·
What is radar rainfall?
·
How is the radar rainfall different from rain gauge
measurements of rainfall?
·
What are the advantages and limitations of radar
rainfall?
·
What does this information mean and how is it
interpreted?
Q: What is radar rainfall?
A: The National Weather Service (NWS)
operates a network of NexRad Doppler radar sites. A radar site emits a beam of
energy that scans the area surrounding it. When the beam encounters rain, a
portion of the beam is directed back to the radar. The intensity of the
returned beam of energy is proportional to the intensity of the rainfall
encountered. The individual radar beams are combined into a “mosaic” that
creates a complete coverage of the radar energy reflected across northeast
Florida. The radar energy values are compared to rainfall measured at the St.
Johns River Water Management District’s network of rain gauges. Using
specialized computer algorithms, the reflected radar energy is adjusted based
on the rain gauge measurements, and the result is an estimate of the radar
rainfall over the District. [Top]
Q: How is the radar rainfall different
from rain gauge measurements of rainfall?
A: The radar rainfall mosaic is made up of
individual units called pixels. Each pixel is two kilometers by two kilometers,
or approximately 43 million square feet. If radar rainfall indicates one inch
of rain fell, it represents an estimate of the average rainfall over the whole
pixel (all 43 million square feet), which equals one inch. If a rain gauge
indicates one inch of rain fell, it represents one inch of rain was collected
in the rain gauge with an opening of only eight inches in diameter. Thus, radar
rainfall estimates over a wide area, while a rain gauge measures for a small,
specific point. [Top]
Q: What are the advantages and
limitations of Radar Rainfall?
A: Radar rainfall has complete coverage of
the whole St. Johns River Water Management District area. Also, radar rainfall
rarely ever has any missing observations. The radar can detect smaller storms
that may miss rain gauges within the network. However, even after careful
adjustment of the radar rainfall with rain gauge measurements, there can be
potential differences and anomalies. It is important to remember that radar
rainfall is just an estimate. However, the radar rainfall is an improved
estimate compared to only using the gauge rainfall.
[Top]
Q: What does this information mean and
how is it interpreted?
A: The radar rainfall amounts displayed on
the map are the daily total rainfall amounts in inches for the chosen date from
12 a.m. to 11:59 p.m., Eastern Standard Time (EST). Each pixel is shaded
according to a standardized set of categories, so the darker the shading, the
greater the rainfall total.
For more detailed information please visit the
National Weather Service’s radar web page [Top]
