Radiative Cooling and Heating within Topography—Parametric studies with a 3D Radiative Transfer Model

Temperature inversions form regularly in topographic depressions and along sloping terrain during nighttime, when the surface energy budget becomes negative. In contrast, superadiabatic layers form above the surfaces during the day as a result of large radiative energy inputs. Several processes contribute to the nighttime cooling and daytime heating of the atmosphere confined within topography, one of which is radiative flux divergence or clear-air radiative cooling.

The radiative cooling and heating in idealized valley and basin topographies of 500 m depth are investigated using the three-dimensional radiative transfer model MYSTIC (Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres). Based on idealized temperature profiles derived from observations in Rocky Mountain valleys, parametric studies are performed to investigate the role of a) the width and shape of topographic depressions, b) atmospheric stability, and c) the surface temperature distribution on the magnitude of radiative cooling and heating.

Under conditions when sidewall surface temperatures in valleys or basins are the same as those of the adjacent enclosed atmosphere, and the valley or basin width exceeds 1 km, radiative heating rates at the basin or valley center are almost identical to those found over flat terrain. When averaged over the entire atmospheric layer from sidewall to sidewall, however, heating rate profiles differ from the idealized flat terrain case, emphasizing the importance of radiative heating in the air layers closer to the sidewalls. For an evening situation with a surface inversion building up over the valley or basin floor, for example, the radiative cooling is weaker than for a similar inversion building up over flat terrain. When more realistic near-surface temperature structures are taken into account - including shallow near-surface inversions over the sidewalls which maintain a surface temperature deficit relative to the enclosed atmosphere - stronger radiative cooling is observed in the valley or basin than over the plain.