Empirical Z-Visibility Relation Found by Fog Measurements at an Airport by Cloud Radar and Optical Fog Sensors

A scanning K-band cloud radar (MIRA-36) was operated at the airport Munich during the fog season 2011/2012. The measurements took place in the frame of the German government funded iPort project (innovative airport) for improving safety and efficiency of air traffic. The purpose of the measurements was to find out, if fog -- being critical for air traffic management -- can be detected by the cloud radar and if airport meteorologists can gain useful information from the radar in addition to available information from runway visual range transmissometers (RVR), ceilometers, and human observers. The radar was tilted to low elevation angles towards the (landing) gliding path for providing measurements with a height resolution of 2.6 m beginning at 16 m above ground. Thus fog could be detected at height ranges most important for the air traffic management. In a 15 min cycle also azimuth scans at 45 deg elevation were performed to get information about the cloud coverage above the airport. In contrast to optical methods the radar provides range resolved information far beyond the range of optical visibility. On the other hand it is not a priori obvious, if there is significant correlation between the radar signal and visibility due to the D^6-dependence (D = drop size diameter) of the radar reflectivity factor Z. After elimination of rain events comparisons between Z at 25 m height and the human observer looking from the tower at 25 m show indeed a useful correlation in the dBZ range from -55 to -20 dBZ. A coarse Z-visibility relation has been determined from the data: visibility length in m ~= -92 dBZ – 1800. The correlation with the RVR instruments installed at 2 m above ground is worse. Particularly in case of deep fog layers -- observed with the radar -- the RVRs tend to indicate much higher visibility than then cloud radar (lowest height 16 m) or the human observer (at 25 m). The process responsible for this discrepancy is probably the near-surface fog dissipation due to fog collection by vegetation (here mainly blades of grass) in conjunction with reduced radiative fog production due to counterradiation by the deep fog layer. A method has been developed to determine the meteorological optical rang (MOR) by combined radar and ceilometer data. Due to the extinction of optical waves this method is of course restricted to ranges in the order of MOR, which is in case of low visibility only a thin layer at the bottom of clouds or fog. In contrast to the Klett-method it does not require the reflectivity in a reference range. Comparisons of MOR computed with the radar/ceilometer-method were also compared with Z. The main advantage of this comparison is that visibility and Z refer to the same measuring volume. The radar/ceilometer combination can also be used as a reliable filter to sort out cases with rain. At the end of the experiment the radar was configured for making 20 RHI scans per hour between 3 and 177 deg elevation. The intention was to observe the advection of cloud banks across the airport for investigating the potential of short range prediction (few minutes) of the local fog development. These preliminary attempts were not conclusive because the spatial extension of the few observed fog events was beyond the maximum range of the radar. A noteworthy side result of these scans is the analysis of Doppler velocities showing the capability to determine horizontal wind velocity profiles (component in the RHI-plane) and even the fall velocity profile of the fog droplets. It seems that the influence of turbulent vertical wind is effectively eliminated due to averaging over large horizontal areas. It can be seen that the fall velocity of the droplets increases with distance from the fog top. Case studies suggest that the accuracy of radar based visibility estimates may be improved by accounting for the droplet fall velocity.