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The Analysis of Convective Storms. Thermodynamic Diagrams. There are three desirable characteristics of atmospheric thermodynamic diagrams: The area enclosed by any cyclic process should be proportional to energy or work. The more straight lines the better.
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Thermodynamic Diagrams There are three desirable characteristics of atmospheric thermodynamic diagrams: • The area enclosed by any cyclic process should be proportional to energy or work. • The more straight lines the better. • The angle between isotherms and dry adiabats should be as large as possible.
Emagram—Abscissa is T, ordinate is proportional to lnp. (From energy/mass). Tephigram—Abscissa is T, ordinate is logarithm of potential temperature, sometimes diagram is rotated (From T-phi) Stuve—Abscissa is T, ordinate is Area is not proportional to energy. Skew T-ln p—Similar to emagram, but temperature lines are skewed to increase the angle with dry adiabats.
Convection Parameters • Lifted Condensation Level (LCL) – level at which a parcel lifted from the surface will saturate • Convective Condensation Level (CCL) –level at which a parcel from the surface heating to its convective temperature will saturate • Convective Temperature – the temperature that the surface layer would need to be heated to to convect
Convection Condensation Level (CCL) • The convection condensation level is the height to which a parcel of air, if heated from below, will rise to until it’s just saturated. It represents the height of the base of cumulus clouds created by surface heating. • To determine the CCL, follow the saturation mixing ratio line upward from the surface dewpoint and find the intersection with the T curve. • The convective temperature can be found by following a dry adiabat downward from the CCL. It represents the temperature that must be reached for the formation of convective clouds.
Lifted Condensation Level • The lifted condensation level (LCL) is the height at which a parcel becomes saturated when lifted dry adiabatically. It is found by finding the intersection of the saturation mixing ratio line through the surface dewpoint and a dry adiabat through the surface temperature. It’s actually often more realistic to use an average dewpoint for the area near the surface.
Level of Free Convection • Level of Free Convection (LFC) – level at which a lifted parcel becomes warmer than its surroundings due to the release of latent heat, and hence buoyant. It is found by starting at the LCL and proceeding upward along a moist adiabat until the temperature of the parcel is greater than its surroundings, that is, it crosses the T curve.
Convection Parameters (cont.) • Level of Free Convection (LFC) – level at which a lifted parcel becomes warmer than its surroundings, and hence buoyant. • Equilibrium Level (EL) – level at which a previously buoyant parcel’s temperature again equals the environmental temperature. This is an approximate height for thunderstorm anvils.
Convection Parameter (cont.) • Convective Inhibition (CIN) – the “negative” energy area below a parcel’s level of free convection. • Convective Available Potential Energy (CAPE) – The “positive” energy area where a parcel is accelerating upward. • Equilibrium Level (EL) – level at which a previously buoyant parcel’s temperature again equals the environmental temperature. This is an approximate height for thunderstorm anvils.
Stability Parameters All indices are useful diagnostics but should not be used blindly • Lifted index (LI) • Showalter index (SI) • Total totals (TT) • Severe Weather Threat Index (SWEAT) • Bulk Richardson Number • Storm Relative Helicity
Thunderstorm Types • Single cell (pulse)—can be strong, but no severe • Multicell—can be severe, but don’t generate strong tornadoes • Supercell—rotating updraft, most severe storms
