Extratropical Cyclones – Genesis, Development, and Decay Xiangdong Zhang International Arctic Research Center Basic Facts Extratropical cyclones is a major weather maker for mid and high latitudes. Size: roughly 1000-2500 km in diameter; Intense: central pressure ranging from 970-1000 hPa; Lifetime: 3-6 days to develop, and 3-6 to dissipate; Movement: generally eastward at about 50 km/hr; Peak season: winter; Formation: along baroclinic zone or from transition of tropical cyclones. Outline Goal: Understand cyclone from simple model to complex dynamics • Classic surface-based polar-front model – Bergen Model • Surface – upper troposphere coupling – understanding from kinematics • Interactions between dynamics and thermodynamics – a more complex vorticity dynamics Bergen Cyclone Model (BCM) Mechanism of cyclone development: Baroclinic instability Z Warm cold Unstable Stable Baroclinic Instability: Available potential energy (APE) kinetic energy (air movement -> wind) Center of Gravity h h≈0 Unstable Stable Are we satisfied with BCM so far? Questions we could not answer: • How do upper level waves disturb the surface cyclone formation? • How can surface cyclone be maintained when air mass fills in? How does ageostrophic wind redistribute air mass and links upper level waves to surface cyclone development? planetary waves at 500 hpa a weather chart at 500 hpa Surface – upper troposphere coupling • Geostrophic wind: the wind when it is in perfect geostrophic balance: 1 f k ´ vg = - Ñp r • Ageostrophic wind: difference between the actual wind and the wind when it is in perfect geostrophic balance: va = v - v g ds dV 1 dp 1 dp =s + (- fV )n Force Balance Free Atmosphere dt r ds r dn s dV d (V s) dV ds = = s +V dt dt dt dt dV 1 dp s=s dt r ds d s = s dq t+1 t s dS s component t dq ds ds = dq ds ds dS dq s dS R n d s dq d s ds ds ds V = = R = dt = n (dt ® 0) dt dt d s dt d s R ds dt R 1 dp V2 1 dp V2 ds V 1 dp - fV = - fVg - fVa =0 V = V n = (- fV )n r dn R r dn R dt R r dn n component V 2 <0: cyclonic curving Ageostrophic wind: Va = fR >0: anticyclonic curving Ageostrophic wind when the air curves cyclonically: • The centripetal acceleration breaks the geostrophic balance; • The ageostrophic wind points the opposite direction of the geostrophic wind. V2 Va = - < 0 fR vt-1 va Low Pressure Pressure Gradient Force Centripetal Acceleration vt vt+1 Coriolis Force High Pressure Sub-geostrophic wind: slower than the geostrophic wind. Ageostrophic wind when the air curves anticyclonically: • The centripetal acceleration breaks the geostrophic balance; • The ageostrophic wind points the same direction of the geostrophic wind. Va = - V >0 fR 2 vt+1 High Pressure Coriolis Force Centripetal Acceleration vt va vt-1 Pressure Gradient Force Low Pressure Super-geostrophic wind: faster than the geostrophic wind. Ageostrophic wind when the air speeds up: • The pressure gradient increases and air blows toward lower pressure side; • The ageostrophic wind points the left of the geostrophic wind. Low Pressure Pressure Gradient Force vt-1 va vt Coriolis Force High Pressure Ageostrophic wind when the air slows down: • Opposite. Summary I: Curvature effects (uniform pressure gradients along the flow) Summary II: Effects from varying pressure gradients along the flow Low Pressure Divergence Convergence Pressure Gradient Force CF > PGF (PGF decrease) PGF > CFP (PGF increases) Coriolis Force Convergence Divergence High Pressure old new Upper level driver From 2007 Thomson Higher Education Are we satisfied with kinematics so far? Questions we could not answer: • How does temperature impact cyclone development? • How does external and internal heating and impact cyclone development? Vorticity dynamics Thermal wind Balance: VT = Vg2－Vg1 = Vorticity: