Su, C.
1996
Modeling Entrainment and Fine-Scale Mixing in Cumulus Clouds
Krueger, S.K.; Su, C. and McMurtry, P.A.
J. Atmos. Sci, (in press), 1996. Funded by National Science Foundation and Office of Naval Research.
The model used by Krueger (1993) to study entrainment and mixing of thermodynamic properties in the stratus-topped boundary layer has been extended to study these processes in cumulus clouds. The new model, called the "explicit mixing parcel model" (EMPM), represents the fine-scale internal structure of a rising thermal in a cumulus cloud using 1D domain. The internal structure evolves in the EMPM as a consequence of a sequence of discrete entrainment events and an explicit representation of turbulent mixing based on Kerstein's (1988) linear eddy model. In this version of the EMPM, a simple parameterization is used to determine the local condensation or evaporation rates. However, the EMPM can incorporate a droplet growth model to allow prediction of droplet spectra evolution.
The EMPM was used to predict the characteristics of Hawaiian cumulus clouds observed by RAGA, et. Al. (1990). All of these quantities required by the EMPM, except for the entrained blob size were obtained form the observations. Profiles of in-cloud mean and variances of thermodynamic properties calculated by the EMPM for entrained blob sized of 50 m, 100 m, and 200 m and by a parcel model with instantaneous mixing were compared to those both mixing representations, but the observed mean liquid water mixing ratio and buoyancy profiles and all of the observed variance profiles are better reproduced by the EMPM. The measurements were not accurate enough to allow further conclusions regarding the entrained blob size.
Additional results from the EMPM suggest that the characteristic entrained blob size may be more precisely determined from aircraft measurements of the clear-air segment size distribution. The model results also demonstrate that the fine-scale structure represented by the EMPM's 1D domain can be directly compared to high-frequency aircraft measurements.
Su, C.
1996
Modeling Entrainment and Fine-Scale Mixing in Cumulus Clouds
Krueger, S.K.; Su, C. and McMurtry, P.A.
J. Atmos. Sci, (in press), 1996. Funded by National Science Foundation and Office of Naval Research.
The model used by Krueger (1993) to study entrainment and mixing of thermodynamic properties in the stratus-topped boundary layer has been extended to study these processes in cumulus clouds. The new model, called the "explicit mixing parcel model" (EMPM), represents the fine-scale internal structure of a rising thermal in a cumulus cloud using 1D domain. The internal structure evolves in the EMPM as a consequence of a sequence of discrete entrainment events and an explicit representation of turbulent mixing based on Kerstein's (1988) linear eddy model. In this version of the EMPM, a simple parameterization is used to determine the local condensation or evaporation rates. However, the EMPM can incorporate a droplet growth model to allow prediction of droplet spectra evolution.
The EMPM was used to predict the characteristics of Hawaiian cumulus clouds observed by RAGA, et. Al. (1990). All of these quantities required by the EMPM, except for the entrained blob size were obtained form the observations. Profiles of in-cloud mean and variances of thermodynamic properties calculated by the EMPM for entrained blob sized of 50 m, 100 m, and 200 m and by a parcel model with instantaneous mixing were compared to those both mixing representations, but the observed mean liquid water mixing ratio and buoyancy profiles and all of the observed variance profiles are better reproduced by the EMPM. The measurements were not accurate enough to allow further conclusions regarding the entrained blob size.
Additional results from the EMPM suggest that the characteristic entrained blob size may be more precisely determined from aircraft measurements of the clear-air segment size distribution. The model results also demonstrate that the fine-scale structure represented by the EMPM's 1D domain can be directly compared to high-frequency aircraft measurements.