| Description: | During periods of extremely cold outside temperatures (-30 C or below), the MPL may suffer
from reduced sensitivity due to obscuration of its viewport window with frost or
condensation.
During the winter of 1998-1999, on-site observers noted frost or condensation obscuring
part or all of the viewport window. A fan providing blowing heated air on the viewport had
failed. Although replacement of this fan reduced the degree of obscuration, the problem
was not entirely eliminated. During the summer of 1999 a more robust forced-air system
was installed and the problem was essentially solved except for during isolated cases of
extremely cold weather. Under these extreme conditions partial or complete obscuration
of the window is still possible with the result that cloud detection sensitivity will be
reduced depending on the fraction of the window surface obscured. Even with significant
obscuration, the MPL may still detect clouds and the cloud base heights of detected
clouds will be reported correctly. This problem is one of detection sensitivity only and does
not affect the range scale in any way. |
| Suggestions: | Being virtually indistinguishable from low ground-level ice fog or blizzard
conditions of blowing snow, this condition is very difficult to definitively identify exclusively
from the MPL datastream. It is not flagged in the lowest level "a1" datastream at all.
In subsequent processing it is typically flagged generally as a "beam-blocked" condition
but it is not distinguished from blocking by atmospheric phenomenon such as fog or
blizzard conditions.
It may be possible for the user manually identify frost-covered episodes in the presence of cold clear skies by observing brightness temperatures from the AERI or IRT. (Cold clear skies will present very low brightness temperatures while fog and blizzard conditions will not.) Alternatively, the user may use the co-located ceilometer as an alternate indication of cloud base. If the ceilometer does not report a beam-blocked condition or obscuration but the MPL does, it is likely that the MPL window is significantly obscured.
Being virtually indistinguishable from low ground-level ice fog or blizzard
cond
itions of blowing snow, this condition is very difficult to definitively identify exclusively
from the MPL datastream. It is not flagged in the lowest level "a1" datastream at all.
In subsequent processing it is typically flagged generally as a "beam-blocked" condition
but it is not distinguished from blocking by atmospheric phenomenon such as fog or
blizzard conditions.
It may be possible for the user manually identify frost-covered episodes in the presence of cold clear skies by observing brightness temperatures from the AERI or IRT. (Cold clear skies will present very low brightness temperatures while fog and blizzard conditions will not.) Alternatively, the user may use the co-located ceilometer as an alternate indication of cloud base. If the ceilometer does not report a beam-blocked condition or obscuration but the MPL does, it is likely that the MPL window is significantly obscured. |