z g1 = z g2 - z T 500 hPa level 2 ¶z1 ¶z2 ¶zT = ¶t ¶t ¶t With certain approximations, we have: Surface level 1 Petterssen’s Development Equation (Carlson (1998)) Cyclone Development Equation Az 2 vorticity advection at 500 hPa AT surface-500 hPa layer-averaged temperature advection S H surface-500 hPa layer-averaged adiabatic heating/cooling surface-500 hPa layer-averaged diabatic heating/cooling Positive Vorticity Advection (PVA) N Negative Vorticity 5x10-5 s-1 10x10-5 s-1 15x10-5 s-1 20x10-5 s-1 Positive Vorticity E Negative Vorticity Advection (NVA) N Negative vorticity 4x10-5 s-1 8x10-5 s-1 12x10-5 s-1 16x10-5 s-1 Positive vorticity E Effects of Vorticity Advection Az 2 Ridge For a Typical Synoptic Wave: 500 mb Trough • Areas of positive Az 2 (PVA) are often located east of a trough axis • PVA increases the surface vorticity ζ1 and leads to the formation of a surface low or cyclone PVA NVA Effects of Temperature Advection AT • Areas with maximum warm AT (WAA), one has , which leads to an increase in surface vorticity ζ1 and the formation of a surface low or cyclone WAA Effects of Diabatic Heating H • Strong diabatic heating (H >0) always helps to increase surface vorticity ζ1 • Diabatic heating includes radiation, latent heat release from cloud and precipitation, and sensible heat exchange Effects of Adiabatic Heating S • When S < 0, there is whole layer (surface-500 hPa) convergence, which leads to a decrease in surface vorticity and unfavors the development of surface low • Upper level (above 500 hPa) divergence is needed for cyclone development! Note: We can have: ¶w ¶u ¶v = -( + ) = -D ¶p ¶x ¶y w = w ps - D*(p - ps ) Therefore: S = w ps (g d - g ) - D(p - ps )(g d - g ) From continuation equation: If there is no surface forced vertical velocity ( w p = 0 ) and the surface-500 pha layer-averaged convergence (D < 0) leads to S < 0 , unfavorable to cyclone development. s Surface Cyclone Development The surface cyclones intensify due to WAA and an increase in PVA with height → rising motion → surface pressure decreases With warm air rising to the east of the cyclone, and cold air sinking to the west, potential energy is converted to kinetic energy (baroclinic instability) and the cyclone’s winds become stronger Rising 500mb PVA WAA SFC Pressure Decrease System Intensifies Surface Cyclone Development Weather of Extratropic Cyclone Occluded Front: Cold with strong winds Precipitation light to moderate Significant snow when cold enough Warm Front: Cloudy and cold. Heavy precipitation Potential sleet and freezing rain From gsfc.nasa Cold Front: Narrow Band of showers and thunderstorms Rapid change in wind direction Rapid temperature decrease. Warm Sector: Warm Potential showers and thunderstorms Surface weather chart 12Z, Wed, Nov 9, 2011 surface cyclone 500 hPa weather chart 12Z, Wed, Nov 9, 2011 How did upper level waves support the developing surface cyclone • Occurred before a trough and after a ridge advection of + vorticity advection of warm air divergence due to curvature divergence due to deceleration 500 hPa trough surface cyclone Single synoptic scale cyclone process can cause highly variable surface wind field and impact sea ice Xiangdong Zhang, IARC Climatological characteristics of northern hemispheric cyclone activity Winter Summer cyclone count/frequency Climatological characteristics of northern hemispheric cyclone activity Winter Summer cyclone central SLP Summary • Cyclone is a prominent element of weather system, impacting our daily life. • Genesis, development, and decay of cyclones result from 3dimensional, interactive processes between dynamics and thermodynamics. • Better understanding of cyclones has important implications for improving weather forecast and climate change assessment.