The multi-angle snowflake camera (MASC) is an instrument that collects stereographic photographs of free-falling hydrometeors from three angles, while simultaneously measuring their fall speed. MASC consists of three cameras, each separated by angles of 36 degrees. The field of view of each of these three cameras is aligned to have a common focus point about 10 centimeters away from the cameras. A ring surrounds this focal point, through which hydrometeors fall. The ring houses a system of two near-IR emitter-detector pairs that are aligned with the camera’s field of view and detect free-falling hydrometeor passage, with the lower emitters configured to trigger the MASC cameras. The IR motion sensors are designed to filter out slow variations in ambient light, and allow for hydrometeor fall speed to be derived through successive triggers along the fall path.
* For more information see the MASC Instrument web page
The exposure times of the cameras are extremely short (approx. 1/25,000th of a second), and allow the MASC to capture snowflake sizes from 30 micrometers to 3 centimeters. Anywhere from zero to thousands of images may be collected in a single day.
As you can see from the figures below, MASC metrics should be either green or orange. Snowflake fall speed is the only metric to look at. Periods of data NA (the orange boxes) indicate that a value is not available and are normal when it is not snowing. These do not need to be mentioned in the DQA. Missing data (gray) or bad data (red) may indicate a problem. Bad data may be tripped in the metrics when the valid maximum of the snowflake fall speed exceeds 10 m/s, or when the valid minimum of the snowflake fall speed falls below 0.00000 m/s.
Plots include the daily and monthly fall speeds of the snowflakes in meters per second. The fall speed of a snowflake is calculated from the time it takes to traverse the distance between the upper and lower IR-triggering array. Falling hydrometeors larger than approximately 0.1 mm in maximum dimension are detected.
A video of the black and white images captured by the MASC instrument are also included with the diagnostic plots. This video may not appear in the diagnostic plot window if there were a low number of snowflakes captured for that day (the daily snowflake fall speed plot in this case would only show one plot point, and the metrics table would only show one green box). The more snowflakes captured, the longer the video should be.
Instrument is located at NSA.
List of known issues for this instrument that MAY NOT need to be mentioned in your DQA's:
The MASC instrument was moved from the OLI M1 to the NSA C1 site fairly recently. As such, there are currently no known issues or problems from the MASC at its current location. The known issues listed below are from its time at the OLI M1 site location, but may be helpful as reference or as clues to any problems that may occur with the MASC at the NSA C1 site.
Trigger Sensitivity Issues
The MASC instrument originally experienced trigger sensitivity issues due to RF (radio frequency) interference from a radar instrument located adjacent to the MASC at its previous AMF-3 (OLI M1) site location. The cameras on the MASC were found to be taking images even on clear, sunny days when there were no snowflakes to trigger the instrument. RF chokes were installed to increase electromagnetic interference protection, and new firmware fixed the false triggering of the MASC instrument due to RF pollution. More detail can be found here: MASC log or in DQPR 4524.
The MASC cameras are very sensitive to external light conditions. The vanes of the wind shielding can sometimes act as a mirror in cases of direct sunlight. These windshield reflections can show up on the top and bottom of the MASC images. Although it may appear as though the images are obscured in cases like this, they are not, and the particles are not affected. This type of issue does not need to be mentioned in your DQAs. See an example below:
If you see images that appear obscured as above, it may be helpful to also check the corresponding NSA TSI movie for that day. If it is very sunny with clear skies, then windshield reflection is most likely the culprit.
List of past problems for this instrument that DO need to be mentioned in your DQA's:
The MASC instrument was moved from the OLI M1 to the NSA C1 site fairly recently. As such, there are currently consistently recurring past problems from the MASC at its current location. The problems listed below are from its time at the OLI M1 site location, but may be helpful as reference or as clues to any problems that may occur with the MASC at the NSA C1 site.
If the MASC instrument is placed in a location that is prone to blowing snow, or if snow and ice are not routinely cleaned off of the instrument, then missing snowflake data may occur as the instrument becomes obstructed. This can be mitigated by placing the MASC instrument within a snow fence. Further detail can be found in DPQR 5201.
A problem with the MASC camera software can also cause MASC snowflake imagery to fail to save to the computer disk. As a result, no snowflake video (masc_camera) would appear in the diagnostic plots, regardless of how many snowflakes/snowflake fall speeds were captured for that day. This may require a DQPR.
Power outages can also cause the MASC to lose power and not record data if the software does not immediately restart. This may be the case if there are no metrics for several days in a row and may require a DQPR.