Snow Donut

An area of snow moved into central Oklahoma Friday morning from the southwest creating a donut-like appearance on radar.


No, the precipitation was not side-stepping its way around the KTLX radar, but instead this illustrates two things:

1) trajectory of a radar beam from the radar, and

2) physical process of moistening the atmosphere

The donut hole is created because the radar display is actually showing precipitation at different vertical levels of the atmosphere relative to the ground.  A radar beam will travel upward through the atmosphere due to the tilt of the radar and, primarily, because of the curvature of the earth.  The radar beam is closest to the ground at the location of the radar. But once a radar beam emanating from KTLX has traveled to the Red River, it is 10,000 feet off the ground or more.  Thus, radar echoes along the Red River represent what is occurring thousands of feet into the atmosphere.  The yellow circle in the image below shows the circular path around the KTLX radar where the beam has traveled to 1800 feet above ground level. Because the edge of the snowfall resides at this distance from the radar means that snow was falling to a level of about 1800 feet before evaporating and thus not reaching the ground in Norman or the OKC metro area, yet.


At that time (10:08am), the temperature in central Oklahoma was around 18 F degrees, but the dewpoint was 3F degrees giving a relative humidity of 50%.  As the snow initially reached central Oklahoma aloft and began to fall to the ground, the column of air from where the snow originates down to the surface would begin to moisten.  This time corresponds to the line labeled “1” in the Norman, OK meteogram below. Once snow makes it all the way to the surface nearly 90 minutes later, the surface temperature and dewpoint profiles would begin to change.  This is illustrated in the meteogram by looking at the red line (plot of surface temperature) and green line (plot of the dewpoint).  The dewpoint began to increase around 11:30am (line labeled “2”) from 3 F to eventually 13 F, increasing the relative humidity from 50% to 85%.  The dewpoint depression, or the difference between temperature and dewpoint (the separation between the red and green lines), decreases as the surface layer begins to saturate.  Evaporation of the precipitation produced a slight cooling at the surface of about 2 degrees over the same time period.


By 11:37am, snow had reached the surface in Norman at roughly the time labeled as “2” in the meteogram.


This same process continues as the region of precipitation propagated northeastward and the atmosphere is moistened starting from the cloud level and then downward to the surface. The snow donut hole completely filled in by about 12:30pm as snow finally reached the surface in the northeastern sections of the Oklahoma City metro area.


– Chris Porter


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