The multifilter rotating shadowband radiometer (MFRSR) takes spectral measurements of direct normal, diffuse horizontal and total horizontal solar irradiances. These measurements are at nominal wavelengths of 415, 500, 615, 673, 870, and 940 nm. The measurements are made at a user-specified time interval, usually about one minute or less. The sampling rate for the Atmospheric Radiation Measurement (ARM) Climate Research Facility MFRSRs is 20 seconds. From such measurements, one may infer the atmosphere's optical depth at the wavelengths mentioned above. In turn, these optical depths may be used to derive information about the column abundances of ozone and water vapor (Michalsky et al. 1995), as well as aerosol (Michalsky et al. 1994) and other atmospheric constituents.
A silicon detector is also part of the MFRSR. This detector provides a measure of the broadband direct normal, diffuse horizontal and total horizontal solar irradiances. A MFRSR head that is mounted to look vertically downward can measure upwelling spectral irradiances. In the ARM system, this instrument is called a "multifilter radiometer (MFR)". At the Southern Great Plains (SGP) there are two MFRs; one mounted at the 10-m height and the other at 25 m. At the North Slope of Alaska (NSA) sites, the MFRs are mounted at 10 m. MFRSR heads are also used to measure "normal incidence" radiation by mounting on a solar tracking device. These are referred to as normal incidence multi-filter radiometers (NIMFRs) and are located at the SGP and NSA sites. Another specialized use for the MFRSR is the narrow field of view (NFOV) instrument located at SGP. The NFOV is a ground-based radiometer (MFRSR head) that looks straight up.
The direct normal component is calculated from the difference of the global and diffuse measurements. At the start of a measurement series, a global measurement is first taken with the shadow band in the home position; fully rotated counter-clockwise without hitting support arm. The shadowband is then rotated from the home position and stops in three positions before returning home. The first and third stops are just before and just after shading the diffuser. At the second stop the diffuser is completely shaded (see picture below). Measurements at the first and third stops are used to correct the error introduced by the shadowband shading a portion of the sky in addition to the sensor.
Filter1 & Filter5 are for aerosol measurements.
Filter2, Filter3 & Filter4 are for ozone measurements.
Filter6 is for water vapor measurements.
Here is a nice plot of the spectral response of each filter for the MFRSR. Notice how each has a curve centered around the wavelengths we describe. It also shows the magnitude difference expected between the different "colors".
The metrics for the MFRSR will have a lot of missing data overnight. Since there is no incoming radiation, the instrument does not collect the data. There are also a lot of nuisance flags that trip with this instrument. The valid min for a lot of the radiation variables is 0 W/m2/nm, but given the noise that can creep in from the instrument, logger, data cables, etc, the values can drop below 0 at times.
The top plot is the Hemispheric Irradiance for all bands. A sunny day is the best time to determine if the instrument is operating correctly. The hemispheric data should follow a nice smooth curve, peaking at solar noon (dashed-yellow line).
The 2nd plot is the diffuse irradiance. This should follow a similar trend as the hemispheric data, but may be a little noisier.
The 3rd plot is the Direct Normal Irradiance. It will have a little broader curve, but should trend similar to the other 2 plots on a clear day!
Head Temperature and Logger Voltage
This is a diagnostic plot of the head temp and the logger voltage. The voltage will appear noisy due to the small y-scale it is plotted on.
MFRSR Cloudy Day
A cloudy day may look like bad data, but if you know the variables it should not be hard to pick out! The Hemispheric and Diffuse should follow the same trend, albeit a little noisier or with spikes. The big clue is that the Direct Normal Irradiance is mostly near 0 W/m2/nm. We are not getting any direct shortwave radiation from the sun, because it is getting diffused by the clouds.
Currently, the only comparison plots are at SGP and NSA C1 as these are the only 2 sites that also have a co-located NIMFR. These pull in data from the nearby MFRSR E13 instrument as well as the NIMFR. Each filter is plotted out along with the broadband comparison. The general trends should be very similar, but the actual values may differ a little depending on the filter.
