¶ Present Weather Detector (PWD)
¶ Overview
The Present Weather Detector (PWD) is an intelligent, multi-variable sensor for automatic weather observing systems (PWD User Guide, Attached). The PWD can measure the intensity and amount of both liquid and solid precipitation. A schematic of the PWD is shown in Fig 1. The PWD uses optical data along with data from the Vaisala Rain Sensor and the Temperature sensor to determine the precipitation intensity and type.
¶ Diagnosing Problems
The optical sensor will be the primary source of possible problems. Here are some possible flag patterns to help diagnose problems.
| Variable | Pattern | Problem |
|---|---|---|
| pwd_err_code | PWD data is missing and there are failing error codes | Lens cleaning issue or change with optics |
| lens blockage by snow, or dust | ||
| major hardware problem | ||
| pwd_* | all are missing including the error codes | communications or power issue |
| pwd_err_code | At NSA (high latitudes when sun does not set) PWD has many alarms | Sun is not setting and may be shining directly into options. No current problem solution. |
¶ Measurements
Visibility
The PWD detects 1 minute and 10 minute mean visibilities, Fig. 2. These averages are calculated from the extinction coefficient values.
Present Weather
The present weather codes are calculated from using all the PWD sensors at instantaneous (15s), 15 min, and 1 hr average intervals. Dust buildup or other contamination on the sensors can cause these to provide false information. Use the temperature, humidity, and KAZR data on these plots to help determine if the instrument is correct. Instances where the PWD is incorrect, will require a DQPR.
Precipitation
The precipitation measurements consist of a few different variables.
- 1 Minute Average Precip Rate
- Cumulative Liquid Precipitation
- Cumulative Snow
The precipitation rate measured from this instrument has not been in good agreement with the MET tipping Bucket Rain Gauge. Use caution when comparing to other instruments as this is a known problem.