Download The Ramdas layer

The Ramdas layer: a micrometeorological paradox
Ponnulakshmi V K and Ganesh S
ABSTRACT: On calm clear nights, a peculiar temperature distribution called the ‘lifted
temperature minimum (LTM)’ develops, in which a local minimum in the vertical
temperature profile occurs a few decimeters above the ground. The LTM was first
observed by Ramdas and Ramanathan in 1932, and has been variously referred to as the
Ramdas layer, the Ramdas paradox, the elevated temperature minimum etc. It is quite
robust and has since been widely observed on varied surfaces. Nevertheless, the
phenomenon remains counterintuitive since it goes against conventional wisdom of
ground being the coldest at night; further, the presence of a colder layer of air amidst
warmer surroundings in an otherwise homogeneous atmosphere is difficult to explain.
Such near-surface temperature distributions are of particular relevance to agricultural and
boundary layer meteorology. The prevailing theoretical explanation for the LTM is the
VSN model (Vasudevamurthy, Srinivasan and Narasimha, 1993) in which the infra-red
radiation balance in a homogeneous water-vapor-laden atmosphere is modeled using a
flux-emissivity formulation. The theory produces an LTM in apparent agreement with
observations, and the underlying physics has been attributed to the presence of a nearsurface emissivity sub-layer. Crucially, the LTM is predicted to occur only above
surfaces that are not perfect emitters (absorbers).
We demonstrate here that the origin of the Ramdas layer remains a mystery. The
theoretical explanation based on the VSN model is fundamentally inconsistent. The
exaggerated effect of the reduced ground emissivity on the near-surface cooling rates
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.