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VERIFICATION OF WARNINGS, ADVISORIES, AND FORECASTS
FORECASTING ALTIMETER SETTINGS

Aerographers Mate 1 & C
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In the next section we will discuss evaporative ducts and  their  importance  to  weather  analysis  and forecasting;  specifically,  the  Atmospheric  Refractivity Profile  Generator. REFRACTIVITY  FORECASTS  USING ATMOSPHERIC   REFRACTIVITY PROFILE GENERATOR (ARPGEN) LEARNING    OBJECTIVES: Identify applications,  limitations,  assumptions,  and functional  description  of  the  ARPGEN  product. The ARPGEN is used for two purposes: 1. To create a refractivity data set (RDS) 2. To place it into the RDS for use by the electromagnetic  propagation  programs various The RDS displays a profile of modified refractive index (M) with respect to height, the height of the evaporation  duct,  and  the  surface  wind  speed. The operator directly enters the necessary surface observation  data  for  all  except  the  historical  option  of the program; the historical option is retrieved from the permanent  data  base  (PDB)  files. APPLICATION ARPGEN is used to create RDS. These data sets describe  the  effects  of  the  environment  on  the propagation  of  electromagnetic  (EM)  energy  in  the microwave portion of the spectrum. LIMITATIONS AND ASSUMPTIONS The  restrictions  as  well  as  principles  taken  for granted in using the ARPGEN product are as follows: l ARPGEN allows a maximum entry of 30 M-unit versus height pairs. Levels with heights >10,000 m are discarded due to insignificant refractive effects at higher altitudes. . The standard atmospheric lapse rate is used to extrapolate  for  a  sea-surface  M-unit  value. .  The  evaporation  duct-height  algorithm  assumes that entered surface weather observations are at a height of 6 m above the sea surface. l The RDS menu selection can accommodate up to  10  refractivity  data  sets.  As  these  sets  are  created, they are placed into higher numbered positions in the RDS. When 10 data sets are present, a newly created data set takes the place of the data set not accessed for the longest period of time. . M-unit profiles must be entered in ascending order. . For historical data sets, the M-unit profile is retrieved  for  the  closest  radiosonde  station  to  the operator-selected location; the surface data are retrieved for the closest Marsden square containing data in the PDB. In many instances, these locations for data may be several hundred miles apart. Data base coverage maps are provided in the TESS (3) Operators Manual. . Four types of historical profiles can be created by this function; standard, surface-based duct, elevated duct, and combined surface-based and elevated duct. .  The  position  of  the  RDS  (for  nonhistorical profiles)  is  specified  when  the  operator  selects  to compute rather than enter an evaporation duct height. This   location   will   be   associated   with   the operator-selected  refractivity  profile. .  The  RDS  (with  the  airborne  microwave refractometer   [AMR])   option   accommodates   five flights containing refractivity information. Different portions of a particular flight can be accessed to generate different refractivity profiles. This function will not appear in the menu if an AMR tape-reading device is not connected. FUNCTIONAL DESCRIPTION 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 10-3







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