Analysis of Flat Convection during the Morning Boundary Layer Transition using Airborne Measurements

The transition of the nocturnal stable boundary layer to a convective boundary layer (CBL) is not very well understood compared to other topics like processes within the daytime CBL (Wayne et al., 2001). A few experimental investigations were already made. E.g. Lenschow et al. (1979) found that during this transition the wind, temperature, and humidity varies with the terrain. But in generall little is know about the flat convection in the early morning shallow boundary layer. This lack of knowledge was the reason to make further studies on this topic.

In summer 2002 the helicopter borne turbulence probe Helipod joined a meteorological field campaign within the EVA-GRIPS and VERTIKO projects funded by the German Government.

The Helipod is attached to a 15 m rope under a helicopter and can operate autonomously since it has its own power supply, navigation, on-line data processing and storage. It is equipped with several sensors to measure the atmospheric wind vector, humidity, air and surface temperature over a wide spectral range. This concept enables the Helipod to resolve small scale turbulence and turbulent transport of momentum, heat, and moisture.

The analyzed experiment took place at Lindenberg near Berlin in Germany. The heterogeneous terrain within the flight area was a mix of grassland, agriculture, forest, lake, and villages which is typical for northern Europe. During the field experiment the Helipod operated together with ground-based stations, a 99 m tower, and remote sensing systems.

On four days the Helipod performed measurements in the early morning on a special grid flight pattern at a constant flight level above the ground. In between several vertical profiles were taken. Due to the data it was possible to observe the transition of the nocturnal stable boundary layer to the convective boundary layer. During the two hours of the flight the boundary layer height of the flat convection crossed the flight level. So the quotient of the flight level and the BLH (z/z_i) became smaller by the time. Therefore it was possible to get different flight levels in terms of local scaling or Deardorff scaling.

Different types of analysis were made that led to systematic and promising results e.g. the Deardorff-scaled sensible heat flux. Furthermore the results from the Helipod measurements were compared with other measurement systems.