Morning transition of the temperature profile close to the surface

At the end of a clear night the atmospheric layer close to the surface usually shows a temperature inversion. At sunrise the radiative component of the surface energy budget changes from net loss at the end of the night to net gain of radiative energy during daytime. This change in turn induces a change in the near surface temperature profile and in the turbulent state of the layer close to the surface. The focus of this paper is the transition period between the strong surface-based inversion existing at the end of the night and the establishment of well developed convective turbulence that occurs about 1 hour later. We hypothesize that the morning transition problem may be related to the problem of entrainment. The latter term is commonly used to describe the mixing process at the top of a well developed convective boundary layer. If, however, we understand entrainment as the turbulent process by which a stable layer is transformed into a convective mixed layer, then the morning transition may be treated as a particular example of entrainment

The work presented here includes data analysis and a simple model of the process. A subset of the Oklahoma Mesonet stations that have temperature measurements at 1.5 and 9 m above the surface is analyzed for two days of July 1997, that show no signs of cloudiness during the morning hours. We present average time series of relevant meteorological variables: temperature at different levels, radiation fluxes, wind speed, and turbulence variables when available. The transition period is characterized by defining several bulk parameters: initial inversion strength, time scale of the transition, maximum heating rate, etc. A basic statistical analysis of these parameters is performed.

A prominent feature of the morning transition is the frequent occurrence of a well defined peak in the time rate of change of temperature close to the surface. A possible explanation for this peak is that during this period two processes are important in warming the air close to the surface: the input of sensible heat flux at the surface and the mixing of the nocturnal temperature inversion profile. The effect of the last process can be significant only during the transition period, before the temperature inversion dissipates. We test this explanation by formulating a simple model of the process, in which a layer with an initial temperature inversion is heated and mixed simultaneously. Under some assumptions the model predicts a relationship between the maximum heating rate, its time of occurrence, the initial temperature inversion strength, and the final heating rate. We test this prediction with 44 cases in which these parameters can be estimated from the data set and find a correlation coefficient of app. 0.6.

Other processes can also explain this morning transition, and due to the variability in the data at these hours the task of testing them is difficult. However, we believe that this is a relevant problem that can increase our understanding of entrainment and convection.