Evaluation of the boundary layer dynamics of the TM5 model over Europe
We evaluate the capability of the global atmospheric transport model TM5 to simulate the boundary layer dynamics and associated variability of trace gases close to the surface, using radon (<sup>222</sup>Rn). Focusing on the European scale, we compare the boundary layer height (BLH) in t...
| Main Authors: | , , , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-09-01
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| Series: | Geoscientific Model Development |
| Online Access: | http://www.geosci-model-dev.net/9/3137/2016/gmd-9-3137-2016.pdf |
| Summary: | We evaluate the capability of the global atmospheric transport model TM5 to
simulate the boundary layer dynamics and associated variability of trace
gases close to the surface, using radon (<sup>222</sup>Rn). Focusing on the
European scale, we compare the boundary layer height (BLH) in the TM5 model
with observations from the National Oceanic and Atmospheric Admnistration
(NOAA) Integrated Global
Radiosonde Archive (IGRA) and also with ceilometer and
lidar (light detection and ranging) BLH
retrievals at two stations. Furthermore, we compare TM5 simulations of
<sup>222</sup>Rn activity concentrations, using a novel, process-based <sup>222</sup>Rn
flux map over Europe (Karstens et al., 2015), with harmonised <sup>222</sup>Rn
measurements at 10 stations.
<br><br>
The TM5 model reproduces relatively well the daytime BLH (within 10–20 %
for most of the stations), except for coastal sites, for which differences
are usually larger due to model representation errors. During night,
however, TM5 overestimates the shallow nocturnal BLHs, especially for the
very low observed BLHs (< 100 m) during summer.
<br><br>
The <sup>222</sup>Rn activity concentration simulations based on the new
<sup>222</sup>Rn flux map show significant improvements especially regarding the
average seasonal variability, compared to simulations using constant
<sup>222</sup>Rn fluxes. Nevertheless, the (relative) differences between
simulated and observed daytime minimum <sup>222</sup>Rn activity concentrations
are larger for several stations (on the order of 50 %) than the (relative)
differences between simulated and observed BLH at noon. Although the
nocturnal BLH is often higher in the model than observed, simulated
<sup>222</sup>Rn nighttime maxima are actually larger at several continental
stations. This counterintuitive behaviour points to potential deficiencies
of TM5 to correctly simulate the vertical gradients within the nocturnal
boundary layer, limitations of the <sup>222</sup>Rn flux map, or issues related to
the definition of the nocturnal BLH.
<br><br>
At several stations the simulated decrease of <sup>222</sup>Rn activity
concentrations in the morning is faster than observed. In addition, simulated
vertical <sup>222</sup>Rn activity concentration gradients at Cabauw decrease
faster than observations during the morning transition period, and are in
general lower than observed gradients during daytime. Although these effects
may be partially due to the slow response time of the radon detectors, they
clearly point to too fast vertical mixing in the TM5 boundary layer during
daytime. Furthermore, the capability of the TM5 model to simulate the diurnal
BLH cycle is limited by the current coarse temporal resolution (3 h/6 h) of
the TM5 input meteorology. |
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| ISSN: | 1991-959X 1991-9603 |