Reconsideration of conditional instability: Static stability of a saturated air parcel in an unsaturated environment

This study examines the stability criterion of a saturated air parcel in an unsaturated environment using the parcel method for parcels of the same virtual temperature as their environment. by considering the lapse rate of virtual temper- ature in a moist adiabatic process,we get the results that the lapse rate of temperature for the criterion is larger than the usual moist adiabatic temperature lapse rate by about 20% of the difference between dry and moist adiabatic lapse rates. the newly obtained criterion was used to examine the frequency distribution of temperature lapse rates observed by sonde in the tropics. the results show that the maximum frequency of stability occurs at neutral,and that about half of the total cases are unstable if we consider the stability for a saturated air parcel in an unsaturated environment. © 2011,meteorological society of japan.