¶ Infrared Sky Imager (IRSI)
¶ Background
The IRSI is an automatic, continuously operating system that measures cloud fraction and precipitable water vapor. It is designed to capture hemispheric sky images and provides corresponding time series retrievals of fractional sky cover during the day, as well as during the night. The IRSI also provides diurnal sky imagery in the mid-IR atmospheric window (from 8-13 microns), and images during daylight hours in the visible wavelengths for cloud retrievals. These images have a hemispheric field of view, centered on zenith. The IRSI software is also capable of automatically identifying cloudy and clear regions of the sky to help calculate fractional sky cover. This allows for a real-time display of sky conditions.
For more information see IRSI.
¶ Additional Information
The IRSI has both an IR and a visible subsystem housed in the same enclosure along with the system electronics. The visible subsystem includes a 180 degree lens, a CCD (charge coupled device) image sensor, and a filter wheel with eight positions. The IR subsystem includes a 180 degree field of view, diamond coated lens, an IR sensor, and another filter wheel with eight positions. Lenses for both the IR and visible subsystem are housed in the top of the enclosure, covered by a radiation shield. Both the visible and IR lens have a cover. The IR lens includes embedded temperature sensors and a heater to act as a blackbody reference. See below for a diagram of the IRSI and its components:
¶ Expected Operations
¶ Metrics
The metrics should be mostly green across the board for the IRSI. A patch of unavailable data for fractional sky cover and hemispheric sky images in the visible wavelengths is normal during the evening hours. In the example below, this occurs between 0100 and 1100 UTC. The sun altitude will also flag as bad during this time, as it is below the horizon.
¶ Plots
Below is an example of the expected multi-panel plot in IR wavelengths for:
- The % of total sky cover for wide field of view and narrow field of view
- The % of high and low emission sky cover for narrow field of view
- The % of high and low emission sky cover for wide field of view
- Precipitable water vapor in mm
- Mean brightness temperature in K at zenith, as well as the color temperature in K at zenith
Below is an example of the expected multi-panel plot in visible wavelengths for:
- The % of total sky cover for wide field of view and narrow field of view (not recorded during nighttime hours)
- The % of thin and opaque sky cover for narrow field of view
- The % of thin and opaque sky cover for wide field of view
Next are diagnostic plots. This multi-panel plot includes:
- The enclosure, camera, reference blackbody, and external blackbody temperatures in Celsius. These won't match exactly but should trend the same. Any large differences or outliers should be reported in your DQAs and in a DQPR.
- The blackbody radiance: This should exhibit a diurnal pattern, decreasing during nighttime hours and increasing once the sun comes up.
- The red to blue ratio for wide field of view and at zenith
These diagnostic plots for temperatures and blackbody radiance are also plotted on a weekly time scale.
There are also several videos included in the plots for IRSI to help with data quality. The first is an image of the sky in the visible wavelengths during daylight hours. The sun should be completely blocked out in these videos, and should be mentioned in your DQAs if this is not the case. Any dirt on the lens should also be reported.
The next video is of clear-sky subtracted radiance, and is recorded for the entire day (both daytime and nighttime hours). This video is obtained through the use of a custom 10.2 - 12.2 micron filter, which optimizes clear-sky cloud contrast. This contrast is important because it allows for the effects of water vapor emission to be separated out from cloud emission, and water vapor absorption lines are the least prevalent between 10.2 and 12.2 microns. So, looking at the sky through a filter that only allows wavelengths between 10.2 and 12.2 microns provides the best way to look at the sky without the effects of water vapor emissions.
The next video is only recorded during daytime hours and shows the Red/Blue ratio N2 mask.
The last video shows the IR clear sky subtracted cloud mask, and is recorded for the entire day. Clear-sky radiance is subtracted from the normalized radiance, and we are left with any clouds that are in excess of the clear-sky emissions. High emission clouds are depicted in white, low emission clouds are depicted in gray, and the sky is depicted in blue.
¶ Comparison Plots
SGP Sky Cover % Comparisons
The top panel in the following multi-panel plot shows all of the sky cover percentages for high emission clouds in IR wavelengths for both wide and narrow fields of view, as well as the sky cover percentages for opaque clouds in visible wavelengths for both wide and narrow fields of view, plotted on the same panel. The corresponding TSI sky cover percentage of opaque clouds is also plotted on the same panel. IR sky cover percentages should be plotted throughout the nighttime hours, while visible sky cover percentages should not. The sky cover percentage for high emission clouds should generally trend the same for both narrow and wide fields of view, as well as the sky cover percentage for opaque clouds.
The second panel shows all of the same sky cover percentages, but for low emission and thin clouds. The TSI sky cover percentage is also plotted for thin clouds.
IR sky cover percentages on each plot
¶ Site-Specific Information
This instrument is currently located at SGP C1.
¶ Known Issues
¶ Known Behaviors
Known issues for this instrument that MAY NOT need to be mentioned in your DQA's can be documented here.
¶ Past Problems
Past problems for this instrument that DO need to be mentioned in your DQA's and possibly requiring a DQPR submittal can be documented here.