SPC Thermodynamics

SPC Thermodynamics Analysis

Data Courtesy Storm Prediction Center

Surface-Based CAPE Surface-Based (Contour)/CIN (J kg-1 - Shaded)

SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to a parcel of air originating at the surface and being lifted to its level of free convection (LFC). No parcel entrainment is considered. The CAPE and CIN calculations use the virtual temperature correction.

CIN (Convective INhibition) represents the "negative" area on a sounding that must be overcome before storm initiation can occur.

Surface Base CAPE (Contour) and Surface Base CIN (Shaded)

100-mb Mixed Layer CAPE/CIN (J kg-1)

MLCAPE (Mixed Layer Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the mean potential energy conditions available to parcels of air located in the lowest 100-mb when lifted to the level of free convection (LFC). No parcel entrainment is considered. The CAPE and CIN calculations use the virtual temperature correction.

CIN (Convective INhibition) represents the "negative" area on a sounding that must be overcome before storm initiation can occur.

Normalized CAPE (J kg-1)

The NCAPE (Normalized CAPE) is CAPE that is divided by the depth of the buoyancy layer (units of m s**-2). Values near or less than .1 suggest a "tall, skinny" CAPE profile with relatively weak parcel accelerations, while values closer to .3 to .4 suggest a "fat" CAPE profile with large parcel accelerations possible. Normalized CAPE and lifed indicies are similar measures of instability.

Downdraft CAPE (J kg-1)

The DCAPE (Downdraft CAPE) can be used to estimate the potential strength of rain-cooled downdrafts within deep convection, and is similar to CAPE. Larger DCAPE values are associated with stronger downdrafts. Likewise, DCIN (downdraft inhibition) is analogous to convective inhibition (hatching at 25 and 100 J kg-1)

Surface-Based Lifted Index (C) & Convective Inhibition (J kg-1)

SBLI (Surface Based Lifted Index & Convective Inhibition) is the Lifted Index at 500-mb, based on the surface parcel, and the convective inhibition for the same parcel. These fields are meant to identify areas of surface-based CAPE and minimal convective inhibition, which suggests some threat for surface-based thunderstorms.

Mid-Level Lapse Rates (C km-1)

A lapse rate is the rate of temperature change with height. The faster the temperature decreases with height, the "steeper" the lapse rate and the more "unstable" the atmosphere becomes.

Lapse rates are shown in terms of degrees Celcius change per kilometer in height. Values less than 5.5-6.0 C km-1 ("moist" adiabatic) represent "stable" conditions, while values greater than 9.8 C km-1 ("dry" adiabatic) are considered "absolutely unstable." In between these two values, lapse rates are considered "conditionally unstable." Conditional instability means that if enough moisture is present, lifted air parcels could have a negative LI (lifted index) or positive CAPE.

The 700-500 mb lapse rates, also referred to as mid-level lapse rates, are meant to identify regions where deep convection is more probable (all else being equal). Likewise, steeper lapse rates correspond to the possibility of larger CAPE and stronger storm updrafts.