Radar Detection of Boundaries

[Mike Johnston]

This is more of a question/observation.

I have noticed this evening that a distinct seabreeze boundary is very well documented out of the KCLX (Charleston, SC) radar at the lowest tilt. Even though the boundary is approximately halfway to KCAE (Columbia, SC) radar site, it isn't picked up at all on the KCAE radar.

Question: is there something inherent in radar technology that would pick up "upstream" boundaries like this but less likely to pick up "downstream" boundaries? I'm not at all an expert in radar interpretation, but there must be some explanation as to why such an obvious boundary would be detected quite clearly by one site, but not at all by another, even though the boundary is equidistant between the two points.

TIA for any insight.

 

[Jeff Snyder]

Mike,

If you are using GR, change it to a color scale that extends to a lower value than the default color scale. I just pulled up KCAE at this time (740 pm EDT), and the boundary does show up, but it's around 0-6 dBZe. I think the default color scale for GR3 starts at 10 dBZ, which means you'll miss that feature.

Anything that affects the height of the beam as it propagates to the target (e.g. this boundary) is likely to affect how that target is sampled. On the most basic level, the two radars are located at different altitudes/elevations above sea level, so the starting height of each beam is different even for the same elevation angle and range. The temperature and moisture characteristics of the atmospheric along the propagation path of the beam can also significantly affect beam height. Depending upon how temperature and moisture change with height, the beam may experience sub- or super-refraction. If the temperature increases with height (i.e. there is an inversion in place) and moisture rapidly decreases with height, the refractivity may change such that the beam experiences superrefraction, resulting in the beam being "bent" downward as it propagates through the atmosphere. In such a case, the radar beam is going to be lower than it otherwise would. If the opposite is true (e.g. dry adiabatic lapse rates and moisture that increases with height), the radar beam may experience sub-refraction, which results in the beam height being greater than anticipated. The fact that refractivity is a function of the temperature and moisture profiles is the reason why you tend to see much greater anomalous propagation (AP, e.g. ground clutter) at night when the nocturnal inversion sets up, and why you can see significant AP after the passage of a thunderstorm's outflow / gust front in a significant cold pool. In both of these cases (nocturnal inversion and convective cold pool), you can see thermo and moisture profiles change significantly such that superrefraction occurs.

In this particular case, only taking a cursory glance at sfc obs, the air south of the sea breeze is a little cooler and more moist than north and west of the sea breeze. So, assuming conditions 1-2 km above the surface aren't terribly different across the sea breeze, the refractivity, and thus propagation tendencies of the beam, will be different ahead of and behind the boundary. Specifically, it seems entirely possible that the beam behind the boundary (e.g. coming from KCLX) will tend to see greater refraction (and thus bend downward more appreciably with distance) than a beam propagating ahead of the boundary (e.g. coming from KCAE), in this case.

If we look at the estimate beam heights on GR3, you'll notice that the boundary is only visible up to ~4300 ft from KCAE, but it's seen from KCLX all the way up to 5800 ft (all of these are above radar level and do not take into account elevation changes). In reality, it's likely that the beam coming from KCAE is higher than that reported value, and/or the beam from KCLX is lower than that reported value. This likely explains WHY the two radars "see" that boundary so differently, even if said boundary is nearly equidistant from each radar -- they aren't actually sampling the same volume of air at the same height within that sea breeze "frontal" zone.

 

[Mike Johnston]

OK, thanks so much for your thorough explanation, Jeff.

I'll probably have to read over it 4 or 5 times to fully understand, but that's a great explanation.

The only reason I noticed is that a bit of a synoptic disturbance looks like it is coming down from the northwest, and we've been without rain for a number of weeks here, so I thought I would look upstream a bit. The sea breeze boundary is almost a a daily occurence here, and visible on radar by about noon - sometimes serves as a localized surface convervence, but mainly not.

Definitely moister and cooler here in the low country on a typical day, than in the midlands and uplands of SC, so thanks for the clarification on how that might effect the reflectivities. Usually, the biggest question with the sea breeze boundary is how far it will propogate on shore. Some days, only 20 miles or so. Other days, it can come inland 70 or 80 miles.

 

[Keith Wilson]

This time of year we have the daily seabreeze here as well. It is interesting to see it so clearly on radar even though there may not be a cloud in the sky.

 

[Dave Gallaher]

Thank you, Jeff--that explained a couple things I'd long wondered about concerning ground clutter at night and following the outflow.