**Summary: NAM-Y Impact on AQ Forecasts**

NOAA's AQ team evaluated the NAM experimental (NAM-Y) Summer 2007
retrospective forecast simulations that were run with NDAS cycling for
the AQ retrospective period (Aug. 5-18, 2007). High ozone events were
observed over most parts of the CONUS during this period. NAM-Y was
evaluated for low-level and boundary layer parameters to which
photochemical processes are especially sensitive. Weather fields shown
below were plotted on the EMC AQ web page:
<http://www.emc.ncep.noaa.gov/mmb/aq>. Also summarized below are
additional weather information from EMC's land-surface group, and
information on comparisons of predicted ozone concentrations.

+-------------------+--------------------+----------------------------+
| Fields Compared.  | Comparison ; NAM-Y | Features: especially SoCal |
| August 5-18, 2007 | vs. NAM            |                            |
|                   | (CONUS-wide)       |                            |
+-------------------+--------------------+----------------------------+
| -   3 hourly 2m & | 2m: slightly       | So. Cal: slightly cooler   |
|     850 mb        | warmer 0.1 deg C   | offshore and along coast;  |
|                   |                    | during episode 2-4 deg C   |
| -   temperatures, | Slightly weaker    | warmer.                    |
|                   |                    |                            |
| -   winds         | Slightly drier     | 850 m: regionally mixed:   |
|                   |                    | slightly warmer 0.5 deg C  |
| -   relative      |                    | (introducing bias E, W     |
|     humidity      |                    | regions); slightly cooler  |
|                   |                    | (0.2 -0.4 deg C) for       |
|                   |                    | sub-regions in SW, Gulf    |
|                   |                    | Mexico) reducing warm bias |
|                   |                    |                            |
|                   |                    | Slightly weaker            |
+-------------------+--------------------+----------------------------+
| -   Surface       | increased          | S. Cal: increased downward |
|     longwave,     |                    | shortwave inland and       |
|     shortwave,    |                    | during ozone episode       |
|     sensible and  |                    |                            |
|     latent heat   |                    | Elsewhere: Increased       |
|     fluxes        |                    | locally ahead of slower    |
|                   |                    | front and Tropical Storm   |
|                   |                    | Erin                       |
+-------------------+--------------------+----------------------------+
| -   Planetary     | Higher over        | Generally lower over       |
|     boundary      | Eastern and        | Pacific Coast by \~200 m   |
|     layer heights | Central US by      |                            |
|                   | 200-400m;          |                            |
|                   | increases high     |                            |
|                   | bias               |                            |
+-------------------+--------------------+----------------------------+
| -   Cloud cover   | Slightly drier     | During Aug 14-15 episode:  |
|     and cloud     |                    | drier, lower cloud water   |
|     water         |                    |                            |
+-------------------+--------------------+----------------------------+
| -   Surface ozone | Increase in bias   | Pacific Coast area, pos.   |
|     conc.: daily  | \~ 2ppb            | bias is reduced            |
|     max 8-hr avg  |                    |                            |
|                   |                    | Spatial patterns of        |
|                   |                    | elevated O~3~ downwind of  |
|                   |                    | LA basin show lower peak   |
|                   |                    | O~3~ concentration: on     |
|                   |                    | some days (Aug 14-17) this |
|                   |                    | improves agreement with    |
|                   |                    | observation                |
+-------------------+--------------------+----------------------------+
| -   Frontal       | Appearance of TS   | TS. Erin: longer           |
|     passage       | Erin and one cold  | ozone-building episode     |
|                   | front are slower   | prior to arrival off Gulf  |
|                   | by \~12 hours.     | Coast, raises high bias    |
|                   |                    | near Houston.              |
|                   | O~3~ enrichment    |                            |
|                   | associated with    |                            |
|                   | advancing cold     |                            |
|                   | fronts is more     |                            |
|                   | pronounced: 5ppb   |                            |
|                   | in NE, 1-2 ppb in  |                            |
|                   | SE                 |                            |
+-------------------+--------------------+----------------------------+
| -   Canopy        | Generally NAM-Y    | Large differences in both  |
|     Conductance   | canopy conductance | directions seem associated |
|                   | is slightly lower  | with differences in clouds |
|                   | in East and very   | and precipitation          |
|                   | slightly higher in |                            |
|                   | west, and much     |                            |
|                   | lower in eastern   |                            |
|                   | Canada.            |                            |
+-------------------+--------------------+----------------------------+
| -   Dry           | Generally slight   |                            |
|     deposition    | decrease in V~d~   |                            |
|     velocity      | for ozone caused   |                            |
|     (V~d~) for    | mostly by slight   |                            |
|     ozone         | increase in        |                            |
|                   | aerodynamic        |                            |
|                   | resistance. This   |                            |
|                   | decrease in V~d~   |                            |
|                   | could contribute   |                            |
|                   | to slight increase |                            |
|                   | in ambient O~3~    |                            |
+-------------------+--------------------+----------------------------+

