# Presentation: 499931

## ParallelNam – SLC Evaluation

- Randy Graham and Alex Tardy
- WFO Salt Lake City
- May 2006

**Notes:**
This presentation will provide a brief overview of some of the changes that we can expect to see with the WRF model and will also highlight some of the differences between the ETA and the WRF. The presentation is 25 slide long and will take about 20 minutes to complete. 

## Overview

- Compared guidance from operational ETA with that from the ParallelNam from early April through mid-May
- Focused on elements that should be impacted by changes implemented in the WRF
- Better terrain
- Less diffusion
- Improved initialization scheme
- Improved moisture analysis
- Elements of primary concern included
- Winds
- Dewpoints/RH
- QPF

## Winds in Well-Mixed Environments 

**Winds in Well-Mixed Environments **

- Not uncommon to see differences in surface winds between the WRF and the ETA
- WRF appeared to frequently have improved valley winds
- Improvement especially noted in well-mixed environments

**Notes:**
We again see differences (albeit frequently subtle) between the WRF and the ETA runs.  One good example is that which appears over the Dugway mesonet.  The WRF (shown in Green) indicates that we will have more northerly winds by 1800 UTC while the ETA (shown in brown) indicates more westerly component to the wind.  Verifying mesonet obs indicate that the WRF was superior in this area on this day.  There we also differences in the Cache Valley and the Uinta Basin on this run. The difference in the Uinta basin were a wash while the WRF was superior in the Cache Valley as well.  

I seen a couple of instances in which the WRF did a better job bring in up-valley flow or a lake breeze in the past couple of weeks.  In these situations the ETA12 was slow to bring northwest winds into Salt lake County while the WRF did a better job depicting the actual evolution of the wind shift. 

## Improved Up-Valley Wind/Lake Breeze Initiation?

- WRF was consistently better with Lake Breeze initiation
- At given time step each model would have improvements over the other in some locale
- Overall, WRF had better surface winds (especially lake breeze initiation)

**Green - WRF**** **
**Brown - ETA**

**Notes:**
Here is an example where the WRF picked up on the up-valley flow better than did the ETA.  This is something that I have seen on a pretty regular basis during the test period.  In this example we are looking at a 12 hour forecast from the 0600 UTC run. The WRF wind barbs are in green and the ETA wind barbs are in brown.  Note the ETA is still indicating drainage winds at this time while the WRF has brought north-northwest winds through nearly all of the Salt lake Valley.  The obs from SLC and Airport #2 indicate that in this instance the WRF had a better handle on the valley winds at this time. 

## It’s not the heat, it’s the humidity...

- ETA struggled with Tds early in forecast cycle...WRF showed improvement
- Significant errors in ETA over West Desert...WRF significantly better in this case
- WRF Tds around GSL were too high at times
- WRF Tds had issues as well...particularly over high terrain

**Notes:**
It has been pretty common to see substantial differences in the ETA and WRF surface dewpoint values.  The WRF has seemed to be too high with its surface dewpoints over southwest Wyoming while in other areas it has been superior to the ETA.  Here is a pretty dramatic example from the 3rd of May.  The ETA has always seemed to struggle early in the forecast cycle (say the first 6 hours) to the point where we have commonly chalked it up to model “spin-up” and ignored it’s output.  Due to the initialization scheme in the WRF it should have much better initialization and ultimately performance in the early portion of the run.  

In this example, let’s look at a 3 hour forecast from the ETA valid at 0300 UTC on May 3rd.  Note that over the west desert there is a large area of surface dewpoints that are below zero degrees F.  If we closely at Wendover we see that the ETA forecast has a surface dewpoint of around -12 F.  The actual observation at Wendover shows are dewpoint +10 F.  Now let’s look at the WRF for the same time frame.  In this case the WRF has a far superior forecast early in the cycle.  The 3 hour surface dewpoint forecast at ENV from the WRF is around +12F.  It’s error is only on the order of 2F while the error in the ETA is greater than 20F.  Think about the impacts that errors such as this would have on RH forecast, CAPE, obs database background fields etc.  It should be noted that the WRF should do a much better job depicting areas of enhanced dewpoints associated with ongoing precipitation at model initialization time and should do a better job with moisture gradients. 

Also note how well the WRF depicts the higher dewpoints around the Great Salt Lake. The WRF has consistently depicted considerably higher dewpoints around the GSL and in some cases they have been considerably too high.  That is something that we will have to watch. It should be noted that in every analysis that I have looked at the ETA was superior to the WRF in some locations while the WRF was superior to the ETA in others.  Subjectively, I have felt that the overall depiction of the surface fields has been superior in the WRF.

## Dewpoint/RH Issues

- Much has been made of late of the high bias in ETA Tds
- Fix to bug in ETA which affects how much water is evaporated by green vegetation should result in better low level Td/RH in the WRF
- However...still see high bias in WRF Td/RH in the low levels, particularly over high terrain

## Dewpoint/RH Issues

- Td/RH Issues in the WRF may be a result of AWIPS display issues (interpolating down to 2m)
- This is important with respect to AWIPS display and GFE Grid Initialization
- SLC looking into creating tools to utilize WRF Boundary Layer RH in GFE to improve initialization
- Overall surface Td/RH values in the WRF appeared to show some improvement over the ETA

## QPF Depiction

- WRF was considerably better with precipitation events in the first 12 hours of the run
- Early run precip/QPF improvements were particularly helpful in two advisory level snowfall events and a warning level lake effect snow event
- Beyond first twelve hours results were mixed
- Some events the WRF was better and for others the ETA was better
- Detail in the QPF from the WRF did add value
- Convective events did not show clear advantages for either the WRF or the ETA

## And the Answer is...

- WRF depicts considerably more detail in QPF fields which was useful
- However...QPF was greatly overdone across southwest Utah in this event

**ETA**

**GFS**

**WRF**

## Reality Ruins Another Great Model Forecast...

- WRF depicted the structure well...but significantly overdid QPF

**Notes:**
One thing that is noticeable is that the WRF displayed structure that mimicked reality as seen in the satellite and radar imagery above.  However, the amounts were drastically overdone.  While the WRF may indicate a great deal of realistic structure is does mean that it will produce an accurate forecast.  In this case in which the 48 hour WRF forecast with maximum amounts in excess of 1.5” the banded structure that it represented produce only light precipitation.  

## And the Answer Is...

- Snapshot from early May event for which ETA produces minimal QPF with FROPA
- WRF much more aggressive with precip coverage
- WRF better in this example...and was consistently better with overall depiction of QPF early in model runs

## Lake Effect Snow Event

- Dominant lake band event in mid April (18th) handled much better by the WRF when compared to the ETA
- WRF depicted narrow swath of heavy precip which coincided with actual band location
- WRF did not capture re-orientation of band in the latter portion of life cycle.
- Significantly better performance than ETA for this event

## Summary

- WRF tended to show improved surface winds
- Improved Lake Breeze/Up-valley wind initiation
- Overall better wind fields in well-mixed environments
- Mixed results with Surface Dewpoints
- Overall better than ETA but still issues with WRF values, particularly over the higher terrain
- Significantly improved handling of precipitation in the first 6-12 hours of model run
- Important contribution during several advisory/warning level events
- QPF beyond first 12 hours of model run was a mixed bag
- WRF did much better job depicting lake band event