Investigation 7B

DataStr= eme Atmosphere Current Weather Studies 7B:

DOPPLER RADAR

As described in the Wednesday, 21 October 2009, Daily Weather Summary, a significant wintry weather situation was developing on the plains east of t= he Rocky Mountains. Low-pressure had formed in southea= stern Colorado area an= d was expected to drift southeastward before heading to the northeast over the ne= xt couple of days. This slow formation process would bring abundant moisture i= nto the storm's circulation and was expected to produce prodigious precipitatio= n.

1= . Image 1 is the surface weather map for 12Z 21 OCT 2009. Precipitation as show= n by radar shadings [(did)(did= not)] cover the area of eastern Col= orado at map time.

2= . Pueblo, located in southeastern Colorado and indicated by a maroon (on-screen) dot,= was situated north of a stationary front that arched from Idaho to western Colo= rado to northern New Mexico and north of a cold front from there across northern Texas into Oklahoma. Based on the Denver w= ind direction, winds across eastern Colorado were probably generally from the [(north)(south)].

3= . At 12Z the temperature at Pueblo<= /st1:place> was 37 °F and the wind was reported as 16 knots gusting to 23 knots from the north (reported elsewhere). Place a wind arrow shaft coming into the Pueblo station do= t on the map showing this reported wind direction. Add feathers to indicate speed. T= his direction [(did)(did not<= /u>)] generally fit the inferred wind pattern across southeastern Colorado.

4= . Image 2 is the Pueblo= , CO National Weather Service Doppler ra= dar displays of the Base Reflectivity on the left and the Base (radial) Velocit= y on the right at 1226Z and 1222Z, respectively, on 21 October 2009, near the ti= me of the Image 1 surface map. The location of the radar site near Pueblo is denoted= by a black dot in the reflectivity display and a yellow dot in the velocity disp= lay. The reflectivity is related to the intensity of radar return signal. The reflectivity display pattern at Pueblo is shown keyed to the scale along the right side. The blue and green shadin= gs (on-screen) widely spread over the area indicates a return of radar energy to the receiver from generally light precipitation. = The yellow shading with a few orange dots indicates moderate precipitation. The precipitation at map and radar display times was reported as light rain.

Comparing the Pueblo reflectivity shading pat= tern in the left side of Image 2 to the region denoting precipitation by radar shadings on the Image 1 surface map showed that they [(generally agreed)(were widely displaced)] in maximum intens= ity and coverage location near the site.

5= . In the Doppler Base (radial) Velocity display on the right, negative numbers/g= reen hues indicate winds with radial components toward the radar and posi= tive numbers/red hues denotes radial components of the wind away from the radar sited at the yellow dot. The scale at the lower right shows direction= s and magnitudes of the radial velocity. The magenta "oval ring" on the periphery of the display area indicates where the radar cannot distinguish between wind motions toward or away from the site.

The green and red radial velocity shadings [(do)(do not)] generally cover most of t= he same areas where the reflectivity return shadings of the left view indicated precipitation near the radar site.

6= . In the Base Velocity view, the lighter shade boundary between reddish (on scre= en) and greenish shadings oriented generally west-northwest / east-southeast indicates 0 "Doppler wind speed". Draw a short straight line segm= ent of "best fit" through the yellow dot to show the position of 0 Doppler wind speed near the radar site. This "zero-speed" situati= on occurs when the radar beam direction is [(perpendicular)(parallel)] to the actual wind dire= ction (or there are calm conditions) and there is no wind motion component direct= ly towards or away from the radar.

7= . In the Base Velocity display, the main area of winds flowing toward the radar site (green shading) is located generally to the [(north= )(south)] of the radar site.

8= . In the Base Velocity display, the main area of winds flowing away from = the radar site (red shading) is located generally to the [(north)(south)] of the radar site.

9= . Draw a short (about 1 cm) arrow perpendicular to the 0 Doppler wind speed bounda= ry (drawn in Item 6) from green to red shadings through the yellow radar site = dot. Place an arrowhead on the red end to indicate the Doppler-detected wind direction at the station. The direction of your arrow signifying the wind direction in the lower layers of the atmosphere sensed by the radar signal = [(is)(is not)] generally along the same direction as the near-surface winds reported in Item 2.

1= 0. As the radar beam travels outward from the radar site, the beam curves downward le= ss than Earth's surface. Therefore, the beam is sampling altitudes at increasi= ng elevations with distance outward from the site and shading patterns give information on wind speeds and directions at higher and higher altitudes.
The 0-Doppler wind speed boundary across the central area displayed by the radar is essentially [(a straight li= ne)(an "S" curve indicating changing wind direction with height)]. The extremes of shape for this boundary can range from a straight line for consistent directions at all altitudes t= o an extremely bent curve for varying wind directions.

Doppler velocity depictions may be quite complex, especially during storm episodes, and require interpretation by trained radar meteorologists. While many TV stations claim they are providing "Doppler" radar information, the views they present are generally of reflectivity. The real Doppler velocity images would prove confusing to most viewers. You might practice calling up NWS sites and viewing both reflectivity and velocity displays where precipitation is occurring in your area. More information on Doppler radar and its imagery can be found at ht= tp://www.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htm.

We will examine additional radar views of air motions in a later investigation dealing with tornadoes. Such radar views can be found from the website Radar section with the NWS Radar Page link. Additional discussion of radar imagery interpretation can be found at: http://w= w2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/home.rxml.

After completing this week's applications, transmit the following work to y= our instructor:

Investigations 7A and 7B = Investigation Answer Form,

Investigation 7B page 7B-= 3.

Current Weather Studies activities 7A and 7B CWS Answer Form.

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