TY - JOUR
T1 - Humidity variations across the edge of trade wind cumuli
T2 - Observations and dynamical implications
AU - Wang, Yonggang
AU - Geerts, Bart
N1 - Funding Information:
We appreciate the assistance of David Rogers and Allen Schanot from the National Center for Atmospheric Research with the use and interpretation of the C-130 variables. This research was funded by National Science Foundation Grants ATM-0342597 and ATM-0849225 .
PY - 2010/7
Y1 - 2010/7
N2 - Aircraft data are used to analyze the composite horizontal structure of shallow tropical maritime cumulus clouds across the cloud edge into the ambient clear air. The emphasis is on humidity variations, and their implications for cumulus dynamics. The Lyman-α humidity probe has the required fast response and is unaffected by wetting in-cloud. On average the water vapor mixing ratio increases gradually from the clear air towards the cloud edge, and air is often sub-saturated in the outer fringe of the cloud, implying that droplets are evaporating. Similarly, conserved variables such as the total water concentration and the wet equivalent potential temperature gradually transition in the "margin" of cumulus clouds. The gradual change of water vapor mixing ratio and conserved variables across the cloud edge highlights the significance of lateral entrainment and detrainment, and it reveals a characteristic penetration depth of mixing eddies of 10 to 15% of the cloud diameter, or about 50. m.An ~100 m wide region just outside the cloud is generally characterized by negatively buoyant, sinking air. The excess water vapor in this region, also documented in several recent studies, confirms that the negative buoyancy is caused by evaporative cooling in the cloud margin. Although rather weak, this cooling appears strong enough to evoke a dynamical response, even in the relative small trade wind cumuli.
AB - Aircraft data are used to analyze the composite horizontal structure of shallow tropical maritime cumulus clouds across the cloud edge into the ambient clear air. The emphasis is on humidity variations, and their implications for cumulus dynamics. The Lyman-α humidity probe has the required fast response and is unaffected by wetting in-cloud. On average the water vapor mixing ratio increases gradually from the clear air towards the cloud edge, and air is often sub-saturated in the outer fringe of the cloud, implying that droplets are evaporating. Similarly, conserved variables such as the total water concentration and the wet equivalent potential temperature gradually transition in the "margin" of cumulus clouds. The gradual change of water vapor mixing ratio and conserved variables across the cloud edge highlights the significance of lateral entrainment and detrainment, and it reveals a characteristic penetration depth of mixing eddies of 10 to 15% of the cloud diameter, or about 50. m.An ~100 m wide region just outside the cloud is generally characterized by negatively buoyant, sinking air. The excess water vapor in this region, also documented in several recent studies, confirms that the negative buoyancy is caused by evaporative cooling in the cloud margin. Although rather weak, this cooling appears strong enough to evoke a dynamical response, even in the relative small trade wind cumuli.
UR - http://www.scopus.com/inward/record.url?scp=77953811291&partnerID=8YFLogxK
U2 - 10.1016/j.atmosres.2010.03.017
DO - 10.1016/j.atmosres.2010.03.017
M3 - Article
AN - SCOPUS:77953811291
VL - 97
SP - 144
EP - 156
JO - Atmospheric Research
JF - Atmospheric Research
SN - 0169-8095
IS - 1-2
ER -