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| Titel |
Air dynamics near the ground surface and convective aerosol emission |
| VerfasserIn |
E. Gledzer, I. Granberg, O. Chkhetiani |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026374
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| Zusammenfassung |
| Field measurements conducted in a Caspian desert and estimates obtained for fluid
dynamic parameters in the viscous thermal boundary layer near the ground surface are
used to derive asymptotes for the mass concentration of fine aerosols. The external
parameters in the problem are the friction velocity and the temperature drop across the
thermal boundary layer. A porous soil layer model with air dynamics governed by the
Darcy equation is used as a possible mechanism of aerosol emission. The critical
parameters are estimated at which the thermal rolling of sand particles takes place in soil
pores.
The underlying hypothesis in this work is that fine aerosol emission from the soil is
proportional to the horizontal air velocity uT at the height of the thermal boundary
layer. In addition to the obvious simplicity of this hypothesis, another supporting
argument is that it implies Bagnold’s law u*3 for relatively high friction velocities:
an increase in uT leads to emission of not only fine aerosols but also larger soil
particles that satisfy this law. However, it should be noted that this empirical law
holds when u* is much higher than the threshold values ~ 0.4-0.5 m/s. The thermal
factors then become not very significant, and sand and aerosol are carried away by
strong turbulent velocity fluctuations ensuring the rolling and saltation of numerous
particles at the ground surface. In this work, primary attention was given to the thermal
factors at relatively low friction velocities associated with the mean wind shear.
A fine aerosol fraction was separated in the soil, which was treated as a porous
medium.
For low and moderate friction velocities u*, formulas show that, as δT grows, uT
increases with an exponent a ranging from 1/2 to 2/3. For large u* as δT grows, uT falls off
like δT-1-2, which is fairly similar to the measured dependencies for the aerosol
concentration. Based on the permeable medium model, we estimate the threshold values of
δT at which sand particles in the ground begin to move. The corresponding formulas show
that the convective emission of fine aerosols in no wind or light wind can be more effective
than in moderate wind.
The investigation was fulfilled under the support of RFBR grant No. 08-05-00942. |
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