NARR-Based Climatology of Estimated Altimeter Error due to Nonstandard Temperatures

Using the North American Regional Reanalysis (NARR) dataset, we calculated estimates of altimeter error caused by non-standard atmospheric temperatures and surface pressures. The estimated error is determined by computing the difference between the NARR-derived heights for a specified pressure level to the heights of the same pressure level calculated using the altimetry equation, which assumes a standard atmospheric temperature lapse rate. This difference is similar to the traditional D-value variable with the exception that we use the actual altimeter setting as the base pressure rather than using the standard datum plane pressure of 1013 hPa. Since altimeter setting is not a variable in the NARR data set, it was calculated using the NARR surface elevation and station pressure. We refer to this altitude difference as the “corrected” D-value, where the correction is for non-standard surface pressure. Using the NARR data for 1989 to 2010, we followed WMO guidelines to produce a 30-year monthly climatology of the 12Z corrected D-value for pressure altitudes of approximately 4,000, 8,000, and 12,000 feet. For comparison, we also created a 30-year climatology altimeter error using the common ICAO rule of thumb where altimeter height is reduced by four percent for every 10K the air at flight level is than standard.

The motivation for the climatology was to create an educational tool for general aviation (GA) pilots and their instructors to increase awareness regarding the magnitude of potential altimeter errors related to non-standard temperatures. Having this knowledge could provide useful in reducing the number of GA controlled flight into terrain (CFIT) accidents.

The presentation will provide details regarding how the climatology fields were constructed as well as show the results. In addition we will provide a brief comparison between the calculated errors and those obtained using the ICAO rule of thumb.