Introduction
The Lews Castle College Metek Micro Rain Radar (MRR) provides start-of-the-art information on the precipitation affecting Stornoway. It is able to perform the following functions (see below for an example image and an explanation of the colours):
- Detects imminent precipitation (rain, snow, sleet, hail)
- Detects imminent ceasing of precipitation
- Detects rate (heaviness) of precipitation (through the bottom 3km depth of atmosphere)
- Detects direction and speed of movement of precipitation (through use of Doppler facility)
- Determines height of freezing layer (level at which snow turns to rain). Therefore, it can warn of imminent snowfall and freezing rain.
- Can determine the type of precipitation e.g. hail, snow, sleet or rain
Radar updates can be seen on Twitter (updated every hour) at: https://twitter.com/lewscastleradar - please follow!
Example Radar Plot, 13 June 2016
Explanation of images (above)
Usually, one of the above four plots are available for real-time viewing. They all show height (vertical left axis) against time (horizontal axis, with the most recent time at the far right). The colours and shading are explained as follows:
| Raw reflectivity values (measured in dBZ): Anything above 35 dBZ is heavy precipitation. | Fall velocity (speed of descent of rain/snow): The largest raindrops fall at 6-8m/sec. Can you spot the 'bright band'? |
|
Rainfall rate (mm/hr): Anything above 0.2mm/hr will wet the ground, above 1mm/hr will make puddles, 10mm/hr is a downpour. |
Doppler Velocity (m/sec): Essentially the same as fall velocity, in a non-moving frame of reference (relative to the radar). |
For more information, please contact Eddie Graham (@eddy_weather). With thanks to Graham Florence (ITLE).
The Science:
The Lews Castle radar offers a temporal and spatial resolution of weather systems (and their precipitation) at least one order of magnitude (or more) greater than conventional observational systems.
The radar operates at a frequency of 24GHz, at a vertical resolution of 30m x 100 steps, giving a maximum height penetration of up to 3100m above the surface. The radar sends a vertical pulse of radiation, which is back-scattered (reflected) by any hydrometeors (snow, hail, sleet, rain) present in the atmosphere. The radar data are averaged over time bins of 1 minute in duration.
As the wavelength of the radiation (approx. 1.3cm) is larger than the droplet size, then the amount of back scatter of the radiation is proportional to the 6th power of the droplet radius Thus, large raindrops or hailstones reflect significantly more radiation than do drizzle or small ice crystals. Sleet or melting snow (snowflakes covered with a thin layer of water) reflect anomalously strongly, leading to a phenomena known as the 'bright band' (see below).
The enhanced reflectivity of the bright band approximates to the height of the freezing level in the atmosphere. Over Stornoway during summertime, the height of this level ranges from perhaps ~900m to ~2400m, and from the surface to ~1500m in winter.
Further reading / resources:
Fukao, S., Hamazu, K. and Doviak, R.J. (2014). Radar for meteorological and atmospheric observations. Springer.
Rutgers University (2016). Remote Sensing of the Atmosphere and Ocean [online] [13 June 2016]