Thursday, December 1, 2016

Comparing the MYNN and MYJ planetary boundary layer schemes for a Lake Breeze event.

I was the lead author for a paper titles "Impact of Lake Breezes on Summer Ozone Concentration in the Salt Lake Valley." This article was recently accepted in the Journal of Applied Meteorology and Climatology. (see article here:

One concern from the reviewers was our use of the MYJ planetary boundary layer scheme. They pointed to evidence that the MYNN scheme was superior, and may improve our results, particularly in the timing of a lake breeze passage on June 18, 2016. We declined to re-run the simulation using the MYNN due to computing resources and time.

I have since re-ran the simulation with the MYNN and found the results are very similar with the MYJ. Below are time series graphs for the stations presented in the paper with the temperature and wind with the MYNN model run overlaid in magenta. The only change between the "WRF" and "MYNN" model data is that the MYNN uses the MYNN PBL scheme instead of the MYJ PBL scheme.
Color and station name denote the observed temperature and wind at the station location. Black dashed is the original WRF simulation, magenta dashed is the same simulation except uses MYNN boundary layer physics.

Temperature and wind patterns between the two runs are very similar. We see that using the MYNN did not change the timing of the lake breeze on this day.

The cause of the observed delay in the lake breeze progression is likely caused by stronger meridional wind component below 5,000 meters (550 mb). In the vertical profiles below, you can see the opposing southerly winds at the Salt Lake City Airport were from the direct south at 10 m/s below an inversion layer at 5,000 m. The HRRR and WRF with MYNN PBL scheme had slightly weaker winds and more from the southwest. This subtle difference is likely the primary reason for the delayed lake breeze on the afternoon of 18 June 2016. The next question is, "how do we fix that subtlety in the model." I'm not sure I know the answer to that.
Vertical profile of potential temperature, mixing ratio, and vector winds at 19 June 2015 00z. Profiles for Salt Lake City Rawinsonde (blue), HRRR analysis (red), HRRR 1-hr forecast (green), and WRF with MYNN PBL Scheme (black) are shown. Notice in the observed sounding the critical level at 5,000 m where there was a temperature inversion. Below this level winds opposed the lake breeze and were directly south while the simulated winds were weaker and more southwesterly than the observed winds. This subtlety is the likely reason why the WRF simulation missed the delayed lake breeze.

Just as a comparison between 3 km and 1 km domains: the lake breeze is much less "sharp" in the outermost domain, run at 3 km (right), than the inner domain run at 1 km (right).
Domain 2 with 1 km grid spacing
Domain 1 with 3 km grid spacing

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