¶ Profiling G-band (183 GHz) Vapor Radiometer (GVRP)
¶ Background
The G-Band Vapor Radiometer Profiler (GVRP) provides time-series measurements of brightness temperatures from 15 channels between 170 and 183.31 GHz. Atmospheric emission in this spectral region is primarily due to water vapor, with some influence from liquid water. The wing channels between 170 and 175 GHz have sensitivity 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 especially useful during low-humidity conditions (PWV < 5 mm).
¶ Additional Information
The GVRP samples at a rate of approximately 2 Hz for a total of around 3000 measurements per day.
For more information on Radiation Variables and the Radiometer Spectrum, see Radiation Information
For more information see GVRP.
¶ Expected Operations
¶ Metrics
The metrics for the GVRP are normally green across the board. Any deviation for more than a few minutes may indicate a potential problem. The brightness temperature variable is stored as a 2D array. We are unpacking the variable into 1D arrays and naming it after the associated frequency (appended at the end of the brightnessTemperature_* variable)
¶ Plots
Other Notes
- In general the brightness temperature time series should be smooth and with low noise levels.
- Brightness temperatures should be greater than 2.75 K and less than approximately 310 K. (We do not expect the ambient temperature in Barrow to be significantly higher than 30°C).
- Surface meteorological readings (temperature, pressure, and relative humidity) can be compared to tower measurements. The agreement should be +/-2 K for temperature, about 5 kPa for pressure, and 5% for relative humidity.
- The instrument mentor routinely compares the measured brightness temperatures with model computations as a general quality check.
All of the plots below are also included as a weekly plot.
Brightness Temperature
A descriptive measure of radiation in terms of the temperature of a hypothetical black body emitting an identical amount of radiation in the same narrow bands of wavelengths. The brightness temperature s measured at 15 different wavelength. The values are only good if the elevation angle of the lens is at 90º (straight up). The data is flagged and removed if the elevation angle is not equal to 90. As of February 24, 2011, the instrument was changed and it started measuring at angles not equal to 90º. This is a common occurrence and makes it look like all times are being flagged, this is not the case. It just looks like that due to the high sample rate.
Since the brightness temperatures are sensitive to water vapor, there will be a noticeable change in temperature during rain events. A blue rain flag will be trip when the instrument is sensing rain.
Surface Temperatures
The GVRP also measures the blackbody and surface temperatures. The blackbody temperature should follow the same sort of overall trend as the surface temperature. The surface temperature is compared with the co-located met observations which are normally within 1K or less.
Surface Relative Humidity
The surface relative humidity is also being compared with the co-located met observations. These values are normally within 5%. The RH sensors is known to become saturated at values over 94%. This is something that is documented in the comments and does not need to be reported. Lately the met RH values have been noisier than the GVRP values for reasons unknown.
Surface Pressure
The comparison of the surface pressure with the co-located met normally is within 0.1 KPa of each other.
¶ 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.
Sky Brightness Temperature Comparison
The brightness temperatures from multiple instruments are compared for 1 day and 1 week. Because the instruments are actually measuring the sky temperature at different bands, the absolute values will not match, but a general comparison can be made to see if the instrument is picking up subtle changes expected when Liquid and Water Vapor changes occur. The relative changes in the plots will not match up directly either. Mainly we are looking for cases where the the instruments detect a significant change in temperature due to the presence of Liquid or Water Vapor, but one of the instruments do not detect this change, or vice versa.
As you can see in the plots below, the GVR has some noise showing up in the data, most notably the +/-14 GHz channel.
The rain flag plotted is either from the MWRHF (1st) or MWR3C (2nd) when available.
¶ Site-Specific Information
This instrument is located at NSA.
¶ 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:
High Noise in the data
From DQR D110927.3: All channels of the radiometer became suddenly very noisy. The receiver noise figure was computed and was found anomalous. Data are unusable during the indicated times. On 10/13 the instrument was removed and will be sent to the vendor for repairs.
This is a hard one to find by just looking at the brightness temperatures (1). It best shows up in the Liquid Water Path (LWP) and Precipitable Water Vapor (PWV) comparison plots (2). It can be seen that the LWP and PWV are noticeably noisier! A problem like this requires a DQPR.
Erratic Surface Temp and RH Data
From DQPR 2603: After a period of missing data, surface temperature and surface relative humidity data returned and were incorrect, flat-lined at 999.990 K and 999.90%, respectively. A removal and cleaning of the temperature and humidity sensor solved the problem.
Invalid surface temperature and relative humidity values can also occur if the sensor is old or defective. Typically, these sensors last several years, and once they begin to give erratic readings, they need to be replaced. Any indication of erratic temp and RH readings such as the example above should be mentioned in your DQAs and in a DQPR.
Large Portions of Missing Data
This can be due to a variety of reasons, but is likely due to a software crash or malfunction. If missing for greater than 24 hours, a DQPR needs to be submitted.