TY - JOUR

T1 - Entrainment and mixing at the interface of shallow cumulus clouds

T2 - Results from a combination of observations and simulations

AU - Hoffmann, Fabian

AU - Siebert, Holger

AU - Schumacher, Jörg

AU - Riechelmann, Theres

AU - Katzwinkel, Jeannine

AU - Kumar, Bipin

AU - Götzfried, Paul

AU - Raasch, Siegfried

N1 - Funding sponsor: Deutsche Forschungsgemeinschaft Fundning number: SI 1534/3 Institution of Civil Engineers

PY - 2014/9/26

Y1 - 2014/9/26

N2 - This study combines observations, large-eddy simulations (LES), and direct numerical simulations (DNS) in order to analyze entrainment and mixing in shallow cumulus clouds at all relevant spatial scales and, additionally, to verify the results by the multiple methods used. The observations are based on three flights of the CARRIBA campaign which are similar to the classical BOMEX case used for LES. Virtual flights in the LES data are used to validate the observational method of line measurements. It is shown that line measurements overrepresent the cloud core, and it is quantified how derived statistics depend on small perturbations of the flight track, which has to be taken in account for the interpretation of airborne observations. A linear relation between fluctuations of temperature and liquid water content has been found in both LES and observations in a good quantitative agreement. However, the constant of proportionality deviates from purely adiabatic estimates, which can be attributed to cloud edge mixing. The cloud edge is compared in detail in observations and LES, which agree qualitatively although the LES cloud edge is smoother due to the model's resolution. The resulting typical amplitudes of the turbulence fields from this comparison are compared with the large-scale forcing model which is used in a series of DNS which study the mixing below the meter scale, which show that LES does not resolve the intermittency of small-scale turbulence.

AB - This study combines observations, large-eddy simulations (LES), and direct numerical simulations (DNS) in order to analyze entrainment and mixing in shallow cumulus clouds at all relevant spatial scales and, additionally, to verify the results by the multiple methods used. The observations are based on three flights of the CARRIBA campaign which are similar to the classical BOMEX case used for LES. Virtual flights in the LES data are used to validate the observational method of line measurements. It is shown that line measurements overrepresent the cloud core, and it is quantified how derived statistics depend on small perturbations of the flight track, which has to be taken in account for the interpretation of airborne observations. A linear relation between fluctuations of temperature and liquid water content has been found in both LES and observations in a good quantitative agreement. However, the constant of proportionality deviates from purely adiabatic estimates, which can be attributed to cloud edge mixing. The cloud edge is compared in detail in observations and LES, which agree qualitatively although the LES cloud edge is smoother due to the model's resolution. The resulting typical amplitudes of the turbulence fields from this comparison are compared with the large-scale forcing model which is used in a series of DNS which study the mixing below the meter scale, which show that LES does not resolve the intermittency of small-scale turbulence.

KW - Airborne observations

KW - Cumulus clouds

KW - Direct numerical simulations

KW - Entrainment and mixing

KW - Large-eddy simulations

UR - http://www.scopus.com/inward/record.url?scp=84925458208&partnerID=8YFLogxK

U2 - 10.1127/0941-2948/2014/0597

DO - 10.1127/0941-2948/2014/0597

M3 - Article

AN - SCOPUS:84925458208

VL - 23

SP - 349

EP - 368

JO - Meteorologische Zeitschrift

JF - Meteorologische Zeitschrift

SN - 0941-2948

IS - 4

ER -