© Jarmo Koistinen
Janakkala, Finland
Latitude: 61° 0' 0'' N
Longitude: 25° 0' 0'' E
22 May 2010 0640 (Local Time)
Camera direction: towards W
Image P/S code: P.7.5
Image I.D.: 5709
CL = 5, CM = 0, CH = 1
This photograph shows weak early morning convection (0640 hours local time). At 1 and 2, we see small tufts with a cumuliform appearance, the lower parts of which are more or less ragged; this is typical of the cloud species floccus. The elements resemble typical Altocumulus floccus elements, but are clearly larger and lower. The cloud base of 2 000 m places this cloud near the upper limit of the low cloud level; thus, it is designated as Stratocumulus floccus. As the surface wind was very light, this cloud should not be confused with Cumulus fractus, which is generally driven by stronger boundary layer winds, nor with Cumulus mediocris, which does not have such a ragged cloud base. A surface temperature inversion was present at the time of the image; therefore, the convection was forced by higher level instability and not diurnal surface heating, which generally initiates Cumulus development over land. Cirrus can be seen through the floccus elements at the top of the image, with Cirrus or Cirrostratus just above the tree line.
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The image location in southern Finland was 100 km north of a weak kata-type cold front. The front was essentially dry, with only a few scattered light showers, and had passed the site the previous night. The inflow of polar air was weak due to light winds through the whole troposphere.
The Jokioinen, Finland (WMO 02963) sounding from 0300 hours local time and 80 km south-west of the image location shows a surface inversion present with a 2-m temperature of 14 °C–16 °C, whereas in a well-mixed boundary layer it would be approximately 19 °C. Thus, the early morning convective instability was driven from above and not by solar heating of the ground. The sounding shows a moist unstable layer from 850 to 750 hPa, the approximate height of the cloud in the image.
The relatively small, ragged tufts of cumuliform appearance are Stratocumulus floccus. They cannot be any of the following:
(a) Cumulus fractus of fine weather, as their frayed appearance was not due to a strong, turbulent wind (this was confirmed by the slow change in outlines and slow movement of the elements);
(b) Cumulus humilis, as none of the elements have completely formed or clear-cut horizontal bases, or appear as if flattened on top;
(c) Cumulus mediocris with irregular bases and ragged in parts, as there is insufficient vertical extent and the wind was neither fresh nor strong;
(d) Stratocumulus castellanus, as the cumuliform towers are not rising from a common base;
(e) Altocumulus floccus, as the elements are too large and their height was estimated to be in the low levels.
Stratocumulus floccus is often seen with Stratocumulus castellanus, a line of which is lower on the horizon. Floccus sometimes forms as a result of the dissipation of the base of castellanus.
Stratocumulus floccus and castellanus generally form in an unstable atmosphere above the planetary boundary layer (PBL). On this morning, significant overnight cooling resulted in a very shallow PBL. The cloud base at 1 400 m (4 500 ft) places the cloud significantly above the PBL.
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