Calculation of detailed surface temperature maps using the Helipod and the Tornado reconnaissance aircraft of the German Airforce

The LITFASS 2003 experiment near Lindenberg (60 km south east of Berlin) was one of the largest campaigns carried out in Central Europe for years. Research groups from all over Germany and the Netherlands observed mainly the water transport in the soil and the atmosphere of a heterogeneous terrain with a distribution of grassland, forest, agriculture, and lakes that is typical for Central Europe. Main topic of this joint field experiment was the energy exchange between the earth surface and the atmosphere. The data sampled using various methods and systems will be used for the initialization and validation of numerical models of the atmosphere.

Beside ground-based stations, wind profiler, LIDAR, SODAR, scintillometers, and a 99 m tower, two airborne systems performed research flights. The helicopter borne turbulence probe Helipod measured wind, humidity, air and surface temperature during mainly low-level flights as before in 1997, 1998, and 2002 at this site. Completely new to the German meteorological community was the use of military jets for research purpose.

The Tornado aircraft of the 51th reconnaissance squadron 'Immelmann' are equipped with high-resolution infrared cameras. Due to its high air speed the infra-red camera aboard the Tornado aircraft covered the experimental site of about 20 km times 20 km in less than 30 minutes. The surface temperature distribution changed slowly with time, so these photographies represent snapshots of the surface temperature of the entire area. The Tornado cameras use achieve a spatial resolution in the order of a meter or (depending on the altitude of the aircraft) even better.

The infrared pictures sampled by the Tornado aircraft provide relative temperature distributions and are not calibrated. For the usual reconnaissance task the camera systems are optimized for a maximum contrast and not for absolute temperature measurements. But via the comparison with simultaneous measurements of other involved systems these gray-scale photographies can be calibrated by the surface temperature. Especially the horizontal flights at low altitude with the Helipod provide an excellent calibration tool. The measurements at several ground stations give the opportunity for data quality control and cross-validation.

The first impressive result of the analysis of the calibrated surface temperature maps is that the heterogeneity of the experimental site was much stronger than assumed by the numerical models used so far. Especially the influence of the water content of the soil is clearly visible. The maps show impressively that even on uniformly used agriculture fields there are large horizontal variations in the surface temperature. This is especially important for the installation of ground-based stations that are intended to measure area-representative characteristics of the vicinity.