A SLANTWISE SHOWALTER INDEX BASED ON MOIST SYMMETRICAL INSTABILITY - RESULTS FOR CENTRAL ALBERTA

The Slantwise Showalter Index (SSI) is defined as follows: A parcel fr om 85 kPa is lifted along a surface of constant angular momentum dry a diabatically until saturation occurs, and then pseudo-adiabatically to 50 kPa. The slantwise lifted 50-kPa temperature is then subtracted fr om the observed 50-kPa temperature. A negative value for SSI indicates the potential for moist symmetric instability. Since a deep layer of moist-symmetrically unstable air is likely to produce significant over turning, we explored the usefulness of SSI to indicate conditions of s teady rain. A complete year of twice-daily soundings from Stony Plain, Alberta, provided the experimental dataset. During the winter, SSI va lues were positive indicating that the 85-50 kPa layer was moist-symme trically stable, with very few exceptions. During the summer, however, about 5% of the soundings indicated deep layers of unstable stratific ation with respect to slantwise overturning (i.e. negative SSI values) . Negative SSI values, combined with relative humidity values greater- than-or-equal-to 60% at low levels, proved to be useful for identifyin g a few of the observed summertime rain events.