122
Air Pollution and Turbulence: Modeling and Applications
of thermals overshooting. Note that recent ED schemes were developed based on
the TKE prognostic equation and a defi nition of the mixing length that involves the
square root of TKE and a timescale (Cheinet and Teixeira 2003; Teixeira and Cheinet
2004). This type of parameterization can produce a realistic top-entrainment, but the
instability in the lower BL (although small) still persists.
The analysis of the performance of the EDMF scheme for the cumulus diurnal
cycle also shows good results. The onset, dissipation time, and cloud cover of cumulus clouds are all well captured. The same applies to the vertically integrated quantities, namely, the LWP. Nevertheless, there are still some unsolved problems in the
cloud layer properties, requiring further research. While the EDMF scheme seems
to lead to realistic potential temperature profi les, there remain some discrepancies
in the mean humidity profi le and signifi cant problems in the momentum profi les. As
the momentum balance was not modifi ed in the EMDF scheme presented here, it
may well be that a nonlocal effect needs to be added to those equations.
It is worthwhile to emphasize that a rather simple entraining parcel model is used
to diagnose the different properties of the ensemble of updrafts, including the mean
profi le of vertical velocity and the condensation level. This was found to provide the
necessary information for the computation of the MF contribution. The cloud base
MF closure allows for a direct link between the convective state of the sub-cloud
layer and the cloud layer. This is based on the concept that clouds are actually the
visible extensions of thermals that penetrate into the stable layer.
Finally, an important operational advantage of the EDMF scheme is associated with
a signifi cant reduction in model computational costs in moist conditions. In the dry BL
case, the inclusion of the EDMF scheme in the MesoNH model implied an increase in
its computational cost of about 7%, when compared to the standard version of the same
model with a TKE scheme. However, in the shallow moist convection case, where the
standard MesoNH also uses the shallow convection scheme of Bechtold et al. (2001),
EDMF leads to a substantial reduction in computational cost to 30% of its standard
value. This important reduction leaves room for further improvements of the code.
As mentioned, EDMF approaches represent a step toward the unifi cation of turbulence and convection parameterizations in weather and climate prediction models.
Hopefully, it will also allow for a better understanding of the physical processes
associated with the CBL.
ACKNOWLEDGMENTS
The authors are grateful to the editors for the opportunity to write this chapter and
for their encouragement in all the writing process. This work has been supported
by Fundação para a Ciência e Tecnologia (FCT) under project REWRITE, PTDC/
CLI/73814/2006, cofi nanced by the EU under Program FEDER.
REFERENCES
Ackerman, S. A. and S. K. Cox. 1981. Aircraft observations of the shortwave fractional absorptance of non-homogeneous clouds. J. Appl. Meteorol. 20:1510–1515.
Agee, E. M. 1984. Observations from space and thermal convection: A historical perspective.
Bull. Amer. Meteorol. Soc. 65:938–949.
© 2010 by Taylor and Francis Group, LLC
Air Pollution and Turbulence: Modeling and Applications
of thermals overshooting. Note that recent ED schemes were developed based on
the TKE prognostic equation and a defi nition of the mixing length that involves the
square root of TKE and a timescale (Cheinet and Teixeira 2003; Teixeira and Cheinet
2004). This type of parameterization can produce a realistic top-entrainment, but the
instability in the lower BL (although small) still persists.
The analysis of the performance of the EDMF scheme for the cumulus diurnal
cycle also shows good results. The onset, dissipation time, and cloud cover of cumulus clouds are all well captured. The same applies to the vertically integrated quantities, namely, the LWP. Nevertheless, there are still some unsolved problems in the
cloud layer properties, requiring further research. While the EDMF scheme seems
to lead to realistic potential temperature profi les, there remain some discrepancies
in the mean humidity profi le and signifi cant problems in the momentum profi les. As
the momentum balance was not modifi ed in the EMDF scheme presented here, it
may well be that a nonlocal effect needs to be added to those equations.
It is worthwhile to emphasize that a rather simple entraining parcel model is used
to diagnose the different properties of the ensemble of updrafts, including the mean
profi le of vertical velocity and the condensation level. This was found to provide the
necessary information for the computation of the MF contribution. The cloud base
MF closure allows for a direct link between the convective state of the sub-cloud
layer and the cloud layer. This is based on the concept that clouds are actually the
visible extensions of thermals that penetrate into the stable layer.
Finally, an important operational advantage of the EDMF scheme is associated with
a signifi cant reduction in model computational costs in moist conditions. In the dry BL
case, the inclusion of the EDMF scheme in the MesoNH model implied an increase in
its computational cost of about 7%, when compared to the standard version of the same
model with a TKE scheme. However, in the shallow moist convection case, where the
standard MesoNH also uses the shallow convection scheme of Bechtold et al. (2001),
EDMF leads to a substantial reduction in computational cost to 30% of its standard
value. This important reduction leaves room for further improvements of the code.
As mentioned, EDMF approaches represent a step toward the unifi cation of turbulence and convection parameterizations in weather and climate prediction models.
Hopefully, it will also allow for a better understanding of the physical processes
associated with the CBL.
ACKNOWLEDGMENTS
The authors are grateful to the editors for the opportunity to write this chapter and
for their encouragement in all the writing process. This work has been supported
by Fundação para a Ciência e Tecnologia (FCT) under project REWRITE, PTDC/
CLI/73814/2006, cofi nanced by the EU under Program FEDER.
REFERENCES
Ackerman, S. A. and S. K. Cox. 1981. Aircraft observations of the shortwave fractional absorptance of non-homogeneous clouds. J. Appl. Meteorol. 20:1510–1515.
Agee, E. M. 1984. Observations from space and thermal convection: A historical perspective.
Bull. Amer. Meteorol. Soc. 65:938–949.
© 2010 by Taylor and Francis Group, LLC