Freezing Level Question

[Brandon Schmidt]

I often notice this at the end of the life of a storm. Anvil blowoff travels along and something (if possible, I'd love to know what) causes the upper levels of the anvil...low reflectivity...to fallout and increase in reflectivity just below the freezing level. Is this due to the fact that ice in the upper layers of the anvil begin to melt at the freezing level, gaining a liquid coating, and remains a liquid below this level resulting in the attached cross-section? Also, what is the significance of the change in ZDR at the freezing level. I'm relatively new analyzing data with dual pol but I love to learn. Thanks - Brandon

 

[Jeff Duda]

I get an error when trying to view your attachments. Try re-attaching them.

Throughout the life of a thunderstorm, cloud water and ice will be streaming upward in the updraft. Some of that leaves through the anvil. If ice crystals can aggregate or rime enough to attain a significant terminal velocity (in this case, "significant" being enough to fall out of the anvil within a few minutes time...or fall speeds on the order of maybe 10s of cm/s), they will indeed fall out of the anvil if that's where the aggregation/riming occurred. I don't think there's anything special about it being at the end of the life of the storm. Perhaps the weakening updraft allows you to see more of the anvil, thus unmasking what was already there.

The phenomenon of increased reflectivity at and just below the freezing line is well-known and observed. It is called the "bright band", and you are spot on with why it happens. S-band radars (i.e., the WSR-88Ds) are much more sensitive to liquid water than to ice, so the sudden change of phase results in more energy returning to the radar. When liquid water coats an ice particle, it appears to the radar to be a very large liquid particle.

ZDR gives a way of estimating the shape of particles. ZDR is typically very low (near zero, both positive and negative) above the freezing level because the particles there are all frozen (except for in thunderstorm updrafts). Ice crystals and snow flakes tend to tumble and their shapes vary enough that, in mass, they appear to be fairly symmetric in the vertical and horizontal planes (the two polarizations the radar uses). ZDR will typically become very large below the freezing level when large rain drops are the main particles in an area. Rain drops tend to flatten out as they fall, thus looking more like hamburgers when cut along a vertical cross section. Thus they tend to return more energy along the horizontal polarization, thus leading to large positive ZDR (since ZDR = log(Zh/Zv), where Zh and Zv are the reflectivities in the horizontal and vertical polarizations, respectively)

 

[Brandon Schmidt]

Thanks for the answer, nice to know!! As for the "end of the life part" I know anvil precipitation can and does occur throughout the life cycle of a storm but I have seen a few instances where, once the updraft has died and the primary core dissipates, an area of low reflectivity remains in the entire shadow of the anvil and travels say, 50 miles, until something triggers much higher reflectivities to show up before the anvil quickly rains out and disappears from the radar completely. The attachments are of a dissipated cell just as the high reflectivities are beginning to show up. Could this be an anvil providing enough moisture to lower levels where stratiform rain just develops over time? This is not the trailing area of stratiform precip behind mcs's btw.
(I will get an image account in the future so better attachments can be uploaded.)