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.
