Emergency NAM model change in coming week or few weeks

NCEP is planning to implement some changes to solve some vexing problems that cropped up under summertime conditions in the NAM since the NAM changed from the Eta model to the WRF-NMM model. The changes will be put into effect "as soon as possible", which generally means on a Tuesday that is not a critical weather day. I will update this topic when I find out it went in or have a scheduled date.

The changes increase vertical and horizontal mixing, which principally have the effect of eroding thin marine layers which have been advecting inland much too far in the Pacific northwest area. The changes will mix the boundary layer deeper in areas of complex terrain and dry climate with marine influence such as around the Great Salt Lake, and it reduces the frequency and intensity of grid-scale precipitation bombs, though there still are often swaths of very heavy precipitation in areas of strong synoptic or mesoscale forcing when precipitable water is high.

Also, the surface roughness is reduced, especially over high terrain, resulting in stronger 10-meter winds. The 10-meter winds over the west had a pronounced low bias and recent stats for the parallel runs show the bias is considerably improved but the RMS error stats are not improved. However, this is a short sample, we need to watch it over a longer period to see whether the RMS may improve from the bias improvement.

These changes may also affect sea and lake breezes and possibly hurricanes and any other phenomena with extremely sharp gradients. I suspect the model will be less likely to produce spurious hurricanes but may also be less effective at simulating the inner core of real hurricanes, however both of those await some examples to see if my expectations are correct or not.

Stephen

The changes mentioned in the previous message are scheduled for implementation Tues Aug 15. Additionally, one more change will also be implemented at the same time: surface (2-meter) temperature observations from land stations, which have not been included in the GSI analysis, will be included in the GSI 3d-var analysis, though with vertical influence confined to be very shallow, primarily affecting the lowest model layer.

Stephen

The operational NAM continues to produce spurious Tropical Cyclones. The NAM has continued to try and spin up the weak little system off the Carolina coast most of the week and now we're seeing the Gulf Convection intensify.

Attached is a 36 hr fcst of NAM at 40 and 80 km (my AWIPS can't see native resolution over the Gulf) with GFS. Looks like a nice little T.S. - unfortunately, I don't believe it will pan out.

Stephen - any idea what's going on here? Our local ARW ensemble run on the 00Z database seemed to point to BMJ and a source.

Jeff

The Aug 15 changes had a coding problem which caused the increase in horizontal diffusion to be overwritten with the previous, smaller value when the model runs. All of the other changes took effect on Aug 15. The horizontal diffusion increase, after recoding, should actually go in on Aug 22. Horizontal diffusion will still be much less than it was in the Eta model, though a bit more than double what has been running in WRF-NMM.

Therefore, the example Jeff showed does not include the increased horizontal diffusion. Parallel tests now show that it will still produce these spurious events as well as real events, though typically somewhat weaker than without the diffusion increase, and a test run for hurricane Dennis from last year (2005) still develops an intense storm with an eye and eyewall, though those features are sharper when the smaller (current) diffusion is used than when the larger (post-Aug-22) diffusion is used.

As for what is causing these events, it does seem to be the BMJ convective parameterization. Most or in some cases nearly all of the precipitation during the deepening phase in the forecast is from the convective parameterization. Two factors absent in BMJ may be crucial: 1) vertical fluxes of horizontal momentum, and 2) cool convective downdrafts

The GFS decreased its false alarm rate when momentum mixing was added to its SAS convective parameterization, and a few (small sample size) tests in the currently under-development Hurricane WRF also show improvement in reducing false alarms without squashing real storms when momentum mixing is in the SAS convective parameterization compared to when it is not included. WRF doesn't presently allow the convective parameterization to pass along momentum fluxes (being worked on), so this you can't easily test in the workstation WRF at your forecast office.

BMJ does not place cool downdraft air in the subcloud layer (as distinguished, for instance, from SAS and Kain-Fritsch which do). Studies of tropical cyclone genesis indicate that convective downdrafts of much lower equivalent potential temperature than the ambient boundary layer air disrupt surface development, and observations generally show deep moisture up to at least the mid-troposphere in the area where hurricane formation occurs (sometimes not extending much beyond the vortex, but at least encompassing it). This you can test in your workstation WRF by running with KF or SAS. However, it may require a big domain because you'll need to include the area where tropical cyclogenesis is occurring in the operational run, and it may take too long for your local computer to run over such a large domain.

Stephen

It turns out that the horizontal diffusion change referred to in my last post did not go in yet and is now scheduled for Sep 5. The NCEP parallel experiments which include this change show many shortwave troughs pass more quickly through a long-wave trough axis and move downstream faster. This should somewhat reduce (not eliminate) the problem of troughs typically being too slow and too deep in the NAM-WRF. I do not know why this problem has been persistent in the NAM-WRF and why the modest increase in horizontal diffusion helps to reduce this problem.

Additionally on Sep 5, the SST analysis used in NAM is reverting from the new 1/12th-deg lat-lon product to the 1/2-deg lat-lon product which was used when the Eta model was running in the NAM slot. Large SST problems on the north and west shores of Alaska have adversely impacted the forecast and in the past some isolated excessively warm SST spots in the Gulf Stream were noticed. These artifacts appear to be a result of some problem in the physical retrieval algorithm applied to the AVHRR satellite data. The 1/2-deg product used an older retrieval method. The new method was believed to be superior but apparently it produces bad results under particular circumstances, perhaps related to handling of sea ice in the retrieval (for the problems near Alaska).

Stephen