A slowly moving upper-level system produced a continuous area of light and moderate rain across central and eastern Oklahoma throughout the day Friday. The reflectivity image below shows a rather innocuous system generating widespread rainfall.
Prior to the implementation of dual-polarization technology within the NEXRAD radar network beginning in 2011, only reflectivity, radial velocity and spectrum width information would have been available. Dual-pol data provides three additional radar variables, and this newly available data can help to reveal physical processes occurring within the atmosphere. Although Friday’s rain was rather benign, it afforded an opportunity for dual-pol technology to clearly locate the melting layer within the atmosphere over Oklahoma.
The image of correlation coefficient below is one of three dual-pol radar variables available. Correlation coefficient can provide information on the homogeneity of radar echoes within a radar volume. A correlation coefficient near 1 indicates a uniform consistency of hydrometeors. When a mixture of different hydrometeors is present, the correlation coefficient will typically drop below 0.98. This transition can easily be seen below using data from the KTLX radar below. The area closest to the radar has correlation coefficient values near 1.0 (red shading). Beyond the white arc, values drop below 0.98 (within the yellow shading).
The transition from red to yellow marks the beginning of the melting layer. The melting layer is a region of the atmosphere where snow and rain coexist as snow is falling aloft and melting before it reaches the ground.
While the image above is of the 0.5 degree elevation scan, the one below is from the 3.4 degree tilt. Higher elevation scans show the melting layer region more distinctly as the radar beam passes upward through the melting layer region more quickly.
The white ring in the image shows the bottom of the melting layer located approximately 6500 feet above the surface. The black ring denotes the top of the melting layer positioned somewhere from 9000 feet (to the west) to over 10,000 feet (to the east) above the surface. The freezing level is located at the top of the melting layer, where temperatures above it are below freezing and temperatures below the freezing level (and within the melting layer) rise above freezing.
An upper-air sounding from central Oklahoma at approximately the same time is shown below. The thick black line is a line of constant temperature; in this case 0 degrees Celsius. The red line shows the change in temperature vertically from the surface (bottom of the graph) to the upper levels of the troposphere (top of the graph). The red line passes across the black line at the freezing level. In this sounding, the freezing level of the atmosphere is located at 700mb or about 10,100 feet.
Thus, the correlation coefficient did indeed provide a very accurate indication of where the freezing level was located in the atmosphere. This also shows the power of using radar data, which can produce a 3D representation of atmospheric processes, while soundings are limited to providing information at only specific points as a weather balloon ascends.
There is great amount that can be said to explain both what dual-pol radar data is and how it can be used to determine what types of hydrometeors are present in various regions of the atmosphere surrounding a radar. And this rather unremarkable weather event in Oklahoma shows just one example of how atmospheric processes can be brought to light using dual-pol technology, which became available to us only within the last few years.