Improving Parameterization of Entrainment Rate for Shallow Convection with Aircraft Measurements and Large-Eddy Simulation

Lu, Chen; Liu, Yangang; Zhang, Guang J.; Wu, Xianghua; Endo, Satoshi; Cao, Le; Li, Yueqing; Guo, Xiaohao

doi:10.1175/jas-d-15-0050.1

Cited by 48 publications

(53 citation statements)

References 49 publications

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“…Entrainment has impacts on updrafts as well. Recent studies using in situ measurements and model simulations suggest that stronger entrainment may result in weaker updrafts (e.g., Lu et al, 2016). In this study, we also find that weaker updrafts are associated with stronger entrainment-detrainment using in situ measurements of relative humidity, equivalent potential temperature, droplet concentration, and LWC (not shown).…”

Section: Discussionsupporting

confidence: 81%

Characteristics of vertical air motion in isolated convective clouds

et al. 2016

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Abstract. The vertical velocity and air mass flux in isolated convective clouds are statistically analyzed using aircraft in situ data collected from three field campaigns: High-Plains Cumulus (HiCu) conducted over the midlatitude High Plains, COnvective Precipitation Experiment (COPE) conducted in a midlatitude coastal area, and Ice in Clouds Experiment-Tropical (ICE-T) conducted over a tropical ocean. The results show that small-scale updrafts and downdrafts ( < 500 m in diameter) are frequently observed in the three field campaigns, and they make important contributions to the total air mass flux. The probability density functions (PDFs) and profiles of the observed vertical velocity are provided. The PDFs are exponentially distributed. The updrafts generally strengthen with height. Relatively strong updrafts (> 20 m s −1 ) were sampled in COPE and ICE-T. The observed downdrafts are stronger in HiCu and COPE than in ICE-T. The PDFs of the air mass flux are exponentially distributed as well. The observed maximum air mass flux in updrafts is of the order 10 4 kg m −1 s −1 . The observed air mass flux in the downdrafts is typically a few times smaller in magnitude than that in the updrafts. Since this study only deals with isolated convective clouds, and there are many limitations and sampling issues in aircraft in situ measurements, more observations are needed to better explore the vertical air motion in convective clouds.

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Section: Discussionsupporting

confidence: 81%

Characteristics of vertical air motion in isolated convective clouds

et al. 2016

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“…As shown in Figure a, w in the cloud cores ( w c ) is negatively correlated with RH. As discussed in Neggers et al () and Lu et al (), entrainment rate is larger for smaller w c . Therefore, higher RH corresponds to smaller w c , and then larger λ mix .…”

Section: Physical Mechanisms For the Positive Correlations Between λMmentioning

confidence: 63%

“…A Rosemount probe was used to measure temperature ( T ), and the Diode Laser Hygrometer was used to measure water vapor mixing ratio ( q v ; Podolske et al, ). Temperature is empirically corrected using liquid water content (LWC; Lu et al, ). Several microphysical, thermodynamic, and dynamical criteria are used to select nondrizzling growing cumulus clouds (Lu et al, ); one important criterion is that

B = \frac{T_{vc} - T_{ve}}{T_{ve}} g > 0.005 normalm / s^{2},

where g is the acceleration of gravity and T v is virtual temperature (Wallace & Hobbs, ):

T_{v} = T ((), 1 + 0.608 q_{v}) .

…”

Section: Data and Approachesmentioning

confidence: 99%

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Observational Relationship Between Entrainment Rate and Environmental Relative Humidity and Implications for Convection Parameterization

Sun

Liu

et al. 2018

Geophysical Research Letters

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Entrainment rate is a critical but highly uncertain quantity in convective parameterizations; especially, the effects of environmental relative humidity on entrainment rate are controversial, or even opposite, in different studies. Analysis of aircraft observations of cumuli from the Routine AAF (Atmospheric Radiation Measurement [ARM] Aerial Facility) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) and Rain in Cumulus over the Ocean (RICO) field campaigns shows that entrainment rate is positively correlated with relative humidity. Physical analysis shows that higher relative humidity promotes entrainment by reducing buoyancy in the cloud cores and by weakening downdrafts near the cloud cores. The reduced buoyancy in the cloud cores and weakened downdrafts surrounding the cores further reduce updrafts in the cloud cores; the cloud cores with smaller updrafts are more significantly affected by their environment, resulting in larger entrainment rate. The relationship between entrainment rate and relative humidity is consistent with the buoyancy sorting concept widely used in convection parameterizations. The results provide reliable in situ observations to improve parameterizations of entrainment rate.

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“…Clouds are known to be affected by atmospheric dynamical processes and microphysical processes and their variations exhibit a great degree of complexity (Lu et al 2016). It is also believed that clouds are one of the most important sources of uncertainty in climate change, because changes in any of the cloud attributes, for example its amount, spatial distribution, lifetime and optical properties, can certainly alter the atmosphere radiative transfer processes and radiation balance.…”

Section: Introductionmentioning

confidence: 99%

Skewness of cloud droplet spectrum and an improved estimation for its relative dispersion

Liu

2016

Meteorol Atmos Phys

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The relative dispersion of the cloud droplet spectrum is a very important parameter in describing and modeling cloud microphysical processes. Based on the definition of skewness as well as theoretical and data analyses, a linear fitting relationship (a = 2.91e-0.59) between skewness (a) and relative dispersion (e) is established and a new method is developed to estimate the relative dispersion of the cloud droplet spectrum. The new method does not depend on any assumption of a particular distribution for the cloud droplet spectrum and has broader applicability than the previous methods. Comparisons of the three methods for the relative dispersion with the observed data supported the following conclusions. (1) The skewness of the cloud droplet spectrum is asymmetrically distributed. An assumption of zero skewness in quantifying the relative dispersion inevitably results in relatively large deviations from the observations. Errors of the estimated relative dispersion due to the omission of the skewness term are not solely related to the skewness, but rather to the product of the skewness and relative dispersion. (2) The use of the assumption that the cloud droplet spectrum takes a gamma distribution is similar to the assumption that the skewness is twice the relative dispersion. This leads to a better accuracy in estimating the relative dispersion than that with zero skewness assumption. (3) Comparisons with observations show that the new method is more accurate than the one under gamma distribution assumption and is the best among all the three methods. (4) It is believed that finding a better correlation between the skewness and the relative dispersion would further reduce the deviations for the estimated relative dispersion.

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Improving Parameterization of Entrainment Rate for Shallow Convection with Aircraft Measurements and Large-Eddy Simulation

Cited by 48 publications

References 49 publications

Characteristics of vertical air motion in isolated convective clouds

Characteristics of vertical air motion in isolated convective clouds

Observational Relationship Between Entrainment Rate and Environmental Relative Humidity and Implications for Convection Parameterization

Skewness of cloud droplet spectrum and an improved estimation for its relative dispersion

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