70
Air Pollution and Turbulence: Modeling and Applications
4.5 Boundary Layer Cloud Parameterizations ................................................... 107
4.5.1 Introduction ...................................................................................... 107
4.5.2 The Slingo Diagnostic Cloud Scheme .............................................. 108
4.5.3 PDF-Based Cloud Parameterizations and a Simple
Iterative Version ................................................................................ 109
4.5.4 A Prognostic Cloud Fraction Parameterization and
a Steady-State Version for Boundary Layer Cumulus ...................... 112
4.6 EDMF Parameterization of Shallow Cumulus ............................................. 113
4.6.1 Introduction ...................................................................................... 113
4.6.2 EDMF Results .................................................................................. 115
4.6.2.1 Dry BL ............................................................................... 115
4.6.2.2 Shallow Cumulus BL ......................................................... 117
4.7 Final Remarks ............................................................................................... 121
Acknowledgments .................................................................................................. 122
References .............................................................................................................. 122
4.1 INTRODUCTION
Earth’s climate system is largely controlled by the transfer of properties across different interfaces. For this reason, boundary layers, which occupy a very small fraction
of the atmosphere and ocean, are disproportionably important. Atmosphere-ocean
and atmosphere-land fl uxes of water, momentum, and energy are mediated by these
boundary layers.
Planetary boundary layer (BL) fl uxes occur at different scales. At very small
scales, smaller than 1 mm, fl uxes are due to molecular diffusion. At larger scales, we
enter the domain of turbulence. The larger BL turbulent structures, which occupy
its full depth up to more than 1 km, may organize themselves in BL convective systems and lead to BL convective clouds. Clouds profoundly modify the surface fl uxes,
interfering not only in the vertical fl uxes of heat and moisture but also in the radiative fl uxes, shortwave and longwave.
Clouds play an important role in the global climate. Stratocumulus and cumulus,
prevalent BL clouds in subtropical latitudes, seem to have a controlling effect in the
atmospheric circulation at the synoptic scale (Philander et al. 1996; Siebesma 1998;
Larson et al. 1999). Any good climate or numerical weather prediction model heavily relies on one or more parameterization schemes, representing the effects of these
subgrid scale clouds (Tiedtke 1987).
Due to the special nature of cloud processes, qualitatively different from other
turbulent fl uxes, such as those involved in dry convection over land, it is not surprising that many atmospheric models use different schemes to deal with saturated and
nonsaturated BLs. However, such procedure implies the switching on and off of the
different schemes, with some sort of discontinuity in model behavior. On the other
hand, results of current parameterization schemes are often poor (e.g., Ayotte et al.
1996; Lenderink et al. 2004).
In this chapter, a short review of the main parameterizations for BL turbulence
and clouds is presented. These are at the base of the development of a set of unifi ed
parameterizations for BL turbulence and convection, applicable to the convective
© 2010 by Taylor and Francis Group, LLC
Air Pollution and Turbulence: Modeling and Applications
4.5 Boundary Layer Cloud Parameterizations ................................................... 107
4.5.1 Introduction ...................................................................................... 107
4.5.2 The Slingo Diagnostic Cloud Scheme .............................................. 108
4.5.3 PDF-Based Cloud Parameterizations and a Simple
Iterative Version ................................................................................ 109
4.5.4 A Prognostic Cloud Fraction Parameterization and
a Steady-State Version for Boundary Layer Cumulus ...................... 112
4.6 EDMF Parameterization of Shallow Cumulus ............................................. 113
4.6.1 Introduction ...................................................................................... 113
4.6.2 EDMF Results .................................................................................. 115
4.6.2.1 Dry BL ............................................................................... 115
4.6.2.2 Shallow Cumulus BL ......................................................... 117
4.7 Final Remarks ............................................................................................... 121
Acknowledgments .................................................................................................. 122
References .............................................................................................................. 122
4.1 INTRODUCTION
Earth’s climate system is largely controlled by the transfer of properties across different interfaces. For this reason, boundary layers, which occupy a very small fraction
of the atmosphere and ocean, are disproportionably important. Atmosphere-ocean
and atmosphere-land fl uxes of water, momentum, and energy are mediated by these
boundary layers.
Planetary boundary layer (BL) fl uxes occur at different scales. At very small
scales, smaller than 1 mm, fl uxes are due to molecular diffusion. At larger scales, we
enter the domain of turbulence. The larger BL turbulent structures, which occupy
its full depth up to more than 1 km, may organize themselves in BL convective systems and lead to BL convective clouds. Clouds profoundly modify the surface fl uxes,
interfering not only in the vertical fl uxes of heat and moisture but also in the radiative fl uxes, shortwave and longwave.
Clouds play an important role in the global climate. Stratocumulus and cumulus,
prevalent BL clouds in subtropical latitudes, seem to have a controlling effect in the
atmospheric circulation at the synoptic scale (Philander et al. 1996; Siebesma 1998;
Larson et al. 1999). Any good climate or numerical weather prediction model heavily relies on one or more parameterization schemes, representing the effects of these
subgrid scale clouds (Tiedtke 1987).
Due to the special nature of cloud processes, qualitatively different from other
turbulent fl uxes, such as those involved in dry convection over land, it is not surprising that many atmospheric models use different schemes to deal with saturated and
nonsaturated BLs. However, such procedure implies the switching on and off of the
different schemes, with some sort of discontinuity in model behavior. On the other
hand, results of current parameterization schemes are often poor (e.g., Ayotte et al.
1996; Lenderink et al. 2004).
In this chapter, a short review of the main parameterizations for BL turbulence
and clouds is presented. These are at the base of the development of a set of unifi ed
parameterizations for BL turbulence and convection, applicable to the convective
© 2010 by Taylor and Francis Group, LLC