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Current Weather Studies 7

PRECIPITATION PATTERNS

& DOPPLER RADAR

Reference: Chapter 7 in the Weather Studies textbook. Complete the introductory and other

appropriate sections of Investigations in the Weather Studies Investigations Manual as

directed by your mentor or instructor. Check for additional Weekly Weather News updates

during the week.

Precipitation generally forms from humid air being lifted to cause adiabatic cooling of the air

leading to condensation and cloud formation. Continuing cloud growth can produce water

droplets and/or ice particles heavy enough to fall, reaching the surface as precipitation.

In mid-March 2019, the central U.S. was impacted by a swiftly developing low-pressure

system...one that flirted with some of all-time lowest pressure values for the states of

Colorado, New Mexico and Kansas. This low-pressure caused severe weather on its

southern flank and blizzard conditions on its northern and western flanks. Flooding was a

concern as well, for those states to the east of its progression (e.g., Nebraska). In the eastern

U.S., chilly air had plunged south from Canada, but given the strength of the high-pressure

that accompanied the chill, clear skies (with generally subsiding air from above) dominated.

This week’s study will focus mostly on the pre-conditions for the continental low-pressure

system that developed in the central U.S. and its accompanying fronts, which aided in lifting

of air. Lifting was also enhanced by converging surface winds and buoyant, moist air. In

this study, you will find that the origin of the intense cyclone; one in the Southwest and one

in the Pacific Northwest. Essentially, over time, the two systems (mostly due to dynamic

processes aloft) merged into a gigantic low-pressure in Colorado and Kansas. This study sets

the stage for that behemoth.

Additionally, as the astronomical winter season transitions to spring, the strength of these

low-pressure systems will gradually decrease as temperature contrasts between lower and

higher latitudes intensifies.

Figure 1A is the weather map from early morning Tuesday, 06Z 12 MAR 2019. At this map

time, a broad, weak, and disorganized area of low-pressure was roughly centered in the

Southwestern states of Arizona and New Mexico, with a central pressure of about 1008

millibars. Multiple fronts meandered around the low-pressure into Texas to the east and

California to the west. An expansive anticyclone (high-pressure) dominated the east coast,

providing evidence this later system produced cool, dry conditions.

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Figure 1B is a weather map of 12Z 12 MAR 2019, which is presented to show the 6-hour

time progression of the pressure systems and the attendant changes in weather. The low-

pressure in the Southwest is, at map time, slightly better organized with a nearly closed

inner-isobar, and more widespread precipitation in Arizona of this latter figure (1B).

Figure 1A. - Analyzed NCEP weather map with Isobars, Radar & Data for 06Z 12

March 2019. Fronts were analyzed four hours prior to other data.

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Figure 1B. - Analyzed NCEP weather map with Isobars, Radar & Data for 12Z 12

March 2019. Fronts were analyzed three hours prior to other data.

1. Comparing the two figures (1A and 1B), the cold front with associated precipitation in

Washington and Oregon in Figure 1A, came further on shore in Figure 1B as the low-

pressure center generally advanced toward the ________. This movement is evidenced

by the decrease in sea-level pressure reports of the stations in both Washington and

Oregon from Figure 1A to Figure 1B. The isobar orientation has also changed due to the

change in observations.

a. east

b. west

c. north

2. The advancing front(s) ________ have provided a lifting mechanism for the air, leading

to clouds and precipitation.

a. would

b. would not

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3. There is ________ evidence in Figure 1B of radar-indicated rainfall surrounding

Spokane, WA, compared to Figure 1A for the same location. Your answers to items 1 and

2 above support this statement.

a. more

b. less

4. The air mass on the southwestern flank of the Ohio Valley anticyclone (high pressure)

and ahead of the advancing frontal boundaries in the southwest U.S. in Figure 1B was

experiencing seasonable temperatures for this time of year. In fact, temperatures in north

and central Texas (to the north of the stationary front) were generally reported in the

________. This air mass was distinct from the air mass to the south of the stationary

front, as temperature to the south (in both south Texas & south Florida), were much

warmer with higher dewpoint values.

a. 40s to 60s (°F)

b. 50s to 70s (°F)

c. 60s to 80s (°F)

d. 70s to 90s (°F)

5. As the low pressure in the Southwest became more organized and moved toward the east,

station models ahead of this low-pressure reported a change in sea-level pressure as well.

