Wall cloud - 9th March 2000

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Wall cloud - 9th March 2000
by Jimmy Deguara

Not much was expected in terms of storms unless the cloud band and rain cleared. But the night before, I noticed that the rain band was indicating signs of clearing. That morning, it was clearing with light west winds. Stratocumulus was widespread but through the breaks I could see spectacular mamatus obviously part of the clearing storms associated with the cloud band.

Throughout the morning, it was obvious that the developing cumulus would develop into storms. But they were aligned along the mountains and moving from the northwest.

During the early afternoon period, there were pulse cells developing and collapsing along relatively thin bands. The tops were crisp though indicating stronger pulses. At about 3pm, I had to make a decision: do I go with my friend for a cup of coffee (which I had suggested) or do I just decline and and grab the camera. "Sorry. Going home", I said. What bugged me is that I had tossed whether I should bring my camera or not that morning. Due to the decision not to bring a camera, I had already missed an opportunity of photographing a spectacular mamatus during the morning without the stratocumulus. I wasn't going to make the same mistake again!

When I got nearer to home, the larger cumulus began to weaken. So rather than go out again, I logged onto the internet and kept watch. ASWA member Mario and I discussed over ICQ what was happening. We checked and discussed the latest AVN model for Lifted Index and wind profile. Well moisture was sufficient with all the rain that had fallen during the week and even early that morning.

The surface lifted index was sufficient for storms, even severe storms. The wind profile though was excellent. We had a jet over eastern and northeastern New South Wales and nice north to north-easterly winds in the lower layers providing some sort of backing. Unfortunately for Sydney, a cloud band was moving south with the jet and therefore cut off the heating source for thunderstorms. But it remained sunny over the region of the storms and being in the right exit region of the jet made it ideal setup.

cumulonimbus southwest of Schofields 4:45pm

Times are local Daylight Saving Time (UTC +11)

As evening approached, the closest large cumulus near Penrith collapsed. It revealed an impressive back shearing anvil from a cell that had gone severe. I took a couple of photographs of it and discussed it with Mario. It was in the next radar scan that Mario suggested "Red on the radar!!!" I reloaded and there it was. Taking into account that it was lagging about ten minutes, it coincided with my photograph of the severe cell. I suppose I had accidently overlooked the amount of high reflectivity of the next scan but still it was signficant. It was at this point that the storm changed direction at about 20 or so degrees to the left of the mean wind flow.

With this scan showing red pixes on the radar, my adreanlin went out of control. But instead of taking off in the car, I grabbed my "essential" equipment and crossed the road.

The storm looked quite impressive particularly on the back end with excellent shear conditions. The anvil faced southeast. There were strong updraughts rising in the same respective area of the storm.

After a short time photographing and video taping the storm, what came into view was unbelievable. I tried to clear my eyes to see if I was seeing correctly. I moved a few metres to make sure the tree wasn't in the way. It seemed that the lowering was a wall cloud. But you guessed it: as I was filming, the video camera shut down. The tape had run out!!! At this point I went crazy not knowing what to do. I seriously wanted to chop my body in half allowing one part to get another tape and I would stand there and watch to see what happened.

It was inevitable: I needed a tape. I ran like a rocket to get that tape and left the neighbours in charge of my equipment whilst I rushed inside. Within five minutes I was setup again. But this time, (and perhaps just as well), I moved further to find a gap between trees (that were situated 200 metres away) so that I could get a better view of the base structure.

The storm was moving reasonably quickly. As I watched, the lowering took more of a familiar shape of a wall cloud. At this point I rang Paul Graham to alert him of the situation, so that he could alert the aussie-weather mailing list and also to make sure a severe weather warning or advice was out. Paul confirmed it a few minutes later.

This wall cloud and associated lowering lasted for at least 25 minutes or more and at one stage had the lowering and scud almost touching the ground! But from about 6pm, the storm began to lean further foward and weakened very rapidly. At this point, I walked back home.

wall cloud still prominent south of Schofields 5:52pm

wall cloud still prominent south of Schofields 5:55pm

dissipating cumulonimbus south of Schofields 5:57pm

dissipating cumulonimbus stage south of Schofields 5:58pm

Radar indicates that the red reflectivity was evident in six consecutive scans. In other words, it lasted about an hour. It seems that the maximum reflectivity came from one main cell which I had been observing for the whole period. This cell existed in a very good shear environment, had strong updraughts and a base structure that fed on the northerly airflow. The fact that the major cell within the storm was long lasting and moved to the left of the mean wind flow indicated that it had supercell characteristics.

I suspect that the storm collapsed probably from insufficient heating with the sun setting and perhaps blown apart by the jet. My belief was that there was just the right balance of wind shear during the period from about 4:30pm to almost 6pm.