Month Polar climate Mid latitudes Tropics
May 1.3 2.5 4
June 2.2 3 4.3
June 2.5 3.5 4.5
August 2.0 3.5 4.5
September 1.5 3 4.3
October 0.5 2.5 4
November -1 1.5 3.5
December -1 1.0 2.5
What if There Is No Melting Level ?
When it’s snowing, there is no melting layer (-1 in the table above). Then the profile becomes
a simple descending line, defined by parameter set.
• Reflectivity gradient above the melting layer 7 dBZ/km (11.3 dBZ/mi)
• Reflectivity gradient below the melting layer 0 dBZ/km (0 dBZ/mi)
• Melting layer height 0 km (0 mi)
• Melting layer thickness 0 km (0 mi)
• Melting layer peak intensity 0 dBZ
The most demanding task is to get the profile right when the melting layer is close to
surface. Even small errors misplace the bright band and thus lead to severe overestimation
and underestimation close to the radar. If you cannot change the profile frequently, and the
temperature fluctuates below and above 0 with bright band appearing and disappearing,
Vaisala recommends you apply the snow profile as described above. You still have the bright
band overestimation problems, but at least you fix all the weakening above the bright band.
Make sure you give the altitude information from external data source referring to the
same reference height (sea level, antenna level) as defined in the IRIS setup. Be careful
when the bright band is close to ground.
3.17.3 Setting the Current Melting Level Without Restarting IRIS
Normally, when you change the Setup files, you have to quit and start IRIS to have the
changes valid.
For SRI this is not practical, since in the case of fast moving front you might want to update
the melting layer height every hour.
To make changes to setups while IRIS is running, you must push information through a pipe
to a program called setup_change.
Chapter 3 – Configuring IRIS Products
RESTRICTED 75
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