Known issues for this instrument that MAY NOT need to be mentioned in your DQA's:
High Latitude 24 hour data comparison
At high latitudes the MFRSR is not able to record correct data for a full 24 hours/day. During days without sunrise/sunset (NSA Summer) the shadow band is not able to make a full rotation because the band would strike the support arm. To ensure the instrument is not damaged, the MFRSR is programmed to stop recording data and enter sleep mode +/- 2 hours of local solar midnight. The NIMFR does not have this problem and will continue to record data. This means the comparison plot of the two will not match +/- 2 hours of local midnight.
Head Temperature - NSA
The head temperature is consistently failing QC at NSA. This is a known issue and does not need to be reported.
Notice that the head temp stays between 41° C and 43° C throughout the day. The head temp is set to keep it at 40° C and this is about as good as one can expect. A prime example of excellent head temperature stability. Though 40° C is the desired temp, head temps in tolerance can range from between 35° C and 45° C. In some particularly hot climates, temps can go above 45° C. Consistency between 35° C and 45° C is desired, but often the daily range fluctuates significantly more than in the above plot. Head temperature is an indicator of instrument health. For the most part, if it stays between 35° C - 45° C things are acceptable.
Logger voltage should ideally be flat between 12 and 14 Volts. It can range a bit higher or a little lower and is sometimes a little noisy. The above example is as good as it gets. Most instruments will have a little noise with an occasional dip or spike.
Head_temp example with problems
Clock reset with brief missing data at C1
The SGP MFRSR at C1 has a daily spout of missing data at approximately 0608UTC. This is due to the clock resetting, and is perfectly normal. See DQPR 4928 or DQR D141207.3 for more information.
Past problems that do need to be mentioned in DQAs:
Oscillation in a channel
Spikes to Negative Numbers
The most common problem with the MFRSR is a misaligned shadowband. There are numerous reason why the band may be off. Below are two pictures of the shading band misaligned.
Obvious shading problem
If only all shading problems were as easy to spot as the following direct and diffuse examples from the same instrument on the same day. The data are from a clear day so the plots should look like the normal plots above.
Less obvious shading examples
More common than the obvious shading problem above, are subtle shading problems as shown below. In reality, however, to a trained eye the following examples are actually fairly obvious. Once a person is familiar with identifying shading issues, even those that truly are subtle can be readily identified. The examples below also show why it is important to view both the direct and diffuse plots. In these examples from the same day, the shading issue is much more prominent in the diffuse data than the direct.
Note that between the dashed lines the data across all channels are noisy in comparison to data outside the dashed lines. A single occurrence like this on one day without similar features on prior or subsequent does not mean there is a shading problem. Look for similar features on prior and subsequent days, and across the same hours, to confirm a shading problem.
An example of normal data that appears to be a shading problem
At times, the diffuse irradiance values may appear a bit noisy and assume a shape similar to what one may expect for a shading problem. For example, check out the plot from the E12 SGP MFRSR from October 2015. While the diffuse irradiance values are indeed noisy during the day, we don't see the same feature reciprocated in the direct normal irradiance data. Thus, the shadow band is likely still in the correct position and accordingly no DQPR was issued. When interrogating data in suspicion of a shading problem, it's always a good idea to consult the other plots, as well as other nearby sites on the same day.
Two Channels Overranging
Below are a few examples of other problems that arise. Generally though, if all the channels do not have the same characteristics as the rest, e.g., spikes or dips caused by clouds, there is likely a problem and it should be reported. Note that 940 (filter 6) can be an exception. This channel is sensitive to water vapor which can be highly variable over the course of a day, while the other channels are not affected by water vapor. Even so, it will never be radically different than the other channels.
Occasionally, a filter will go squirrely-squirrel. This means that the filter behaves quite anomalously compared to the same filter at surrounding sites AND other filters at the same site. Some problems include excessive noise, extended drop-offs to very low values, and frequent reports of -9999 values. Two examples are shown.