Determining Stability » Lapse Rates Superadiabatic Lapse Rates
Superadiabatic lapse rates rarely persist for any length of time in the environment. Any vertical perturbation will spur the release of the instability. Genuine superadiabatic lapse rates frequently occur within the first 300 meters of the surface (due to surface heating). However, when absolutely unstable lapse rates are reported in the troposphere above the boundary layer, they are often rejected as erroneous data. Such lapse rates are typically ascribed to measurement defects or spurious effects. However, there is evidence to indicate that valid cases of superadiabatic lapse rates can occur through a fairly shallow layer sandwiched between stable layers in the troposphere, well above the surface layer.
Two processes can explain the occurrence of such real superadiabatic lapse rates:
- Significant destabilization due to rapid lifting at a saturated/dry-air interface within the lifted layer
- High rate of evaporation at the top of a cloud layer
The simultaneous occurrence of both processes is also a possibility.
Thus, when a superadiabatic lapse rate is included in a RAOB report, you should not disregard it as bad data. Pay close attention to the synoptic conditions occurring in the area of the RAOB ascent. When rapid lifting of tropospheric layers is likely, due to either the rapid movement of a cold front or orographic lifting, there is a significant probability of finding a shallow layer with a superadiabatic lapse rate. Altocumulus castellanus reports on the surface chart are often noted in the area of such activity.