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
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