ARPGEN provides four methods in which refractivity data sets can be created:

1. M-Unit Profile Entry - This option allows the operator to create refractivity data sets by entering M-unit profiles directly. After the M-unit profile and the appropriate surface observation and location information are entered, the profile is checked to determine if an M-unit value at the sea-surface level is present. If one is not present, a surface value is determined by extrapolation, assuming a standard atmosphere gradient.

The evaporation duct height is calculated using the operator-entered air temperature, relative humidity, wind speed, and sea-surface temperature. These parameters are used to determine the bulk-Richardson number, the vapor pressure at the sea surface and at the observation altitude, and the near-surface N-unit gradient. If the N-unit gradient is positive, the evaporation duct is zero. When the N-unit gradient is zero or negative and the atmosphere is stable (positive bulk-Richardson number), the evaporation duct height is a linear function of the N-unit gradient and the atmospheric stability; when the atmosphere is unstable, the evaporation duct height is a power function of the N-unit gradient and the atmospheric stability. When the computed value of the evaporation duct height is AO m, it is set to 40 m.

2. Radiosonde Data Set Selection - Using this option, an M-unit profile is entered by operator selection of a radiosonde data set from the atmosphere environmental file (AEF). M-unit versus height pairs are extracted for the first 30 levels of the sounding or for levels between 0 and 10,000 m heights. When the lowest sounding level is not at the 0 m height, a sea-surface M-unit value is extrapolated using the lowest M-unit value/height pair in the profile, assuming a standard atmospheric gradient. The surface wind speed and evaporation height complete the information required to generate an RDS. The evaporation duct height is computed in the same manner as when an M-unit profile is entered. The location and the date and time of the RDS are specified by the operator on the Evaporation Duct-Height Parameters Input form.

3. Historical Refractivity Data Set - Using this option, a historical RDS is generated for an operator-specified location, month, and profile type (standard atmosphere, surfiace-hosed duct, elevated duct, or combined surface-based and elevated duct). The upper air data used to specify the M-unit profile with respect to height are retrieved from the Radiosonde Data file. The mean surface wind speed and evaporation duct height are retrieved from the Surface Observation Data tile. Note that the closest long-term mean radiosonde observation location and Marsden square containing the information desired are retrieved from the PDB. In some data-void regions, this may result in an inappropriate RDS being created. Use the climatological electromagnetic propagation conditions summary function to evaluate the general climatologic electromagnetic propagation conditions before using this option. The use of climatological refractivity data sets should be limited to planning functions.

4. Analysis of an AMR tape - This option allows the operator to create an RDS by analyzing a tape generated by the AN/AMH-3 electronic refractometer set. These devices are routinely flown on E-2C aircraft. Refer to the functional description for additional information.

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