Misconception #1: Small values of vorticity indicate strong negative vorticity advection. Explanation: Small values of vorticity indicate the atmosphere is dynamically stable. The upper levels of the atmosphere are generally stable with geostrophic flow. Strong negative and thus strong sinking motion is associated with a vorticity maximum that is advecting away from a fixed point. The strongest sinking motion and negative vorticity advection is associated with the upstream portion of a vorticity maximum. Misconception #2: Rising air occurs both behind and in front of an advecting vorticity maximum. Explanation: Rising air occurs in the region of the vorticity maximum where the vorticity maximum is approaching a fixed point (downwind flow through vort max). Once the vorticity maximum moves overhead then downstream, negative vorticity advection (conducive to sinking motion) occurs. Over the US, in general, rising motion is to the east (right) of the vorticity maximum. Misconception #3: Vorticity maximums are always associated with rising air from the surface. Explanation: This is not always the case. Other factors influence the strength of rising air. Three of these are the low-level convergence, thermodynamic buoyancy and the amount of thermal advection (cold air or warm air advection). Positive vorticity advection (conducive to rising air) may be outweighed by low level cold air advection, weak surface convergence and negative buoyancy (all conducive to sinking air). Misconception #4: The higher the value of vorticity, the faster the air will rise on the synoptic scale. Explanation: Not true for the same reasons given in misconception #3. High values of vorticity may be offset by cold air advection, weak buoyancy and a lack of low level convergence. If the low levels of the atmosphere are stable and strong positive vorticity advection overrides this stable layer, precipitation will occur as "elevated precipitation or convection". Keep in mind that strong vorticity overriding strong thermodynamics (positive buoyancy, low level convergence and warm air advection) will lead to huge values of upward vertical velocity and an extremely unstable atmosphere. Also keep in mind that precipitation might not occur with a strong vorticity maximum. If moisture is lacking and the thermodynamics are stable, even strong vorticity advection may not produce precipitation. To produce PVA, there must be a wind flow through the vort max. No matter how large the vort max is, no wind flow through the vort max will result in no PVA uplift. Misconception #5: A low value of vorticity indicates no chance for rain. Explanation: Don't fall into this trap. Strong thunderstorms and rain can occur without the aid of positive vorticity advection. This is especially true in the quasi-geostrophic air mass. Plenty of low level moisture and positive buoyancy can lead to thunderstorms without the aid of vorticity. Low level WAA often produces a net uplift even when the vorticity advection is neutral or negative aloft. Misconception #6: Divergence is the same thing as vorticity. Explanation: Mathematically they are different quantities although they are related to each other. Advection of a parcel experiencing decreasing values of vorticity are associated with divergence. Increasing values of vorticity are associated with negative vorticity. Since air parcels move through the vorticity field at 500 millibars, divergence is generally found to the right and convergence found to the left of an advecting vorticity maximum over the United States. Divergence is a proxy for vorticity since they are mathematically related. Misconception #7: Vorticity and vorticity advection only occurs at 500 millibars. Explanation: Vorticity occurs in all levels of the atmosphere. It just happens that 500 millibars is the best atmospheric level to accurately represent vorticity advection. 500 millibars is near the level of non-divergence. See the following link for more details: http://www.theweatherprediction.com/habyhints/65/ Misconception #8: What is the difference between relative and absolute vorticity? Explanation: Relative vorticity considers shear and curvature vorticity while absolute vorticity considers shear, curvature AND earth vorticity. |