You know it's winter in Fairbanks when…the RAOB shows a better than 10C surface based inversion and the warmest temperature in the sounding is more than a kilometer above the ground: here's Friday morning's temperature trace from the 3am AKST sounding:
This is reflected in the surface observations, with 5am temperatures ranging from 14 below in Goldstream Valley to 10 above at some of the personal weather stations in the hills around town. The very shallow inversion also being reflected in the hourly observations at the Airport, with temperatures changes of as much as five degrees per hour in an otherwise very quite night, with clear skies, faint aurora and no large scale winds.
Rick, in the past I have noticed the seemingly large swings in hourly observations on otherwise calm nights in the winter. Are there some katabatic drainage winds that cause warmer air to be advected from higher elevations or is it some sort of mixing effect?ReplyDelete
I'm sure Rick will have more info on this, but there is periodically a drainage wind that comes down through the Goldstream valley and spreads out across the field in front of the University and the Chena Pump area. Sometimes this will make it to the Fairbanks Airport and impact the temperature. I used to walk across that field to work and sometimes the wind was fairly strong, but not many stations are in the right area to measure it.ReplyDelete
The effect of flow and mixing can be observed when ground fog forms, if observed from a commanding view above. On calm days after a recent snow event, followed by radiational cooling to the dew point, the low fog layer behaves like the surface of an open water body does on a windless day.ReplyDelete
There's continual upper surface ripples that result in thin spots of the fog over the ground, that to me infer mixing. On my remote lake surrounded by hills where I've seen this frequently, slow drainage winds (presumably from cooling) descend onto the basin and further ripple and vary the fog's layer.
I assume without the visible fog the behavior is similar and results in a periodic variation of surface temperature even during reportedly calm conditions.