The Ramdas layer remains a micro-meteorological puzzle

Abstract

We demonstrate here that the origin of the Ramdas layer or the lifted temperature minimum(LTM), first observed by Ramdas and co-workers in the 1930’s ([21]) still remains a mystery. A recent theory ([2]) that purports to explain the preferential cooling of the near-surface air layers leading to the LTM, based on radiative transfer processes in a homogeneous water-vapor-laden atmosphere, is shown to be fundamentally inconsistent. The exaggerated effect of the reduced ground emissivity on the near-surface cooling rates (infrared flux divergences) predicted by the theory is spurious, and is due to a physically incorrect ‘band cross-talk’. The error arises in the treatment of the reflected radiation. This is shown by comparing the flux divergences obtained using gray and band model formulations for the simplistic case of a participating medium with only two bands. It is then argued that the cross-talk error in varying amounts is, in fact, inherent in a naive radiative heat transfer model, involving non-black emitting surfaces, that does not fully resolve the emission spectrum of the participating medium. For the Ramdas layer in particular, the discrepancy between a naive gray model and more accurate band models is greatly magnified due to the rapidly fluctuating nature of the emission spectrum of the principal radiating component (water vapor). It is finally shown that a careful treatment of the reflection term, even within the purview of a gray theory, eliminates the aforementioned spurious cooling. The inevitable conclusion from our analysis is that radiative processes, acting in a homogeneous isothermal atmosphere, will not lead to a preferential cooling of the air layers near the ground, and thence, to an LTM. The origin of the LTM must therefore lie in an atmosphere that is heterogeneous on the same length scales. We discuss the role of aerosols as a likely candidate for this heterogeneity.