1. OC3570 Operational Meteorology and Oceanography LCDR John A. Okon
Good Morning, I will be looking at the events surrounding the clearing event on 21JUL. Our group was onboard from 18JUL to 22JUL with the Operational METOC cruise.
Case Study on the presence of afternoon coastal fog
21JUL02

2. Again notice no significant change in the surface parameters over the three days.

4. On the 20th we remained under the low stratus deck for all but about a brief 2 hour period in the late evening. At 21Z, 1300 Local we began to break out of the low deck. We saw air and sea temps rise. The low level wind shifted from the NW to the east then veering to the SW as we cont to clear

5. The satellite image from 0730 local to 1800 local on the 20th of JULY show a pattern consistent with the clearing we saw on Point Sur, just south of San Nicholas Island. Notice the clearing is only from the land to the sea, this is due to the daytime heating and breaking of the inversion.

6. 20JUL 22Z Composite
**20JUL14Z Composite Sounding is very similar to the 22Z sounding.
This is the upper air sounding on the afternoon of 20JUL, just after we started to clear. Notice a strong inversion at 940mb with a fairly well mixed layer.

7. NCEP Surface anal from 18Z on 20JUL show a standard summertime coastal california pattern. Subtropical ridge, undergoing some modification by a upstream midlat system and some troughing through the central valley and along the coast.

8. This is the ship data from the 21st notice the clearing stayed with us til about 1900 local on the 20th. We remaing under dense overcast until about 1200 local on the 21st. The two black lines indicate where the two soundings I will show you were taken.

9. Satellite loop for 21JUL. This starts at 0730 local and ends at 1800 local on the 21st. Interesting difference in this loop as compared to the loop on the 20th. The clearing of the low deck does occur from the land to the sea but it also clears from the west to the east over the sea.

10. This just shows that we remained clear through the evening, where we had near overcast skied in the previous two day and remained clear through the 22nd.

11. 21JUL 15Z
This sounding is from the morning of the 21st. Important features are the height of the inversion, midafternoon on the 20th it was at 940mb now its at 900mb, and the slope of the potential temperature curve in the boundary layer is giving evidence to the fact that we have a less well mixed boundary layer.

12. 21JUL 22Z
Just seven hours later, the inversion is much weaker and the structure is more complicated than we saw before. The question is why such a big change on the 21st. We saw nothing in the surface obs to suspect the change, we didn’t change geographic locations. We we still just south of the channel islands.

13. No real change in the surface anal valid 18Z 21JUL
No real change in the surface anal valid 18Z 21JUL. Subtropical rigid is hanging on with a minor shift in the orientation of the troughing along the coast.

14. _____ 20/22Z
_____ 21/15Z
_____ 21/22Z
This is the composite which shows the three sounding together. We can see visually the lifting and weakening of the the inversion from 22Z on the 20th to 22Z on the 21st. Notice only minor changes in the dewpoint structure.

15. Still the question remains why did we clear on the 21st but didn’t cloud over that evening. The answer is a weak upper level low moved into the area from the west, providing enough lift of the inversion , mixing the marine layer out of saturation. The other event occuring at the same time is the warming of the land causing the clearing from the east.

16. Potential Temperature
___ 20/22Z
___ 21/15Z
___ 21/22Z
The change of the theta curve is well behaved from 22Z on the 20th to 15Z on the 21st. Notice the lifting of the layer with no evidence of mixing. However, from the 15Z sounding to the 22Z sounding there is definetly mixing from above the inversion as we see warming in the low level but we don’t see much cooling above. I’ll get to that point in a minute. Also, the boundary layer has become a bit more complicated. Without making a thesis out of this it looks we have multiple air mass interaction with the old air mass lifted at the 950mb level and a new air mass forming at the surface

17. Specific Humidity
___ 20/22Z
___ 21/15Z
___ 21/22Z
My reasoning for the two airmass interaction is in the change in specific humidity profiles. If it were a pure mixing event we would not have seen the increase in specific humidity below the inversion as we do here from 22Z on the 20th to 15Z on the 21st. Also the change in the specific humidity structure from the 15Z on the 21st to 22Z on the 21st is more interesting with finer details.

18. ___ 20/22Z
___ 21/15Z
___ 21/22Z
In the potential temperature curves we didn’t see cooling above the inversion as we would expect. In fact there probably was some cooling from below, but it was offset by warm air advection from the east.

19. The apparent cooling of the surface layer is due to the weak cold air advection.

20. The apparent warming of the 950 to 700mb level is due to weak WAA from the offshore flow. Granted this is model output, we don’t have any vertical profilers or offshore data to support

21. Conclusions
No significant changes in surface parameters were evident in the clearing on 21JUL02
Upper level low over Eastern Pacific weakened the subsidence over the channel islands
Multiple dynamic effects caused a unique change in UA profile