Download Interference by water vapor and other atmospheric effects

Interference by water vapor and other atmospheric effects
The UV and IR volcanic cloud observations each have known shortcomings. VOLCAM
is designed to use both techniques simultaneously for improved eruption coverage and
scientific yield. The UV technique is solar reflective and thus is limited to daytime
observations. However, its very high sensitivity to sulfur dioxide and the robust ash
retrievals makes it an important component of the VOLCAM system for both initial
detection and for evaluating when an airspace can be reopened. The IR technique
works day or night but is ineffective if thick high meteorological clouds underlie the
volcanic cloud, or if the volcanic cloud is small and the lower atmosphere moist. Yet
night time coverage is essential for mitigation of volcanic hazards. The known limitations
of the IR ash retrieval method are partially mitigated by the addition of the IR SO2 band.
Both techniques have reduced sensitivity to ash or SO2 in the boundary layer (well below
normal aircraft cruising altitudes) although for different reasons. The simultaneous use
of both IR and UV detectors from the same platform will make available a redundancy
and coherence in data that has never before been achieved. We expect that this will be of
specific advantage in many cases:
 The dual sensors will compensate for environmental conditions that affect the IR split
window method. For example, high cold clouds can make IR ash detection
problematic due to lack of thermal contrast, but the UV detection of ash and SO2 is
enhanced by the higher reflectivity of the high clouds.
 High water vapor density interferes with IR ash retrievals in the tropic troposphere
but does not affect the UV ash and SO2 retrievals.
 Detection of eruption clouds by SO2 will reduce the false alarm rate for magmatic
eruptions to nearly zero. Magmatic eruptions pose a higher risk to aviation because
they emit far larger quantities of ash than steam-driven phreatic eruptions. The SO2
channel can also detect an eruption sooner than the ash channel. In rare cases, the
sulfur dioxide in magmatic eruptions can be partially removed by entry of sea water
into the volcanic vent, such as the case of Rabaul in 1994 (Rose et al, 1995). Ice in
the resulting cloud also masked the IR split window signal but did not interfere with
the detection at UV wavelengths. Effusive eruptions, such as Mauna Loa in 1984 and
Nyamuragira in several eruptions since 1981, produce little ash, but still may pose an
aviation hazard due to large concentrations of gases and acids. Finally, phreatic
eruptions do produce ash clouds which pose problems primarily for general aviation
and airport operations. Therefore, detection of ash clouds remains a primary mission
requirement that needs redundant detectors to minimize both false alarms and missed
detections.
 The redundancy provides a built in capability for the improvement of algorithms
because the data from both instruments can be directly compared. This has been
possible only rarely in the past (Krotkov et al 1999).
Modtran radiative transfer studies by the VOLCAM Science Team show that the split
window IR data can be corrected for the effects of water vapor interference if the water
vapor distribution is known. One of the algorithm development tasks is to determine the
best way to make the correction; whether from a climatological database or from current
data from NOAA satellites. This correction is also facilitated by the redundancy of the
VOLCAM UV camera which provides accurate locations of the ash clouds in daytime.
We will also use the extensive data base of AVHRR and GOES split window data to test
and make further improvements in our IR algorithm for ash during the startup period of
VOLCAM. This will mainly extend the IR capability to smaller eruptions in the tropics,
and improve the accuracy of all IR retrievals.