Conventional doppler radar (left) transmits only a horizontal beam. Dual-polarization radar (right) transmits beams in both the vertical and horitzontal directions. (NOAA)

The existing doppler radar in Sterling, known as WSR-88D (Weather Survelliance Radar, 1988 Doppler), was placed into operation on June 12, 1992. It sends out horizontal pulses to detect precipitation in the air. The radar also senses whether winds are moving towards or away from the radar site, which is critical for estimating thunderstorm wind speeds and detecting possible tornadoes.

But the dual-polarization (dual-pol) upgrade, to be completed by February 24, will transmit radar pulses in both the horizontal and vertical direction, providing significantly more detailed information about the structure and consistency of precipitation and clouds.

NOAA’s National Severe Storms Laboratory (NSSL) notes the following benefits of dual-pol radar (also known as polarimetric radar):

* It can significantly improve the accuracy of the estimates of amounts of precipitation

* It can tell the difference between very heavy rain and hail, which will improve flash flood watches and warnings

* It can identify types of precipitation in winter weather forecasts, improving forecasts of liquid water equivalent or snow depth

* Its data are more accurate than conventional radar, saving the forecasters the step of having to verify radar data

From NOAA: An amusing, educational overview: What is dual polarization technology? Why should you care?

These enhancements should improve forecasters’ confidence in precipitation forecasting contributing to increased lead time in flash flood and winter weather hazard warnings NSSL says.

The technology also provides improved information about non-weather matter in the air like insects, birds, tornado debris and ground clutter.

As Sterling’s WSR-88D radar is upgraded, it will be out of commission. The NWS will rely on radar from surrounding sites (Wakefield, Va., Dover, De., Roanoke, Va., Charleston, Wv., Pittsburgh, Pa., State College, Pa., and Mt. Holly, Nj.) to fill the gap.

The neighboring radar sites in Pittsburgh, and Mt. Holly, Nj. have already been upgraded to dual-pol. The installation in Wakefield, Va. should be complete this week.

All 160 NWS doppler radars will have dual-pol by May 2013 at a cost of $50 million. 109 NWS sites are scheduled for the upgrade in 2012. NOAA estimates the dual-pol upgrade will bring $690 million in annual economic benefits from improved forecasts.

Related: U.S. weather radar network gets upgrade | NWS upgrades to dual-polarization radar for better look at precipitation, tornadoes | Dual-pol overview from NWS | Dual-pol training