) Mesoscale Convective Complex (MCC) Sources: Texas A&M University Nolan Atkins Morris Weisman NOAA/NESDIS NCAR NASA Chris Davis NCAR Hurricane Katrina > 100 km Convective phase of Madden- Julian Oscillation
Due to their energy sources, as well as the smallness of the Coriolis parameter, large-scale equatorial motions have several distinctive characteristic structural features that are quite different from those of mid-latitude systems. (Mass-wind relation!)
Wind speed, temperature, potential temperature, and potential vorticity
The contemporary structure model of upper-level frontal systems Absolute momentum Geostrophic momentum Frontal zone is described by m. Frontal zone is characterized by large magnitude of the absolute geostrophic vector vorticity. The frontal boundaries can be represented as discontinuities in the gradient of m.
Two-dimensional frontogenesis model Semi-geostrophic model Adiabatic Frictionless (Hoskins(1971,1972), Hoskins and Bretherton (1972)) Simulate the structure of the upper level frontal zone 1. Jet 2. Tropopause folding 3. Frontogenesis at the tropopause Fig.5 Composite cross section of upperlevel frontal and tropopause structure Jet formed along the tropopause 450 hpa Isotachs Isentropes Wind PT Shortcoming: Folded tropopause does NOT extend to the 600-700 hpa levels. Definitions of direction: Cross-contour (transverse) Along-contour Fig. 24
囚錮氣旋 : 冷鋒, 暖鋒, 囚錮鋒之衛星影像, 複雜之中尺度現象 L
溫帶氣旋, 鋒面, 與鋒面雨帶 1. 窄冷鋒雨帶 2. 寬冷鋒雨帶 3. 鋒後雨帶 4. 暖區雨帶 5. 暖鋒雨帶 6. 湧衝雨帶 ( 囚錮 )
溫帶氣旋之降雨特徵 垂直指向都卜勒雷達觀測之降雨終端速度 ( 上圖 ) 和雨量計量測之地面降水強度 ( 下圖 )
窄冷鋒雨帶之運動場特徵 暖輸送帶 ( 高相當位溫空氣之輸送帶 ) 經常伴隨低層噴流鋒後為低相當位溫空氣
窄 / 寬冷鋒雨帶之降雨特徵 塔狀積雨雲, 崁入對流, 冰水混合體
暖鋒雨帶和 seeder-feeder process 暖鋒之上產生激發胞 (generating cell), 為供水雲 (feeder) 之來源, 暖鋒下為種子雲 (seeder).
窄冷鋒雨帶 (NCFR) 未降雨個案
雷暴之外流邊界 ( 密度或重力流 ) The x-component Boussinesq momentum equation in the absence of Coriolis forces: Du / Dt = -1/ρ 0 p/ x. If this expression is applied to a steady state horizontal flow in a coordinate system moving with the gust front, we have (u 2 /2) / x = -1/ ρ 0 p/ r.if the gravity current has depth h and density ρ 0 + ρ, where ρ 0, and is moving through a stagnant base-state environment, integrate above equation across the front edge of the gravity current and obtain U f 2 / 2 = (1/ρ 0 ) g ρ h. The speed of the gust front is determined by the depth of the gravity current and the density differences across the interface.
窄冷鋒雨帶 : 低層 噴流與強上衝流 The maximum inflow toward the front is >20m/s at a height <500m. At the leading edge of the cold front, this inflow converges with the advancing cold air mass to produce an intense (shallow max at 2.1 km height) updraft; average ~7 m/s and max>20 m/s.
擾動壓力和浮力 場結構 注意到鋒前浮力為負, 因此鋒前強上衝流主要經由氣壓梯度力來維持 ; 另外鋒後有最大虛位溫負值, 可能是溶解蒸發非絕熱過程所導致.
鋒生動力機制 跨鋒方向風切帶 v/ x ( 鋒前低層噴流 ) 的維持 d/dt [ v/ x]= / x [dv/dt] - ( v/ x u/ x + v/ y u/ x + v/ z w/ x) Time rate of change of along-front velocity v is equal to the cross-front gradient of along-front acceleration and the confluence, along-front variation, and tilting terms.
