more on why don't pingos disappear in winter?

As I mentioned in my brief reply to Inessentially Speaking (see posting "why pingos don't disappear in winter" April 26, 2010) I had included in a research grant application to the UK’s Engineering and Physical Science Research Council a proposal to include some new field measurements using remotely monitored GPS records to track the timing of the growth of pingos throughout the year. For reasons of extreme weather during anything other than a few summer months such readings had not previously been possible.

My thinking was that at least during the early growth period, when the thickness of permafrost and ice beneath the pingo was relatively low, the time lag between the mid-summer maximum surface temperatures and the through-thickness maximum average temperature would probably be relatively small. This would mean maximum compressive energy would reach its maximum fairly soon after the mid summer surface maximum temperatures. This would imply a rapid build-up of compressive strain energy in the permafrost over the period April through to roughly the July - August period (at least in the Northern Hemisphere). It would be this rapid build-up of compressive strain energy that would be causing any incremental increase in the thermal upheaval buckle deformation. This is depicted in Figure 1 below, which suggests that maximum incremental uplift would be reached at around July - August. Equally, once the compressive strain energy had all but expended itself in producing the thermal uplift the subsequent cooling over the August to January period would fairly quickly be expected to produce high tensile strain energy much of which would be released through thermal cracking. While it is suggested the upward growth of the underlying ice lens would prevent the summer growth in uplift from being entirely lost during the winter months, it is likely that some slight settling back would occur. This is suggested in the attached sketch. Over a period of virile growth it might be anticipated that if thermal upheaval were the cause of pingo growth, or at least a major contributory cause, then a year on year pattern of growth something like that shown would be observed.

In contrast, any spike in the pore water pressure would occur when the cold wave reaches the underside of the permafrost. Allowing for thermal lag this would mean maximum growth occurring nearer to mid winter.

It is a pity we were unable to carry-out such measurements. But perhaps by now someone else has?