Radiative and turbulent heating rates in the clear‐air boundary layer

Abstract

AbstractThe diurnal evolution of a clear‐sky midlatitude summertime boundary layer (BL) was studied using a column model over smooth and homogeneous land, subject to weak, moderate, and strong winds. The high‐resolution BL model (lowest point at 30 cm) was equipped with an adequate turbulence scheme and a narrow‐band long‐wave (LW) radiation scheme, the latter validated using data from the International Comparison of Radiation Codes in Climate Models (ICRCCM).In off‐line ICRCCM experiments, ground emissivity ε < 1 led to extra LW cooling of air near the surface compared to ε = 1. However, much stronger LW cooling at heights of 1–3 m, and warming below 1 m, was obtained by setting the ground colder than air at screen height, a typical condition during clear nights. Conversely, a warm surface anomaly typical of sunny days leads to strong LW warming at 1–3 m, with LW cooling just above the ground. These ground temperature anomalies dominated the LW heating/cooling patterns at heights of up to 3–4 m, perhaps explaining controversies in the observed LW flux divergences close to the ground.Interactive model results indicate that the middle part of a windy clear‐air nocturnal BL (NBL) is dominated by turbulent cooling, while the upper and lower NBL is dominated by LW cooling. Below about 1 m, a fourth layer is formed with LW warming and turbulent cooling, in agreement with the off‐line experiments. When the surface winds fall below about 1–1.5 m s−1 LW cooling dominates in the whole NBL, except very near the surface. In these light wind conditions the Monin–Obukhov theory should be revised to include radiative effects.In clear‐air daytime conditions strong convective BL heating dominates over weak LW cooling except at 1–3 m heights where the cooler air absorbs the thermal emission of the hot ground. The subsequent LW warming of the superadiabatic surface layer appears to be strong enough to induce local turbulent cooling (despite the hot surface) in an ‘hour glass’ pattern independently of wind speed, such that the total diabatic heating rate remains nearly constant with height. Copyright © 2006 Royal Meteorological Society