It can be noted by examining sea-level pressure in Figure 1A, compared to the same

stations (e.g., Oklahoma City, OK) in Figure 1B. In Figure 1A, Oklahoma City had a

pressure of 1023.2 mb but the pressure decreased in Fig 1B by ________ .

a. 0.1 mb

b. 1.4 mb

c. 3.4 mb

d. 8.3 mb

6. By the time of Figure 1B, six hours after Figure 1A, saturated conditions closer to the

surface moved eastward toward Oklahoma City as well. This fact is supported by the

dew point changes as indicated on the representative station model. Figure 1B indicated

that the observed dewpoint in Oklahoma City had increased to ________ at 12Z 12 MAR

from 49°F at 06Z.

a. 46°F

b. 48°F

c. 50°F

d. 52°F

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7. Given the saturated conditions, particularly in Oklahoma City, what was the reported

present weather conditions at 12Z on Figure 1B, indicated by two green lines?

a. snow

b. rain

c. fog

d. ice pellets

8. Many of the reporting sites in that state of Texas in both Figures 1A and 1B were

reporting significant cloud cover, due to the increasing moisture content from the south

and west. In fact, the only Texas station to neither report overcast conditions or a sky

obstruction in either Figure 1A or 1A, is ________ located in the far ________ portion

region of the state.

a. Dallas; southern

b. Amarillo; western

c. Lubbock; northern

d. Brownsville; southern

9. Radar shadings in Figure 1B indicate where the NWS network of weather radars was

detecting precipitation. The intensities of the echoes, according to the scale at the left

edge of the map, were related to precipitation rates. From the map display in Figure 1B,

precipitation on the map was broadly located ________.

a. in a broad area of Arizona mostly enclosed by a series of frontal boundaries roughly near a

low pressure system along the New Mexico-Arizona border

b. in the Pacific Northwest ahead of an approaching cold front

c. in both of these locations

Next, we turn our attention to a specific radar site in the Southern Plains instead of the full,

composite, U.S. view. We’ll focus on Kansas, which is out ahead of the frontal boundaries

described to this point in the study. Individual radar sites can provide greater detail on the

intensity and positioning of precipitation.

Figure 2 is a water vapor image acquired at 1202Z 12 MAR 2019, about 2 minutes after the

Figure 1B surface weather map. The animation link (provided in the caption) begins about

the same time as the Figure 1A map time and ends at about three hours after the Figure 1B

map time. The brighter, blue-to-white pixels in the image indicate where there is a greater

concentration of moisture content in the lower troposphere (but above the surface). Some of

the brightest white pixels are shaded green, indicating an even greater concentration of low-

level moisture. Clouds can be generally assumed to exist where brightest white pixels are

observed. Yellow and orange shading indicate a general lack of low-level moisture in the

troposphere.

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Figure 2. Remotely sensed lower tropospheric moisture concentration, as

identified from NCEP satellites (GOES-16) 1202Z 12 March 2019. To

visualize the dynamic nature of the atmosphere and acquire a greater

perspective on moisture trajectory, please visit this link (this may be a

downloadable file, pending which browser you use). The link provides

a visual understanding of how the atmosphere evolved in the hours

between Figure 1A and 1B.

Animation provided by: College of DuPage Meteorology – NEXLAB.

10. Lumpy bright blue-white (some green) shadings in Figure 2 (and animation) generally

________ match the orientation and progression of the fronts from their surface map

symbols (Figure 1B).

a. did

b. did not

11. There was an extensive area of blue-white-green shadings from southern New Mexico,

west Texas, and Oklahoma into Kansas in Figure 2 that ________ generally coincident

with the Figure 1Bb fronts along the southern border of the U.S. as low pressure and

southerly air ascended along the boundary, lifted and saturated.

a. was

b. was not

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12. On the eastern flank of the broad high-pressure area in Figure 1B, wispy arcs of yellow

shading is apparent in Figure 2, which extends along the U.S. East Coast into the Mid-

Atlantic region from Delaware to South Carolina. Using these geographic references in

Figure 1B and Figure 2, the station models in Figure 1B (states along the Atlantic coast)

indicate that surface stations in these states ________ observing generally clear skies.

a. were not

b. were

13. The blue-white-green satellite pixel shadings in Figure 2, which were moving into Texas

and Oklahoma as a slightly curved line, ________ approximately coincide with the

Figure 1B surface map’s display of precipitation as shown by its radar shadings.

a. did

b. did not

Today, many people have the ability to view radar imagery on their mobile devices. Some

mobile apps have become quite robust, with the more complex ones incurring a subscription

cost. One popular mobile radar app among amateur and professional meteorologists is

RadarScope. The following radar images (Figures 3A & 3B), while captured with desktop-

grade software, are also available in the RadarScope app on your mobile device. One

advantage of these products, relative to some local NWS-office products commonly found on

the web, is the “super-resolution” (or finer detail) provided by these images. The additional

detail viewed with super-resolution aids in severe storm analysis, as well as detecting other

smaller weather features.

