¶ G-band (183 GHz) Vapor Radiometer (GVR)
¶ Background
The G-Band Vapor Radiometer (GVR) provides time-series measurements of brightness temperatures from four double sideband channels centered at ±1, ±3, ±7, and ±14 GHz around the 183.31-GHz water vapor line. Atmospheric emission in this spectral region is primarily due to water vapor, with some influence from liquid water. The 183.31 ±14-GHz channel is particularly sensitive to the presence of liquid water. The sensitivity to water vapor of the 183.31-GHz line is approximately 30 times higher than at the frequencies of the two-channel microwave radiometer (MWR) for a precipitable water vapor (PWV) amount of less than 2.5 mm. Measurements from this instrument are, therefore, especially useful during low-humidity conditions (PWV < 5 mm).
¶ Additional Information
Generally speaking, tbsky1 and tbsky3 are sensitive to water vapor only while tbsy7 and tbsky14 are sensitive to both vapor and liquid.
There are multiple levels of the GVR that we have in DQ Explorer. We are currently only working with the b1 level.
For more information on Radiation Variables and the Radiometer Spectrum, see Radiation Information
For more information see GVR.
¶ Expected Operations
¶ Metrics
The GVR metrics should look mostly green as below. There will be nuisance flags from time to time, but as long as they are not for extended periods of time, the data should be ok. Double check the plots though to make sure nothing is out of the ordinary.
¶ Plots
Please see Brightness Temperatures for more information on the variables below.
In general:
- The filtered brightness temperature plots should be smooth and with low noise levels.
- Brightness temperatures should be greater than 2.75K and less than approx. 310K (remember that the GVR is in Alaska; temperatures there don't get much higher than this!)
- ext_temp readings can be compared to tower measurements and should agree to within +/- 2K
- The difference between temp_hot1 and temp_hot2 should be less than 2 degrees C
- temp_warm should be greater than 10 degrees C, but less than 20 degrees C
GVR Sky Temp
DQO Calculated Liquid Water Path
The DQ Office has employed algorithms developed by the GVR mentor, Maria Cadeddu, to calculate the liquid water path (LWP) from the brightness temperatures. These data do generally look noisy, but should notice patterns when there is actual precip or clouds.
Diagnostic Plots (Temp Hot, Diagnostic, External)
These are plots of various diagnostic variables. The first plot below is of the absorber temperatures used in the calibration of the instrument. Below that is different temperatures for different components of the instrument and below that is a comparison plot of external temperatures from the MWRP and MET. All of these temperatures should follow the same general trend throughout the day. The comparison plot with the MWRP and MET should agree to within 3 oC.
¶ Comparison Plots
Liquid Water Path and Precipitable Water Vapor Comparison
This is a comparison plot of all the different instruments that measure/calculate Liquid Water Path (LWP) and Precipitable Water Vapor (PWV). The sonde data is plotted as a dot on the precipitable water vapor plot. There is normally some disagreement between the different instruments as they all have different ways of measuring/calculating these values.
Precipitable Water Vapor vs. SONDE Comparison
The sonde is an in-situ measurement and the data from it can be used to calculate the precipitable water vapor. This value is then compared to the other instruments that calculate the value, including GVR, MWRLOS, MWRP, and more. The values should follow the 1-to-1 line, but may deviate more at higher values.
¶ Site-Specific Information
The GVR is currently located at NSA.
¶ Known Issues
¶ Known Behaviors
Known issues for this instrument that MAY NOT need to be mentioned in your DQA's:
Possible RF Interference
At times the GVR experiences possible RF interference. This interference is characterized by spikes in the GVR sky temperatures. When the interference is severe, such as the example below, it should be mentioned in the DQA. When the interference is low and there are only a few minor spikes no mention is needed.
¶ Past Problems
Past problems for this instrument that DO need to be mentioned in your DQA's and possibly requiring a DQPR submittal:
GVR Calibration Effects on Data
Starting about 18:00 GMT on 5/11/2007 the calibration procedures were started and lasted until 5/16/2007. The DQO was not notified before this procedure, but filing a DQPR for this problem is not a bad thing. It starts the process of writing the DQO to inform the data user that the data during calibration period is not correct. See DQPR 1757.
Spikes in temp_ext
Spikes in the temp_ext value exceed the MAX limits (set by the DQO not DMF) suggest an electrical problem. In this case a wire had broken causing the spikes in the temp_ext field. The comparison with METTWR4h easily show they are not correct. See DQPR 1778. temp_ext values can also spike or behave abnormally due to wire corrosion or a bad temperature sensor.
Abnormal Sky Brightness Temperatures
All sky temperature data become nearly equal and abnormally constant starting at around 12:00 UTC before returning to normal a few days later. In this case, water or dew deposits on the window likely caused the saturated brightness temperatures. See DQPR 2496. Issues like this are more likely to occur during the summer months, as melting snow and precipitation occur more frequently during this time. Abnormal sky brightness temperature behavior can also occur due to incorrect positioning of the instrument mirror (see image below - at 18:00 UTC, the mirror was adjusted), or a power outage (causing the need for a software restart). Issues such as these will require a mention in your DQAs, as well as a DQPR.
