CSIR NET Dec 2015 Q134: Meteorology

The buoyancy frequency (Brunt-Vaisala frequency), N, is defined from the vertical gradient of potential temperature: N^2 = (g/theta)(d theta/dz). It represents the natural oscillation frequency of a vertically displaced air parcel in a stably stratified atmosphere, i.e., how rapidly the parcel oscillates (is restored) after being displaced vertically. A larger N^2 indicates stronger static stability (a more strongly stratified, stable atmosphere that strongly resists vertical displacement), while N^2 approaching zero or becoming negative indicates neutral or unstable stratification. Because N is derived directly from the vertical potential temperature gradient, which is the fundamental measure of atmospheric static stability, the buoyancy frequency is fundamentally a measure of the static stability of the atmosphere, matching option 2.

Option 1 (mass of the atmosphere above a level) describes atmospheric pressure, not buoyancy frequency, which is a differential/local stability measure rather than an integrated mass quantity. Option 2 is correct: the buoyancy (Brunt-Vaisala) frequency is defined explicitly through the vertical potential temperature gradient and directly quantifies atmospheric static stability. Option 3 (speed of the gravity wave locally) is related but secondary; while the buoyancy frequency does set an upper bound on internal gravity wave frequencies and influences their phase speed, N is fundamentally defined as a stability measure, and gravity wave speed is a downstream consequence of stability, not the primary defining quantity. Option 4 (moisture content of the air) is unrelated; while moist static stability calculations do modify the effective buoyancy frequency, the basic buoyancy frequency itself is defined from the dry (or virtual) potential temperature profile as a stability measure, not as a direct measure of moisture content.