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Overview of Tropical Cyclones AOS 453 April 2004 J. P. Kossin CIMSS/UW-Madison HOT COOLER Genesis: Easterly waves African easterly jet hot Sahara vs cool temps along coast of Gulf of Guinea coast reversal of meridional PV gradient combined barotropic-baroclinic instability. April-October. Period ~ 3-4 days. ~ 2000-2500 km. N ~ 60/year. Genesis: may also be instigated by local baroclinic or upper-level trough forcing; along southermost remnants of fronts. Or perhaps through barotropic instability of ITCZ (Ferreira and Schubert 1997). Persistent convection diabatically produces PV and forms Mesoscale Convective Vortices (MCV) in the mid-levels. Multiple vortices are formed within tropical cloud clusters. Mid-level vortex cold-core system (tangential wind increases with height). Tropical cyclone warm-core. How does the conversion occur? Modified Rossby-Burger-Prandtl relationship: Vertical influence D = (flocal a)1/2 L / N Merger (self organization) Environmental requirements (necessary conditions) for genesis: 1) Warm water [SST > 26.5C (80F)] 2) Low vertical wind shear (~10m/s bottom to top) 3) Ambient rotation ( f ) - off equator. 4) Moist mid-levels. Surface swirling flow How does disturbance amplify? Intensification: Conditional Instability of the Second Kind (CISK) has fallen out of favor. Convergence related mechanism. Downdrafts kill moist energy of boundary layer. Convergence is inefficient at raising air to LFC. Wind Induced Surface Heat Exchange (WISHE). Real CISK • Scale-dependent feedback from cumulus to system by: 1. 2. • momentum forcing thermal forcing Response of system to cumulus: 1. 2. Thermal field (mass) adjusts to momentum forcing (L<LR , disturbance is smaller than local Rossby radius) Wind field (momentum) adjusts to thermal (mass) forcing (L>LR) Effects of Heating (Global) When Q represents the diabatic latent heat release of convection, this is sometimes called "up moist down dry" Where does the warming occur? Not so easy..... Axisymmetric Dynamics From Hack and Schubert 1986 Local Response to Local Heating; Linear vs. Nonlinear Efficiency: There is a nonlinear feedback mechanism at work. The more intense the local swirling flow is, the more efficiently the heating can warm locally. More local warming increases pressure gradients which further intensifies the local flow. This can be studied in the context of an axisymmetric balance model (Schubert and Hack 1982). Heating efficiency as function of inertial stability: Transverse Circulation Warming Less inertial stability More inertial stability Schubert and Hack 1982 Real CISK (continued) • • • Since the heating of cumulus projects on to multiple scales on either side of LR, a multiple of responses to cumulus occur; some gravity and some rotational. Because the properties of the rotational response are so different from the gravity wave response, the evolving system can be complex. Normally, the system is defined by a slow mesoscale response that defines the system organization over time. Slant-Wise Convection • Two competing stabilities present in the atmosphere: 1. Static Stability (vertical planes) 2. Inertial Stability (horizontal planes) • Stability in one plane limits instability in the other • Both stabilities are represented by gradients of a conservative potential Slant-Wise Convection (continued) • There is free movement relative to a particular stability along iso-lines of constant potential. • There is stability induced oscillation for movement perpendicular to iso-lines of constant potential. Slant-Wise Convection (continued) • The potential for dry static stability is potential temperature (q) • The potential for moist static stability (saturated air) is equivalent potential temperature(qe) • The potential for inertial stability is angular momentum given by m ( f ) y where y is the radius from the center of rotation. Slant-Wise Convection (continued) • Lines of constant (q) are usually horizontal but dip downward (due to thermal wind balance) into the center of a cyclonic vortex whose strength decreases with height (warm core) and rise upward into the center of vortex whose strength increases with height (cold core). • Lines of constant inertial stability (m) are usually vertical, but tilt away from the center of a cyclonic warm core vortex because of the thermal wind effect and vice versa in a cold core vortex. Slant-Wise Convection (continue) • Hence if we have a saturated warm core vortex, neutral inertial upward movement (movement along an “m” surface) experiences less static stability than pure vertical upward movement . • Likewise, neutral horizontal movement along a q surface, experiences less inertial stability than pure horizontal movement • If vortex is strong enough momentum lines and q lines can cross, creating static instability along m surfaces or inertial instability along q surfaces (isentropes). Slant-Wise Convection (continued) • Hence convection erupting up the tilted momentum surface is called slant-wise convection • Slant-wise convection is due to “symmetric instability” or inertial instability relative to the symmetric vortex that defines the radius of curvature for the momentum lines. Conditional Symmetric Instability • Conditional Instability along a momentum “m” surface, ie condition for slantwise moist convection • Alternative way of looking at the same thing: Inertial Instability along a theta_e surface Tropical cyclone structure: Axisymmetric Structure as Deduced from Rawinsonde Composites (Frank 1977) Axisymmetric Structure as Deduced from Aircraft Flight-Level Data Hawkins and Imbembo 1976 Jorgensen 1984 In an average sense, updrafts in t he eyewall tend to flow along sloping surfaces of constantangular momentum and equivalent potential temperat ure (moist neutralit y). Tropical cyclones are warm core systems. Pressure surfaces become "flatter" (thickness or hypsometric equation), and tangential winds consequently decrease, with increasing height. T he pert urbation (or dynamic) pressure at the vort ex center increa with increasing height . Any resulting subsidence increases the upward directed buoyancy force just enough t o reestablish a hydrostatic balance (Smit h 1980).