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.