Dual Polarization, July 7

Many Tucsonans, including me, tend to keep in mind a typical monsoon day. The typical day begins with abundant sunshine, continuing at lower elevations through lunchtime. By that time isolated storms are maturing over the highest mountains. About mid-afternoon new storms assemble southeast of Tucson, then proceed to sweep across town in an organized line, providing most locations along the way with a brief period of moderate to heavy rain. In some monsoon years there might be ten or twenty days approximating that description. Other years there are only a handful. So far this year we have had a good share of reasonably typical days. One of those days was Thursday, July 7.

The rain started at my place just before 6 PM (a little late for a typical day). Over the next 25 minutes the rain rate averaged about 1.5 inches per hour. There was no hail at my house, not even the tiniest trace of slush. There were reports of at least pea-sized hail nearby in several directions. A lady who lives about three miles east told me that her hail was long-lasting. From her description it sounds like some of her hail must have measured at least half an inch.

The time for the two images above is within a minute of when the rain started at my place. In the southwest quadrant of the top image, there is a reflectivity radial which alternates between green and yellow pixels. Beside it is another radial alternating yellow and red. My place is along the gradient between those two radials. At this distance from the radar, about 75 miles southeast of KIWA, the pixels are about 1 km wide in the azimuthal direction. Not surprisingly, given the first-hand description of the hail, there is an area of high reflectivity 4 to 5 kilometers east of my house. For simplicity, I'll call that cell the hailstorm. Fifteen minutes earlier, when that cell had just developed, it was more or less surrounded by a ring of high differential reflectivity. By the time of these images the high differential reflectivities (second image) had consolidated into two arcs, one extending northwest from my house, while the other, smaller area is southeast of the hailstorm. The rain began abruptly at my house with a brief barrage of big drops (higher differential reflectivity). That quickly changed over to a mixture of large and small drops (lower differential reflectivity), all wind-driven from due east.

Looking at what others are saying about dual-polarization, it appears that the emphasis is on dual-pol being an improvement in the classification of precipitation type. Incorporated in that classification is a distinction between different drop size distributions, and that distinction contributes to improved estimates of rainfall rates, for which it is straightforward to quantify the estimated benefits. But in addition, I think that differential reflectivity by itself will provide new information that will contribute to improved nowcasts of storm evolution.

With the heavy rainfall rates and the diversity of hail reports on July 7, it would seem to have been a good day for the other two dual-polarization products, the correlation coefficient and the differential phase, to shine. My initial impression is that there are problems interpreting them, perhaps because my house is too far from the radar. The differential phase seems to be overwhelmed by non-meteorological spatial variability. For the correlation coefficient, there seems to be a stronger meteorological signal. Here is an image of the correlation coefficient at the same time as the previous images.

The correlation coefficient never dropped below 0.97 at my house. It was lower around the hailstorm.

Phoenix Dual-Pol Radar, June 30

I stumbled on how to access the dual-polarization radar data from the archive at NCDC. Their viewer, the Weather and Climate Toolkit (WCT), in its Moment: pull-down-list conveniently and automatically adds the available dual-pol products to the original three (single-pol) products.

Although there have been several exciting days over the past week and a half, my previous post explained what was interesting about June 30. The day started for me with loud drops falling on the skylight shortly after 4 AM (11 GMT). It was the first rain in a long time. To make the morning even more interesting, the local media thoroughly covered the first uprooted tree of the season.

The two radar images below, which are zoomed into the Tucson area, were saved from the WCT. The top image is reflectivity and the bottom is differential reflectivity. The time is eight minutes before the reported time of the tree uprooting, and it is about a half hour before the big drops woke me. The thin red lines, conveniently provided by the WCT, are the highways I-10 (extending NW-SE) and I-19 (extending to the south). The marker labeled J is my house. The marker labeled M is the corner of Mission and Drexel, near the report of the tree toppling. The distance between M and J is about 12 miles.

The markers labeled with T's are prominent peaks in the Tucson Mountains. I imagine that the heights of the cloud bases were well above those mountains. Yet when viewing animations of both the reflectivity and the differential reflectivity, it's apparent that the cells were feeling the effects of the underlying terrain. Most of them were moving from southwest to northeast, along paths approximately perpendicular to the Tucson Mountains. The cell at M was slow moving, giving the impression of being attached to the southeast end of the Tucson Mountains. It's interesting to notice that the M cell is flanked by an area of bigger drops (higher differential reflectivity) on its southwest side. Now shift attention to an area directly north of M, where there is a cell centered on I-10. Ten minutes later that cell had moved to near my house (point J). By then the reflectivity had decreased by a good 5 dBZ and the differential reflectivity to mostly 2 dBZ or less. A second cell, approaching the top of the Tucson Mountains in this image, was near my house a half hour later. It too had reduced reflectivity by then, and its differential reflectivities had decreased to either slightly above or slightly below 2 dBZ. Perhaps the radar is telling me that the big drops had just finished falling onto my skylight.

Coincidentally, the upper air flow that morning was similar to what is expected for Tucson this coming Tuesday morning. I can see that the models are driving dry air well south of San Diego, then across Baja. There certainly are going to be changes, but I'm suspicious that the break in the monsoon is being exaggerated.