Radiation in fog: quantification of the impact on fog liquid water based on ground-based remote sensing
Radiative cooling and heating impact the liquid water balance of fog and therefore play an important role in determining their persistence or dissipation. We demonstrate that a quantitative analysis of the radiation-driven condensation and evaporation is possible in real time using ground-based...
| Main Authors: | , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-09-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/10811/2017/acp-17-10811-2017.pdf |
| Summary: | Radiative cooling and heating impact the liquid water
balance of fog and therefore play an important role in determining their
persistence or dissipation. We demonstrate that a quantitative analysis of
the radiation-driven condensation and evaporation is possible in real time
using ground-based remote sensing observations (cloud radar, ceilometer,
microwave radiometer). Seven continental fog events in midlatitude winter
are studied, and the radiative processes are further explored through
sensitivity studies. The longwave (LW) radiative cooling of the fog is able
to produce 40–70 g m<sup>−2</sup> h<sup>−1</sup> of liquid water by condensation when
the fog liquid water path exceeds 30 g m<sup>−2</sup> and there are no clouds
above the fog, which corresponds to renewing the fog water in 0.5–2 h.
The variability is related to fog temperature and atmospheric humidity, with
warmer fog below a drier atmosphere producing more liquid water. The
appearance of a cloud layer above the fog strongly reduces the LW cooling
relative to a situation with no cloud above; the effect is strongest for a
low cloud, when the reduction can reach 100 %. Consequently, the appearance
of clouds above will perturb the liquid water balance in the fog and may
therefore induce fog dissipation. Shortwave (SW) radiative heating by
absorption by fog droplets is smaller than the LW cooling, but it can
contribute significantly, inducing 10–15 g m<sup>−2</sup> h<sup>−1</sup> of
evaporation in thick fog at (winter) midday. The absorption of SW radiation
by unactivated aerosols inside the fog is likely less than 30 % of the SW
absorption by the water droplets, in most cases. However, the aerosols may
contribute more significantly if the air mass contains a high concentration
of absorbing aerosols. The absorbed radiation at the surface can reach
40–120 W m<sup>−2</sup> during the daytime depending on the fog thickness. As in situ
measurements indicate that 20–40 % of this energy is transferred to the
fog as sensible heat, this surface absorption can contribute significantly to
heating and evaporation of the fog, up to 30 g m<sup>−2</sup> h<sup>−1</sup> for thin
fog, even without correcting for the typical underestimation of turbulent
heat fluxes by the eddy covariance method. Since the radiative processes
depend mainly on the profiles of temperature, humidity and clouds, the results
of this paper are not site specific and can be generalised to fog under
different dynamic conditions and formation mechanisms, and the methodology
should be applicable to warmer and moister climates as well. The retrieval
of approximate emissivity of clouds above fog from cloud radar should be
further developed. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |