How the heights of the base and top of the icing derived -------------------------------------------------------- Once the icing potential is calculated for each AK-ETA column, the base and top of the icing layer are determined by simply finding the height of the lowest and highest grid points which have icing indicated. Using the SLD potential field for each AK-ETA column, the same technique is applied to find the base and top of the SLD layer. How the cloud top height field is derived ----------------------------------------- Once a cloud top temperature is established for each horizontal AK-ETA grid point, the approximate height of cloud top can be determined. This is done by looping through the AK-ETA model temperature data in the vertical column, beginning at the highest altitude, and finding the first AK-ETA temperature which is higher than the cloud top temperature. Since the temperature at this level in the AK-ETA model is higher than the cloud top temperature, we know that this height is just below that of cloud top. Therefore, we choose the height of the next grid point above this level as the cloud top height. We choose to err on the conservative side when estimating the height of cloud top. We do this because we do not want to inadvertently cut off a cloud top due to a slight error in either the AK-ETA temperature or in our cloud top temperature, since significant icing often occurs near cloud tops. Thus, the height of cloud top indicated here will tend to be a bit on the high side. This technique was applied successfully by Thompson et al. (1997) in their efforts to remove icing diagnoses made by the NCAR-RAP algorithm that were above cloud top. How the cloud base height field is derived ------------------------------------------ The cloud base height field is based solely upon reported cloud base heights from surface observations (METARs) across Alaksa and Western and Canada. Each AK-ETA model grid point is assigned the cloud base height which is equivalent to the lowest cloud base height reported by any surface station within the closest concentric circle, as described in "THE ALASKA IIDA CONCEPT". It is important to note that a cloud base height will only be assigned if a cloud is indicated at grid point from the combination of satellite and surface observations. When no surface observations are available within 125km of the site (e.g. over open water), a cloud base height is determined using model output alone. How the cloud top and base height fields are used ------------------------------------------------- The application of the cloud base and top heights to the integrated algorithm is quite simple. Icing is not allow to be diagnosed above cloud top. Icing is also not allowed to be diagnosed below cloud base WHEN PRECIPITATION IS NOT PRESENT. If precipitation is present, it is possible for icing to be diagnosed down to the surface, since the precipitation may contain supercooled water drops. Eliminiating icing above cloud top and below cloud base (when precipitaiton is not present) keeps the algorithm from forecasting icing where liquid water could not possibly be present (i.e. in clear air). This is an important difference between the integrated icing algortihm and those which use only model relative humidity to determine the bases and tops of the icing layer. The integrated algorithm can accurately delimit the vertical extent of the cloud deck, decreasing the vertical extent of the icing forecast, while not losing any of the altitudes where icing is possible. Algorithms which are purely model-based cannot do that, and thus misdiagnose the vertical extent of the icing layer. The same is true for the horizontal extent of the icing. Since the integrated algorithm will only diagnose icing where cloud is present, it will never indicate icing in cloud-free areas. Those algorithms which use model generated RH fields to identify icing areas will often indicate icing in cloud-free areas. Overall, the data from the satellite and surface observations are used in combination with the AK-ETA model height and temperature fields to produce a 3-dimensional "screen" outside of which icing will never be diagnosed. REFERENCES ---------- Thompson, G., T.F. Lee, R.T. Bruintjes, and R. Bullock, 1997: Using satellite data to reduce area extent of diagnosed icing. Weather and Forecasting, 12, 185-190.