The Distromet disdrometer measures the character and amount of liquid precipitation. The main purpose of the disdrometer is to measure drop size distribution, which it does over 20 size classes from 0.3mm to 5.4mm. The data can be used to determine rain rate. The disdrometer results can also be used to infer several properties including drop number density, radar reflectivity, liquid water content, and energy flux. Data are collected once a minute. Rain that falls on the disdrometer sensor moves a plunger on a vertical axis. The disdrometer transforms the plunger motion into electrical impulses whose strength is proportional to drop diameter. Precipitation amounts measured by the rain gauge are reported once a minute with an uncertainty of 0.001mm.
The disdrometer needs a level firm base and a quiet environment because acoustic noise can be detected by the sensor. Strong winds which produce turbulence at the edges of the sensor, are a source of error as well. The sensor can not correctly operate when flooded, and the top of the sensor needs to be free of snow. For more information see Disdrometer.
The 4 main variables being monitored are
The data should be 100% good. If metrics are showing Bad or Indeterminate, investigation is needed.
This plot shows Rain Rate (top), Max Diameter, and Energy Flux. Each plot has an flag overlay when wind speed measured by the local MET stations is greater than 5 m/s.
This is a 2D plot with axes of Time (x) and Drop Diameter (y). The coloring of the plot indicates the number of drops for each Diameter Bin at each time. If there's no rain, then the plot will be blank.
Full Precipitation Instrument Comparison
A comparison plot showing all the precipitation instrument daily accumulations. A plot of the KAZR reflectivity is inlaid at the bottom of the plot to help determine when it's raining. At the bottom of the plot are some statistics for each instrument. It includes the period total, percent of the local MET data (TBRG most of the time), mm removed by a threshold, and the maximum precipitation rate.
The disdrometer is not intended to be a device to measure rain accumulation, but rather a method of checking the calibration of the co-located KAZR. The reason why the disdrometer is not intended for measuring rain accumulation has to do with the "dead time" problem where the instrument is unable to correctly detect smaller rain drops simultaneously with larger rain drops. Therefore when the rain drops are integrated the end summation will be less than a co-located rain measuring device (up to 10mm in a 50mm event!). There are some corrections that can be applied to the data to account for the underestimation, but those corrections will need to be added via a VAP. However under certain raining conditions the disdrometer can be compared with a co-located rain instrument to verify that it is operating correctly. According to the instrument manufacturer these conditions are:
Behaviors that may not need to be mentioned in DQAs are mentioned below:
Noise from Wind
Wind causes the sensor to vibrate, resulting in the detection of false max drop diameters. These drop diameters are usually ~0.4mm. Since they are wind dependent, the number seen per day has a wide range. On calm days there may be as little as ~5, but on windy days there may easily be 100+. The example below resulted from a windy day. This issue is a known problem and should only be mentioned in extreme cases that persist for an extended period of time.
Drop Size Discussion
The drop sizes are recorded by binning the data into 20 bins. The bin sizes are not equal. This is mostly not going to be an issue, but is worth understanding if you plan to use the data. This plot tries to show how the bin size changes. The black line is "Diameter of drop size class", mean_diam_drop_class (value of the center of the bin) and the blue line is "Diameter interval between drop size classes", delta_diam (or bin size). Notice how the mean diameter of the bin does not follow a straight line when plotted against bin number.
Daily Manual Check
Approximately once a day, the site technicians will test the instrument by physically pushing a button on the instrument. This has the side effect of adding values to the #7 bin even if there is no rain! Currently ARM does not filter this out of the data file so we need to understand and expect this. The technicians have been instructed to not test the instrument during a rain event.
Noise on a Clear Day
This does not happen to often, but can at times. The instrument will record some slight noise that may not be attributed to wind.
Problems that do need to be mentioned in DQAs are mentioned below:
Precipitation accumulation flatlined
The comparison plots are a great resource for determining problems with the disdrometer. In this case, all the other instruments are recording precipitation, but the disdrometer is flatlined. A DQPR should be filed!