# Need help understanding variables that relate to meterology terminology

I am working on a project in predicting the weather and I had a few brief questions.

1. I understand that radar works by bouncing waves off of precipitation and measuring reflectivity. I have a variable and the definition for the variable is as follows, "10th percentile of reflectivity values in 5x5 neighborhood around the gauge." Can someone please explain this? I am guessing that this means it measures the strength of the signal returning from a 5x5 area around the measuring gauge? Is this correct? What does it mean that it is in the 10th percentile? The 50th percentile? The 90th?

2. The next variable is, "Maximum reflectivity values in the vertical column above gauge in dBZ". I know that dBZ is used to measure reflectivity, but what does it mean in the vertical column above the gauge?

3. I understand Correlation Coefficient to be the measure of how similarly the horizontally and vertically polarized pulses are behaving within a pulse volume. Can someone please tell me what a pulse volume is?

I really appreciate any help. Most answers I found through my own digging, but I really need help to understand these few questions.

1) Regarding the question about 10th percentile: The 10th percentile of reflectivity in a 5x5 neighborhood of the gauge means that they take the 25 reflectivity values surrounding the gauge (horizontally), sort them, and return one of the lowest values. The table in the attached image shows an example of this. In this case, the gauge is assumed to be in the green box, and bold value to the bottom right is the 10th percentile value returned by Excel.

2) Regarding the question about the vertical column: The radar scans at multiple elevation angles (between 0.5 and 19.5 degrees above horizontal), and so each one of those scans will be at a different height when it passes over the location of the gauge, sampling a different set of particles (rain, hail, etc). So the vertical maximum reflectivity at that point is simply taking the maximum value at the gauge location from all the available elevations within a complete set of scans. This “maximum reflectivity in the vertical column” variable is sometimes referred to as “composite reflectivity”.

3) Pulse volume refers to the volume of air that is sampled by the radar beam at a given instant. This volume is defined by a horizontal extent, a vertical extent, and a range extent. The horizontal and vertical extents are determined by the characteristics of the antenna and the range being considered. For most radars, this cross-section is circular, since the horizontal and vertical beamwidths are the same, often around 1 degree. The range extent is determined by the amount of time the radar transmits between periods of listening. The physical length of the pulse is the speed of light times the amount of time the pulse is being transmitted, and is typically on the order of a couple hundred meters. The formula for pulse volume is pi(range*vertical beamwidth/2)(rangehorizontal beamwidth/2)(pulse length/2).

For authoritative answers you should consult the producer of the dataset.

1. A 5x5 neighborhood sounds like a 25 point sample of values around your gauge. If you take those values and assume some distribution, the 10th percentile value would be the point on that distribution where 10 percent of the values are smaller than it and 90% are larger. It isn't clear, however, if they assume a Gaussian distribution or some other distribution.

2. Take a vertical column above your gauge from the gauge to infinity. Sample the reflectivity at all heights in that column and report the highest value observed. Vertical column just means the atmosphere directly above the gauge extending to the top of the atmosphere.

3. A radar pulse doesn't measure a point. The pulse has width and height that it samples. These dimensions are specified as the distance that the power of the pulse along its centerline is reduced to half. This is specified as an arc length as the pulse energy spreads out with distance from the radar antenna. Additionally the pulse has a length and this gives the pulse a depth. Combined this is the pulse volume and the volume varies with distance from the radar antenna and with characteristics of the radar itself. You can read more about this in a NWS Weather Decision Training Branch radar training course (Image below from this site). 