Scientific objectives of SoWMEX/TiMREX 1. Mei-yu front and low-level jet 2. MCSs dynamics and microphysics 3. Terrain effect on flow and MCSs/QPE 4. Mesoscale data assimilation/qpf 5. Convection initiation/diurnal cycle/boundary layer processes Multiscale interaction problem
Environment and precipitation characteristics: U and V components of wind, theta, theta-e, lightning, and conv/stra partition for land and ocean events total 40 SCRPS (May 19 to June 26, 2008) Property SCRPs Duration/ diurnal cycle (h) Lightning Frequency (#/h ) Lightning Density (10-3 /h km 2 ) Conv / Stra rain area in % All 9.6 95 92 59 / 41 Land (18) 6.6 48 181 64 / 36 Oceanic (17) 12.1 113 24 54 / 46 Mixed (5) 11.8 189 25 61 / 39
Meiyu frontal rainband G E I
A B
a d b W h c L g H
溫度 溫度 W W W W C 氣壓 氣壓 H L H L L H L L E
中尺度氣象學 229M2030, Spring 2011 任課老師 : 周仲島教授上課時間 : 星期二上午 08:10-10:00 星期五上午 09:10-10:00 地點 : 大氣系館 A104 教室國立台灣大學大氣科學系
課程大綱 中尺度的定義 Holton, J. R., 2004: An introduction to Dynamic Meteorology. 4 th Ed., Academic press, San Diego 中尺度方程組和中尺度不穩度 Markowski and Richardson (2010): Mesoscale Meteorology in Mid-latitudes (2010), Wiley- Blackwell, Royal Meteor. Soc., 407 pages 對流雲與層雲之觀測與動力性質 ( 雲動力基礎 ) Rogers, R. R. and M. K. Yao, 1989: A Short Course in Cloud Physics, 3 rd Edition. Pergamon Press, Oxford 積雨雲 雷暴 和劇烈對流風暴 Cotton, W. R., and R.A. Anthes, 1989: Storm and Cloud Dynamics. Academic Press, San Diego 中尺度對流系統 Houze, R. A. Jr., 1993: Cloud Dynamics. Academic Press, San Diego 鋒面動力 ( 鋒生過程與中尺度雨帶 ) Bluestein H. B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Oxford University Press 熱帶對流 ( 雲與熱帶氣旋赤道波 ) Wakimoto R.M., and R. Srivastava, 2003: Radar and Atmospheric Science: A collection of essays in honor of Davis Atlas. Meteorology Mono., AMS, Boston 邊界層大氣與局部環流 Stull, R. B., 1988: An Introduction of Boundary Layer Meteorology, Kluwer. 地形影響之中尺度現象 Banta, R. M., 1990: The role of mountains in making clouds. Atmospheric Processes over Complex Terrain, Meteorological Monograph, No. 45. AMS, Boston
Definition of Mesoscale convective systems (MCSs) From Zipser (1982): Cloud and precipitation systems, together with their associated circulation systems, which include a group of cumulonimbus clouds during most of the lifetime of the system. The cumulonimbus cloud group must exist for several lifetimes of its constituent clouds (say for at least two hours), and the cumulonimbus group must contribute at some time to a common upper-tropospheric shield of outflow air. Its convective-scale downdrafts must also merge at some time to form a continuous zone of cool air in the low troposphere. Normally, extensive stratiform precipitation would fall from the outflow shield, evaporating to a greater or lesser extent before reaching the ground.
Definition of Mesoscale convective systems (MCSs) From Houze (1993): A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area 100 km or more in horizontal scale in at least one direction.
Definition of Mesoscale convective systems (MCSs) From Johnson (2005): One rather general definition of an MCS is a group of interrelated and coexisting convective clouds, with or without a specific organizational pattern, having a lifetime longer than that of each of its component convective elements.
Global distribution of mesoscale convective complexes (dots) and regions of widespread frequent deep convection as inferred by outgoing longwave radiation (OLR) minima. Light shading indicates OLR minima. OLR measurements in W/m2 are obtained from Earth-Atmosphere Radiation Budget analyses, for June- August (above the line) and December-February (below the line), 1974-78.
莫藍第 颱風
氣象局 09/09/18:00-20:30 QPESUMS 雷達整合回波圖 /30min ( 動畫 ) Typhoon Meranti
The Clouds in an Intensifying Depression -- MCS Life Cycle The VHTs cease forming, but the stratiform cloud region containing MCV vorticity remains for some hours. larger-scale environment rich in vorticity at lower levels. (Houze et al. 2009)
3-D Wind Field of the Mature Tropical Cyclone Gray (1979) RMW: max. ~500 m ASL (Franklin et al. 2003) Frank (1977) The strong convergence in the eyewall at low levels and the strong outflow aloft must be balanced by strong upward motion. The overall cloud and precipitation amounts are determined by this vertical mass transport (within ~400km).
The eye - clouds Outward-sloping eyewall cloud Subsidence mixed layer Low topped St and/or Sc (Willoughby 1998) Subsidence in the eye produces a stable layer at the top of a vigorous mixed layer over a warm ocean surface. (Lilly 1968; Stevens et al. 2003; Bretherton et al. 2004)
Connecting the boundary layer processes with the balanced dynamics of tropical cyclone Modification by Smith et al. (2008) of Emanuel s conceptual model. feed extremely high-θe air into the base of the eyewall cloud frictionally induced cross-isobaric flow 1) The eyewall cloud vertical motions are intensified by the entrainment of high-θe air from the boundary layer of the eye region 2) local convective cells of buoyant updraft motion can form within the eyewall in locations where the entrainment particularly strongly enhances the buoyancy.
Eyewall asymmetry owing to storm motion and shear Precipitation particles occur typically on the downshear left side of the storm. Cloud and precipitation particles generated in the updraft (Black et al. 2002.) Ice particles are advected outward and cyclonically around to the south side of the storm and exit in a massive cirriform plume on the southeast side of the storm.
Electrification of the eyewall cloud ~0 and -5 temperature layer,and the liquid water contents there are nearly always low (,0.5 gm -3 ). Graupel : positive charge Small colliding ice particles : negatively charged Negative strike : Component Ez (electric field vector) points upward from a lower region of less negative to the upper zone of more negative charge. Positive surface lighting strikes (rare) : When a portion of the eyewall cloud becomes buoyantly unstable, with larger vertical velocities and larger liquid water content (>0.5 gm -3 ), the small ice particles may become positively charged when they bounce off graupel particles.
Summary Rapid intensity changes often occur in conjunction with rapid reorganization of the cyclone s mesoscale cloud and precipitation structures. Realistic simulation of the clouds and precipitation, therefore, is a particularly important aspect of the models ability to make these forecasts accurately and for the right physical and dynamical reasons. Forecasting the probabilities of extreme weather and heavy precipitation at specific times and locations will become an increasingly feasible goal for landfalling cyclones. Future works To synthesize and organize the available information on the diverse cloud processes within tropical cyclones