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DiagnosticEvaluation(continued)Decatur,GARecall that previous diagnostics showed that peak ozone as well as NOx and VOC are overpredicted at Decatur in August (with the exception of a few days). CO is also overpredicted (see below).
Aug. 08 Aug. 15
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August 08 12Z
August 15 12Z
Modeled NOx in the morning is less dispersed on August 15 compared to August 8, probably due to less mechanical mixing (lighter winds). Morning NOx is trapped in a very shallow layer on both days. On August 8, there is less than 0.015 ppm NOx in Layer 5, which is capped at 160 m (see Table below). On August 16, the same is true for Layer 6, which is capped at 220 m.
If early morning PBL heights were higher, then NOx, VOC and CO peaks would agree better with the observations.
132470001 current location 132470001 actual location
132470001 (Decatur)
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The predicted PBL heights at Decatur are low compared to what the 12Z Peachtree City (FFC) soundings suggest both on August 8 and on August 15 (in terms of ozone, CMAQ performs better on this day). WRF predicts PBL heights of about 10 m while the soundings suggest 400 to 500 m around 07:00 EST.
The difference in NOx concentrations on these two days may be related to the winds. On August 15, surface WS (at 10 m) is almost zero in the morning and remain relatively small during the day.
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Table 1. Configuration of first 17 CMAQ vertical layers
Level Height (m) Depth
(m)
17 3504 880
16 2624 700
15 1924 552
14 1372 228
13 1144 200
12 944 174
11 770 150
10 620 128
9 492 108
8 384 90
7 294 74
6 220 60
5 160 48
4 112 38
3 74 30
2 44 24
1 20 20
5
August 08 12Z
August 08 12Z Aug 07 00Z
6
August 15 12Z
August 15 12Z Aug 14 00Z
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8
Nashville, TN Greensboro, NC
Birmingham, AL Atlanta, GA
Tampa Bay, FL Wallops Island, VA
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Based on the observations, we propose to set the minimum PBL height to 60 m for coastal areas and 200 m for all other areas (no urban-rural distinction).
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CMAQ Version Minimum PBL Lower Minimum Kzz Upper Minimum Kzz 4.7.1 300 m for urban
50 m for other 0.5 m2/s 2.0 m2/s
5.0 20 m 0.01 m2/s 1.0 m2/s
Kzz_min = Kzz_low + (Kzz_up – Kzz_low)*Urban_Fraction
Minimum Kzz for Decatur is 0.427 while for Conyers it is 0.023 (in the current cell; 0.01 for correct cell).
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Using a simple model, we can estimate a minimum value for Kzz that would match (approximately) modeled precursor concentrations (NOx, VOC, CO) with the observations. Fine tuning (i.e., a second sensitivity run) may be needed after the first trial.
Currently, precursors are trapped in the first layer, which is 20-m thick (recall that WRF predicted PBL heights are about 10 m in early morning). Even if we raise the minimum PBL height, with a Kzz of 0.01 m2/s, in 1 hour (3600 s) the precursors would only mix up to 26 m (20 + √0.01 × 3600). Suppose we want to reduce the precursor concentrations by a factor of 3. This can be achieved by mixing them in a 60-m thick layer (third model layer is 74 m high) instead of the current 20-m thick first model layer. This can be achieved by a Kzz of 0.45 m2/s (20 + √0.45 × 3600). Recall the recommended minimum PBL height is 60 m. Kzz_low and Kzz_up have to be adjusted to yield Kzz_min of 0.45 for the given location.