Figure 3A is a zoomed-in perspective of Kansas (with bits of a few surrounding states) and

one of the National Weather Service (NWS) Doppler radar sites near Wichita, KS (KICT),

displaying a Reflectivity product at 1159Z 12 MAR 2019, about the same time as Figure 1B.

This image serves to provide the reader with a better geographic perspective, relative to the

Figure 1A and 1B map observations. The state borders of Kansas, Oklahoma & Missouri are

outlined in white, with numerous cities and small towns identified on the radar image.

Noted, with a small blue box, the center of the Wichita radar is identified as the middle of the

multi-colored radar reflectivity observations. No other radar observations are present on this

image, only the Wichita radar data]. This is why the periphery of the figure displays more

black background color of the base map. In contrast, in Figure 1A and 1B, all individual

radar images created from U.S. based radar sites are “stitched” together to form a composite

image, showing all of the radars' data on a single map. Thus, the Radarscope image is much

more detailed to a specific radar site.

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Figure 3A. Base (super-resolution) reflectivity radar display from 1159Z 12

MAR 2019. Graphical product courtesy of RadarScope.

14. The reflectivity as seen in Figure 3A is related to the intensity of the radar return signal,

and hence the precipitation rates, according to the scale along the lower margin of the

reflectivity display. The elongated, narrow band of orange-amber shadings north, east

and west of the KICT-radar site were returns of moderate-to-heavy rates of precipitation,

embedded in the area of otherwise generally scattered precipitation (denoted mostly by

green and yellow returns). There ________ as many moderate-to-heavy reflectivity

shadings northwest of the radar site.

a. were

b. were not

The reflected energy from rain, snow, and other particles in the atmosphere can also be used

to determine particle motions. Figure 3B displays the Doppler Base (Radial) Velocity

display for the Wichita, Kansas radar site at 1159Z 12 MAR 2019, and can be interpreted

using the color scale to its lower margin. With an interrogation tool, the pixel values with

negative numbers, or green hues, indicate winds (with imbedded particles) with radial

components toward the radar site. Positive numbers, or red hues, denote radial components

of the wind away from the radar. In other words, green = toward the radar; red = away.

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Again, the center of the radar site is marked on this figure with a blue box and station

identifier noted within that box (KICT – Wichita). The continuous white boundary line of the

Oklahoma-Kansas border is located in the lower one-fifth of the image running east-west &

Kansas-Missouri border runs, north-south through the eastern one-eighth of Figure 3A.

North is considered to the top on all map views.

Figure 3B. Base (radial) super-resolution velocity radar display from 1159Z

12 MAR 2019 for KICT (Wichita, Kansas). Graphical product

courtesy of RadarScope.

The radial velocity color scale further shows magnitudes of the radial velocity in knots. The

purple shade that sometimes appears beyond or imbedded in the red and green shades

represents locations where the radar cannot distinguish whether the motions are uniquely

toward or away from the radar. In fact, in Figure 3B, there is a sizeable ring of purple

shading at the far edge of the figure, but that is only because some of those values lie beyond

the operational range of the radar. The purple pixels are in a bit of a “ring” shape, since the

radar beam is transmitted 360° around a single point.

As the radar beam pulses travel outward from the radar site, the beam curves downward due

to refraction, but with less curvature than the underlying Earth’s surface. Therefore, the

beam is sampling air at increasing altitudes as distance away from the radar site increases in

any direction. Shading patterns therefore give information on wind speeds and directions at

higher and higher altitudes as distance from the radar increases.

