Differential Reflectivity in Summertime Showers

With the newly upgraded dual-polarization weather radars it is possible to glean information about the distributions of water and ice inside of storms. This is very advantageous when trying to determine if a storm might be producing hail or just rain. Here is an example from the Vance AFB, OK radar of a few light summer time showers.

You can see in the reflectivity, located in the top panel, that one of the storms has reflectivity values of over 50 dBZ. In fact, looking at the contours on the bottom panel it is easy to see that there is a small region of >55 dBZ reflectivity. Now normally with reflectivity values >50 dBZ you might start to worry hail with the storm. However, in this case you can use the differential reflectivity (also called ZDR), located in the bottom panel, to distinguish between large rain drops and hail.

This brings us to a side discussion of what exactly is differential reflectivity? Well differential reflectivity is the log of the ratio of horizontal reflectivity to vertical reflectivity. The dual-pol upgrade to the radars modifies them so that they can transmit energy that is oriented both vertically and horizontally. This transmitted energy bounces off the cloud drops, rain drops, hail, etc located inside of a storm and returns to the radar. The radar is then able to measure how much if the returned energy is oriented vertically and horizontally, we refer to these values as the vertical reflectivity and horizontal reflectivity. The horizontal reflectivity is what we all know and love as the standard “radar reflectivity” product. Now the amount of energy returned to the radar that is oriented vertically depends the height of the rain, hail, etc. Likewise the amount of horizontal energy returned depends on the width of the rain, hail, etc.

So given this, if we have high differential reflectivity it means the rain, hail, etc is much wider than it is tall. This occurs when you have large rain drops, which given that they are falling end up elongated and wider than they are tall. If you have small rain drops they depend to be able to hold their spherical shape better which means the differential reflectivity is close to 0. What happens in the case of hail? Well hail tends to tumble as it is falling, which means that to a radar it will appear to be spherical thus giving you a differential reflectivity value close to 0.

Going back to the example figure above you can see an area of high reflectivity with correspondingly high differential reflectivity. This means that the most likely hydrometeor is a large elongated rain drop. In the example figure there is also a region of low differential reflectivity and correspondingly low reflectivity which can be associated with small spherical rain drops. If hail was present anywhere it would appear as a region of high reflectivity collocated with low differential reflectivity.

This is just one example of what you can do with the dual-polarization products!

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