Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 9:05 PM GMT on December 14, 2012
The surface analysis over the North Atlantic Ocean at 12Z today (courtesy of the Ocean Prediction Center) shows an occluded low-pressure system just south of Greenland. Extending eastward from its center, an occluded front stretched toward a triple-point low centered near Great Britain.
The corresponding 12Z visible satellite image from Meteosat shows the cloud structures associated with the occluded and triple-point lows (full disk). Note the enduring darkness of winter at high latitudes.
The most interesting features on the visible satellite image are the hexagonal, ring-like patches called open-cell cumulus clouds (or open-cell convection). To get a better sense for this open-cell convection, check out the cropped image below (here's a slightly pixelated, zoomed-in version).
A portion of the 12Z visible satellite image from Meteosat on December 14, 2012, shows an area of open-cell convection over the North Atlantic Ocean. Courtesy of NERC Satellite Receiving Station, Dundee University, Scotland.
The corresponding 12Z infrared satellite image from Meteosat (full disk) indicates that the convection associated with these open-celled cumulus clouds was relatively shallow (warmer cloud tops). As their shape and name suggests, open-celled convection appears as a ring of cumuliform clouds. Note that the centers of individual patches are indeed "open," indicating that air was sinking in the core of each cell and rising around the edges. Such a mesoscale-alpha pattern of vertical motion creates a lacy, hollow-looking array.
For the record, an individual ring in the array of open-cell convection has a diameter that typically ranges from 20 to 200 kilometers (so each cell is a meso-beta feature). Such intricate structures in cumulus clouds were unknown before the advent of weather satellites because the cell-like patterns were simply too small to be resolved by the fragmented observational network. And the closely packed mesh of clouds was just too large to be recognized from aircraft.
Open-cell convection forms over the oceans when the lower troposphere is unstable and local isobars (or height contours on a constant pressure surface) are cyclonically curved (courtesy of the Penn State Certificate of Achievement in Weather Forecasting). By way of clarification, cyclonically curved contours are a proxy for low-level convergence (which is consistent with synoptic-scale upward motion). The instability that supports this rather low-topped convection typically goes hand-in-hand with moderately strong cold advection over relatively warm water. Weather forecasters also look for winds with speeds greater than 25 knots in the marine boundary layer.
To seal the deal on the environment favorable for open-cell convection, check out the 12Z GFS model analysis (below; larger, annotated image) of 850-mb heights (black, thick contours), 850-mb wind barbs, and 850-mb isotherms (thin, colored contours). The circle I drew roughly indicates the area where open-cell convection occurred. The local 850-mb height lines are indeed cyclonically curved, 850-mb wind speeds are as high as 55 knots, and 850-mb wind barbs cross 850-mb isotherms from lower to higher values (cold-air advection). The bottom line is that all the criteria on the checklist for open-cell convection are met.
The 12Z GFS model analysis of 850-mb heights (black contours), 850-mb wind barbs, and 850-mb isotherms (thin, colored contours) on December 14, 2012. Larger image. Courtesy of the National Centers for Environmental Prediction
Just to convince myself, I retrieved the 12Z GFS model temperature and dew-point soundings (red and green, respectively) at 45 degrees North Latitude and 40 degrees West Longitude (where there was open-cell convection). Perhaps I'll teach beginners how to interpret skew-T's on a later blog. For now, it suffices to say that the temperature at 850 mb was -6.6 degrees Celsius and the temperature at 1000 mb was 6.1 degrees Celsius (essentially the model's sea surface temperature). The 850-mb height was 1349 meters. Thus, the lapse rate between 850 mb and the sea surface was 9.4 degrees per kilometer, which is strongly unstable.
Here endeth the lesson.
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