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15. The green and red radial velocity shadings, and the purple areas, in Figure 3B ________

generally cover the same areas shaded in the reflectivity return display depicted in

Figures 3A.

a. do

b. do not

In the Velocity view (Figure 3B), the gray boundary curve between light reddish-gray and

light greenish-gray shadings, oriented generally northwest and east-southeast through the

radar site, indicates 0 radial “Doppler wind speed.” What this means is that the movement of

precipitation in this region is neither moving toward the radar or away from it. Essentially,

it’s a tangential motion to the radar site.

16. In Figure 3B, the area of greatest radial wind components flowing away from the radar

site (brighter red shading) is located generally to the ________ of the radar site.

a. south-southwest

b. north-northeast

17. In Figure 3Bb, the area of radial wind components flowing toward the radar site (brighter

green shading) is located generally to the ________ of the radar site.

a. south-southwest

b. north-northeast

18. Imagine a straight line about 3 cm in length along the gray 0-Doppler wind speed

boundary, centered on the radar site. This “zero-speed” situation occurs where the radar

beam is generally ________ to the actual wind flow.

a. parallel

b. perpendicular

19. Imagine or draw another straight line perpendicular to your 0-Doppler wind speed

boundary line through the Wichita radar site. Imagine or draw an arrowhead on the end of

the line just imagined or drew in the red area 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 as sensed by the radar signal, is generally from the

________.

a. northeast

b. northwest

c. southeast

d. southwest

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20. Looking back at Figure 1A, the station model for Wichita, KS (the closest station plot to

the KICT radar) showed the plotted surface wind direction as generally coming from the

________.

a. northwest

b. east

c. southeast

21. The surface wind direction from the Wichita station model (Figure 1A), was ________

the arrow you drew or imagined on the radial Velocity display of Figure 3B, indicating

the general low-level wind direction changed slightly with heigh. It is also worth

reminding the reader at this point that Figure 3B represents precipitation above the

ground (with increasing distance from the ground the further away from the radar site).

Winds are often changing direction with height. So, surface winds (Figure 1) are not

always consistent with radar imagery.

a. the same direction as

b. in the opposite direction from

c. approximately 90° different than

Sometimes, the movement of precipitation, in particular item #21, can be better explained by

the wind flow a few thousand meters off the ground (Figure 4), rather than surface wind

motions. It’s important to point out that differences in wind flow occur with height.

Examine the wind barbs in Figure 4 in the state of Kansas. They more closely follow the

height contour lines (in blue). Winds on this figure represent flow at about 3,000 meters

high, and have much less frictional interaction with the surface. Additionally, an animated

version of Figure 3A can be found here, helping to identify the changing precipitation

patterns (video may be slow to load).

22. The wind flow represented in Figure 4 is ________ the movement of precipitation

represented in Figure 3A compared to Figure 1B.

a. much more similar to

b. very different from

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Figure 4. - Analyzed NCEP weather map with height contours (isohypses),

isotherms & station data for 12Z 12 March 2019. 700 millibars of

pressure is typically around 3000 meters in height (or approaching

10,000 ft.)

Doppler velocity depictions may be quite complex, especially during storm episodes, and

require careful interpretation by trained radar meteorologists. While many TV stations claim

they are providing “Doppler” radar information, the views they present are generally of

reflectivity (similar to Figure 3A and 3B); the real Doppler velocity images would prove

confusing to most viewers. Additionally, light-colored reflectivity values in “clear air”

situations can even come from dust, insects, and temperature gradients that provide wind

information in non-precipitation cases would not ordinarily be shown on local TV

broadcasts, as they can be misleading.

You might practice viewing NWS sites with radar and viewing both reflectivity and velocity

displays when precipitation is occurring in an area. More information on Doppler radar and

its imagery can be found at https://www.weather.gov/jetstream/doppler_intro.

We will examine additional radar views of air motion in a later investigation dealing with

tornadoes. Such radar views can be found from the course website, Weather Studies Maps &

Links, Radar section with the NWS Radar Page link. Additional discussion of radar

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imagery interpretation can be found at:

http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/home.rxml.

To recap, this Current Weather Studies showed some of the ingredients for precipitation,

namely sufficient water vapor in the air to form clouds and precipitation, advancing frontal

systems for lifting the air, and converging winds into a central low-pressure system. The

synergistic effects of these ingredients lead to a better understanding of: 1) the location of

where precipitation occurs; and 2) its general intensity. A later Current Weather Study will

focus on thunderstorm formation and mechanisms of intense precipitation formation.

©Copyright 2019, American Meteorological Society