SEVERE CONVECTIVE STORMS -- AN OVERVIEWChapter 1 inSevere Convective StormsA Meteorological MonographTo be published by:The American Meteorological SocietyCHARLES A. DOSWELL IIINational Severe Storms LaboratoryNorman, Oklahoma 73069Review Panel:FREDERICK SANDERS (Chair), ROBERT A. MADDOX, JOSEPH ZEHNDERSubmitted: May 2000_______________________Corresponding author: Dr. Charles A. Doswell III, NOAA/National Severe Storms Laboratory, 1313Halley Circle, Norman, OK 73069 e-mail: [email protected]. Basic concepts of convectionA What is convection?In general, convection refers to the transport of some property by fluid movement,most often with reference to heat transport. As such, it is one of the three main processesby which heat is transported: radiation, conduction, and convection. Meteorologiststypically use the term convection to refer to heat transport by the vertical component of theflow associated with buoyancy. Transport of heat (or any other property) by the non-buoyant part of the atmospheric flow is usually called advection by meteorologists;advection can be either horizontal or vertical.Convection takes many forms in the atmosphere; a comprehensive treatment of thetopic can be found in Emanuel (1994). This monograph's concern is severe convection:that is, the variety of hazardous events produced by deep, moist convection. Hazardousweather events (large hail, damaging wind gusts, tornadoes, and heavy rainfall) are generallythe result of the energy released by phase changes of water. A circular convective cloudwith a 5 km radius and 10 km deep contains, at any given instant, about 8 x108 kg ofcondensed water, assuming an average condensed water content of 1 g m-3. During thecondensation of that water, roughly 1014 J of latent heat energy is released over a time scaleof roughly 25 min (see below). For comparison purposes, a "one-megaton" bomb releasesabout 4 x 1015 J of heat (see Shortley and Williams 1961; p. 903), albeit in a tiny fractionof a second. Thus, at least in terms of released energy, this modest convective cloud iscomparable to a "25 kiloton" bomb. It is this released heat that powers convective storms.Most of the energy is expended against gravity, but some portion also may create hazardousweather.The released heat contributes to buoyancy, B, an essential aspect of convectivestorms. Buoyancy is defined most simply by2B ≡ gT - ¢ T ¢ T , (1)where g is the acceleration due to gravity, T is the temperature of a parcel, and T' is thetemperature of the surrounding environment. Buoyancy, of course, can be either negative orpositive. If B is integrated from the level of free convection (LFC) to the equilibrium level(EL) above the LFC, the result is convective available potential energy (CAPE). It is thisenergy that is responsible for the convective updraft and for many of the hazards producedby the convection. Downdrafts have their own source of energy, however. Downdrafts aredriven by negative buoyancy, also derived from phase changes (mostly evaporation) andfrom the "loading" effect of precipitation. Whereas updrafts transport warm air upward,downdrafts transport cold air downward. Downdrafts also are responsible for somehazardous weather. In either updrafts or downdrafts, the net result is stabilization but themechanisms are distinct, to be shown later.B. Thunderstorms and deep, moist convectionI will use "deep, moist convection" (DMC) frequently in what follows, in lieu of themost common word associated with DMC: thunderstorms. This is because, in someinstances, hazardous weather can be produced by non-thundering convection. Irrespectiveof this minor semantic distinction, DMC is the result of a type of instability. Consider theinviscid vertical momentum equation:dwdt= -1r∂p∂z- g, (2)where w ≡ dz/dt is the vertical component of the flow, z is geometric height, r is the density,and p is the pressure. The vertical pressure gradient force is the first term on the rhs of (2).Since the vertical acceleration is zero in a hydrostatic atmosphere, buoyancy is associatedwith an unbalanced pressure gradient force, caused by density perturbations. Equation (2)3can be transformed using the definition of vertical motion and textbook linearizationmethods (e.g., Hess 1957; p. 95 ff.) to yield:dwdt=d2zdt2= B = -g¢ T G -g( )z, (3)where T' is the environmental temperature, G is the parcel lapse rate (-dT/dz), and g is theenvironmental lapse rate (-dT'/dz).1 When the coefficients are constant, Eqn. (3) is asimple, second-order differential equation that has a simple solution:z(t) = zoexp iNt[ ], (4)where zo is the initial height of the parcel and N is the so-called Brunt-Väisälä, or buoyancyfrequencyN2=g¢ T (G -g)z. (5)The solution (4) implies an instability (i.e., exponential growth of an infinitesemal upwarddisplacement) whenever the square root of N2 is imaginary; this occurs whenever theenvironmental lapse rate exceeds that of an ascending parcel (typically assumed to beadiabatic). Since g is normally not greater than the dry adiabatic lapse rate, the contextclearly is associated with conditional instability (i.e., g > Gm, where Gm is the moist adiabaticlapse rate). The analysis of parcel instability leading to (4) is only appropriate when thetextbook linearization assumptions are valid. Actual parcel instability leading to DMC isprimarily associated with finite vertical displacements; hence, the key to the possibility forgrowth of convective storms is the presence of CAPE, not the environmental lapse ratesalone (see Sherwood 2001; Schultz et al. 2001). Not all situations with conditional 1 The linearization leading to (3) ends up equating buoyancy with conditional instability,which is not quite correct. See Hess’s textbook and Schultz et al. (2001) for a discussionof the approximations used to linearize the equation this way.4instability are characterized by parcels with CAPE.2 Thus, the moisture content of the air iscritical in knowing whether conditional instability actually contains the potential for parcelsto become buoyant (i.e., to have CAPE). In most cases, energy must be supplied to lift theparcel through its condensation level to its LFC; the amount of this supplied energy isknown as the convective inhibition (CIN). From the LFC to the EL, the parcel acceleratesvertically, drawing the energy for this acceleration from the CAPE. Generally, parcels willovershoot the EL and then experience the stable