Daily Archives: February 17, 2014

The Wintertime Diurnal Cycle in Coastal Sonoma

Greetings from Bodega Head!

We are having another quiet, sunny weather day here and with no precipitation or polluted airmass to discuss I am going to do something a little different.

Yesterday we noticed that our NW to WNW winds really picked up in the afternoon. As I write this post at 2 pm PST, the same has begun to happen today. Lets look at the surface met station output for the last 24+ hours from Bodega Bay (BBY) and Santa Rosa (SRS):


BBY Surface met variables for the 2-day period ending at 1 pm PST on Feb 17, 2014. Credit: NOAA-HMT

This first figure is our local met station at BBY. Notice that the wind direction was steady from 300 degrees or so (WNW wind) yesterday afternoon, while the wind speed steadily rose. Overnight the wind calmed and the wind became almost entirely northerly. Note that the maximum wind occurred a couple hours after the daily maximum temperature was reached. Stronger, WNW wind conditions returned this afternoon at about the same time.

Now compare to the surface met timeseries from SRS:


SRS Surface met variables for the 2-day period ending at 1 pm PST on Feb 17, 2014. Credit: NOAA-HMT

Since yesterday morning in Santa Rosa, the winds have been very weak near 2 m/s. Wind direction has meandered near northwesterly.

The difference between two relatively close sites under high pressure and clear skies is due to the sea-breeze circulation. Santa Rosa is approx. 20 miles inland from the coast. Santa Rosa has reached a warmer high temperature each of the last 2 afternoons. Currently it is 56 degrees Fahrenheit on Bodega Head, while the Santa Rosa regional airport is reporting 64 degrees. The regional temperature differences are summed up nicely in this short range 4-km WRF forecast valid at  1 pm PST today.


WRF-4km 10 meter winds and temperature valid at 2100 UTC on Feb 17, 2014. Credit: NOAA-NWS Sacramento WFO.

The thermal contrast between the coastal waters and the warm inland of Sonoma County causes a shallow overturning circulation to form, with relatively low pressure over the warm land and high pressure over the cool sea during the daytime. Winds at the daytime land-sea boundary (our site) are onshore at the surface and offshore overhead. This circulation reverses itself overnight.

The entire sea-breeze circulation may only extend 50 km horizontally, however this is still large enough for the coriolis effect to become important. Because of the coriolis effect, the sea-breeze circulation turns away from coast-normal, and the direction of this turning reverses itself in a predictable diurnal fashion as well. To see how, I have included this figure from Haurwitz (1946!):


Theoretical sea breeze diurnal hodograph. Credit: Haurwitz, B., 1946, Comments on the Sea Breeze Circulation, J. Meteorology, Vol. 4, No. 1, 1-8.

The above is a hodograph, or a trace of the path that the head of a wind vector centered at the origin will take over time. In this case, the graph depicts the relative wind direction and strength over a 24 hour period indexed to the time of maximum thermal contrast between the land and ocean. The point labelled “0” corresponds to the time of maximum temperature gradient.

Haurwitz created this plot while working at MIT, so his frame of reference has the ocean to the East (the positive “x” axis in this plot). It takes a bit of mental gymnastics, but one can superpose this diurnally varying sea-breeze vector onto a NW wind vector like the one that exists for our broad local area. If we reverse the x-axis to account for land being to the east on the West Coast and also reverse the y-axis to account for the fact that coriolis acts to the right of the flow, we see that at the time of maximum thermal contrast between land and sea (late afternoon) we should expect a strong wind which is more westerly than the prevailing flow. On the other hand, overnight we should expect a much weaker and more northerly wind. This is precisely what we have seen in our local wind timeseries over the past 36 hours.

What does this mean for aerosol-cloud interaction? Not much, since we do not expect to have any clouds under these weather conditions. However, we are measuring the ambient aerosol chemistry, sizes and the trace reactive gas concentrations. We should expect that overnight when the flow comes straight out of the north we are more likely to measure terrestrial and anthropogenic aerosols and gases. Conversely, during the late afternoon when the wind is stronger from the northwest, we should only expect marine influences.

Next chance for rain:

We are still expecting a slight chance of rain tomorrow (Tuesday, February 18) evening. Chances are not great, but we may get some periods of light drizzle after 9 pm PST. Ill have a more thorough write-up of the coming weather system here tomorrow.