Readers Eric and Gary have commented recently on the dependence of Fairbanks precipitation on wind direction. It's widely understood (at least among weather enthusiasts) that westerly or southwesterly flow aloft tends to bring moisture to Fairbanks-land, but to what extent is this relationship borne out in the historical data? This is a pretty straightforward analysis: I calculated the vector average wind speed at 700 mb (about 3 km MSL) from balloon soundings each day since 1948, and then I categorized the data into 30-degree direction windows, e.g. 0-30°, 10-40°, and so on. For each direction window, and requiring a minimum wind speed of 10 knots, I then extracted the frequency of measurable precipitation and the average daily precipitation; the results are shown below.
As expected, the highest frequency of measurable precipitation is found for near-westerly flow at 700 mb; for summer the peak is west-southwesterly (230-260°), but for the other seasons, it's close to westerly. All seasons also show a secondary peak in precipitation frequency; again the summer is distinct, with a frequency peak for easterly flow, but for the other seasons the secondary peak occurs for southeasterly flow.
The second chart, showing average daily precipitation amount for all days (not just non-zero precip days), also indicates a peak in precipitation amounts for westerly or west-southwesterly flow. However, in summer the average amounts are nearly as high for northwesterly flow (310-340°) and for north-northeasterly flow. The other seasons show much lower amounts in non-westerly flow regimes, although there is still a secondary peak for southeasterly flow in spring. It's interesting to observe the extreme dryness of northerly and northeasterly flow in winter: there's no moisture to be had in that situation.
Finally, it's interesting to see the total contribution of each flow direction to the total precipitation at Fairbanks - see below. The peak is pronounced in all seasons for flow just south of westerly (240-270°), because the preferred wind direction is out of the southwest; so even though other flow regimes can also be favorable for precipitation, they are less common and so contribute less to the total. (The most common wind direction is 200-230° in autumn and winter, 190-220° in spring, and 230-260° in summer.)
This is a very nice analysis Richard. I personally think that 700 mb is a good proxy for wind direction but am curious how it would look at 850 mb or 500 mb. I am not suggesting additional research, just an opinion.ReplyDelete
Brian, a belated thanks for your comment (I've been out of town for a few days). I agree it would be interesting to look at different levels - and also the amount of veering or backing with height, which is related to the thermal advection pattern. Presumably significant winter precipitation events are strongly associated with warm advection.Delete
It would be very interesting to see the different in stratiform vs. cumuliform precipitation in the summertime primary and secondary peaks. I'm guessing that the secondary peaks for the wind directions anywhere from NW to E/SE are mostly for showers or thunderstorms forming over the Tanana uplands.ReplyDelete
No guess, that's my observation as well. Early to receive solar heating and resulting lift, those hills are CU and precip producers.Delete
Why their products tend to than move in a southerly direction would be worthy of investigation, or at least speculation.
Andy, thanks for the comment. I agree it would be interesting to look at the wind differences between convective and stratiform rainfall in summer. I'll have to read up on possible methods to classify the events without looking at radar presentation.Delete