Domain-wide comparisons of NAM and NAM-Y were supplemented with
comparisons for sub-regions, with emphasis on challenging areas for O3
predictions: the LA basin, and Central California valley and Gulf Coast
regions. For the Pacific region, NAM-Y low level winds were slightly
weaker especially on the windward side of the mountains probably in
response to the new gravity wave processes introduced. Lighter winds may
help CMAQ forecasts in the California Central Valley where ozone is
often under-predicted. NAM-Y temperatures were cooler off-shore and
along the Northern and Central California Coast, possible because the
marine boundary layer and resulting cloud cover in NAM-Y was able to
move on-shore earlier. However, during the very stagnant and high ozone
event of Aug. 14-15, 2007, the NAM-Y forecasts were much warmer (as much
as 2-4 deg C) with less cloud water especially inland and more downward
shortwave radiation in Southern California. The changes in cloud water
and cloud cover may have an impact on CMAQ photolysis and aqueous phase
chemical reactions.

NOAA's AQ team noted that NAM-Y - CMAQ ozone was over-predicted ahead of
the cold front that affected the Midwest and North East from Aug
12^th^-15^th^. Warm temperature biases for the [Northeast
sub-region](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/TSFC_cmb36_meanSEC.gif)
were worsened by about 1 deg C as the cold front approached with NAM-Y,
implying that its movement and associated cloud cover was too slow.
[Over the
Midwest](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/TSFC_cmb36_meanLMV.gif),
the NAM-Y temperature errors were too warm on Aug 14 and 16^th^ by about
0.5 deg C. No consistent differences in winds were noted for these
regions.

The EMC land surface group performed a quantitative evaluation of NAM-Y
2m temperature, RH and 10 m winds for August 1-31, 2007 over the larger
Western and Eastern US sub-regions. Overall differences for these large
domains were small with both areas showing slightly warmer and dryer
conditions. 10 m wind biases were increased in the East but little
impact was noted in the west. At upper levels, EMC noted that [warm
biases of about +0.5 deg.
C](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/TUPA_cmb36_mean.gif)
were introduced for the Eastern US at 850 mb and for the Western US at
850 and 700 mb. [RH moist
biases](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/RHUPA_cmb36_mean.gif)
were also reduced slightly (3-5%) for the Eastern US at these levels.

EMC then produced sub-region statistics for the Southwest and Gulf of
Mexico regions. For the Southwest coastal region that includes Central
and Southern California, the [warm biases were
reduced](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/TSFC_cmbDIU_meanGMC.gif)
by around 0.2-0.4 degrees C and the [RH dry
bias](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/RHSFC_cmbDIU_meanGMC.gif)
was improved slightly. Wind errors were similar. Over the Gulf or
Mexico, the impact was small for temperature and RH, but the low wind
bias was made slightly worse. However, the [day-time temperature
error](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/TSFC_cmb36_meanGMC.gif)
increased by about 1 degree C from the 16^th^ -- 18^th^ during the time
that TS Erin was affecting the Gulf. [Low wind
biases](http://wwwt.emc.ncep.noaa.gov/mmb/aq/verif/namy/VWNDSFC_cmb36_meanGMC.gif)
were also worsened by about 1 m/s during this time.

In addition, the AQ group quantitatively evaluated the predicted NAM-Y
PBL height for the larger sub-regions. NAM-Y TKE derived boundary layer
heights in the late afternoon were generally higher over the Eastern and
Central U.S. by as much as 200-400 m on average. These characteristics
result in larger boundary layer height errors since the operational NAM
boundary layer heights were already overestimated for this period.
However, the impact on PBL heights generated by the bulk Richardson
number method was smaller. For the Western U.S. NAM-Y did not exhibit
any bias except along the Pacific Coast where boundary layer heights
were generally lower for San Diego. These patterns are in agreement with
the cooler, cloudier conditions noted above for this coastal region.

Surface temperature and relative humidity forecast errors are shown
below for NAM (black line) and NAM-Y (red line) over the CONUS

![](media/image1.png){width="5.5in" height="3.772222222222222in"}

![](media/image2.png){width="5.502083333333333in"
height="4.258333333333334in"}

Surface temperature and relative humidity forecast errors are shown
below for NAM (black & blue lines) and NAM-Y (= NAM-E; red & green
lines) over the SW coast and Gulf of Mexico regions respectively.

![](media/image3.png){width="6.125in" height="4.59375in"}

![](media/image4.png){width="6.120833333333334in"
height="0.4395833333333333in"}![](media/image4.png){width="6.170833333333333in"
height="3.404166666666667in"}Comparisons of predicted ozone are shown
below. ![](media/image5.wmf){width="6.248611111111111in"
height="5.15625in"}

![](media/image6.png){width="5.375in"
height="4.03125in"}![](media/image7.png){width="5.375in"
height="4.03125in"}