CSIR NET Dec 2015 Q124: Meteorology

The Rossby radius of deformation (lambda_R) is the characteristic length scale at which the effects of Earth's rotation (Coriolis force) become comparable in importance to those of buoyancy/gravity wave restoring forces in a rotating, stratified fluid. When the characteristic length scale of a flow (L) is much larger than the Rossby radius of deformation (L >> lambda_R), the flow is on a scale where rotational (Coriolis) effects dominate over gravity wave dynamics, placing the flow well within the geostrophic (rotation-dominated) regime typical of large-scale synoptic weather systems. Conversely, for L << lambda_R, gravity/buoyancy effects would dominate (as in small-scale convection or gravity waves). Since L is stated to be much larger than lambda_R here, rotation effects are more important than gravity effects, matching option 3.

Option 1 (both gravity and rotation effects are important) would be more applicable when L is comparable to lambda_R (of the same order), not when L is much larger. Option 2 (both effects unimportant, can be neglected) is incorrect since at large scales (L >> lambda_R) rotation is in fact the dominant controlling effect, not negligible. Option 3 is correct: for length scales much larger than the Rossby radius of deformation, Earth's rotation (Coriolis force) dominates the force balance, making rotational effects more important than gravitational/buoyancy restoring effects. Option 4 (rotation effects less important than gravity effects) is the reverse of the correct relationship; this would instead describe the regime where L is much smaller than lambda_R.