Radar interpretation of rain and snow

Radar Interpretation

What is a PRECIP Product? (Original content - prior to November, 2013)

The type of image you are viewing is indicated at the bottom of the right hand legend. The standard radar product Environment Canada presents on this website is called a "PRECIP" (PRECIPitation) product.

PRECIP Image

The PRECIP image is designed to show the precipitation close to the ground, by using Doppler technology processing for echoes within 128 km from the radar site. Doppler technology allows for better resolution of the precipitation echoes and also provides the ability to detect the movement of precipitation in relation to the radar (i.e. are the raindrops or snowflakes moving towards or away from the radar and at what speed). Beyond this limit, the echoes are displayed using the more conventional CAPPI processing which is explained below.

Three factors distinguish the PRECIP product:

1. Weather radars can receive false echoes from ground objects such as buildings and towers; the PRECIP product uses Doppler processing to edit out most of these false echoes. Doppler processing can detect these false echoes because they are not moving in relation to the radar as raindrops and snowflakes would. So with the clutter caused by ground objects filtered out, the PRECIP product provides a cleaner image close to the radar.
2. Trees and hills around radars can block some or the entire radar beam, resulting in a decreased ability to detect precipitation close to the ground near the radar. This problem can be particularly evident during the winter months when a lot of the precipitation forms and falls close to the ground. With some or most of the radar beam being blocked in the very low levels during the winter, the radar may not do a good job of detecting light precipitation near the radar.
3. Doppler processing does not do as good a job, in general, at detecting precipitation as conventional radar processing. Since the PRECIP product is a combination of Doppler processing (first 110 km from radar) and conventional processing (beyond 110 km from the radar), there can sometimes appear to be a sudden discontinuity or increase in the intensity of precipitation as one goes beyond 110 km from the radar.

What is the difference between PRECIP-Rain and PRECIP-Snow images?

In general, rain is more easily detected by radar than snow. In order to better represent the precipitation during the warm season, a certain relationship is used to relate the reflectivity to a rainfall rate (in mm/h - PRECIP-Rain). During the cold season, a different relationship is used to relate the reflectivity to a snowfall rate (in cm/h - PRECIP-Snow). However, it should be noted that just because the reflectivity is being represented in mm/h doesn't mean that the precipitation shown on the radar is rain or if the reflectivity is being represented in cm/h that the precipitation shown on the radar is snow. The precipitation in most of the more densely populated areas of the country can still show a high degree of variability throughout a good portion of the fall, winter and spring allowing for rain, freezing rain, ice pellets and snow to occur alone or in some messy combination. Therefore, it is always important to relate the precipitation you are seeing on radar with what the current weather conditions are and what the forecast is in the coming hours to get the best sense of what may fall out of the sky in your area.

 

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