Observational estimates of detrainment and entrainment in non-precipitating shallow cumulus

Abstract

Abstract. Vertical transport associated with cumulus clouds is important to the redistribution of gases, particles and energy, with subsequent consequences for many aspects of the climate system. Previous studies have suggested that detrainment from clouds can be comparable to the updraft mass flux, and thus represents an important contribution to vertical transport. In this study, we describe a new method to deduce the amounts of gross detrainment and entrainment experienced by non-precipitating cumulus clouds using aircraft observations. The method utilizes equations for three conserved variables: cloud mass, total water and moist static energy. Optimizing these three equations leads to estimates of the mass fractions of adiabatic mixed-layer air, entrained air and detrained air that the sampled cloud has experienced. The method is applied to six flights of the CIRPAS Twin Otter during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) which took place in the Houston, Texas region during the summer of 2006 during which 176 small, non-precipitating cumulus were sampled. Our analysis suggests that, on average, these clouds were comprised of 30 to 70% mixed-layer air, with entrained air comprising most of the remainder. The mass fraction of detrained air was less than 2% for a majority of the clouds, although 15% of them did exhibit detrained air fractions larger than 10%. Entrained and detrained air mass fractions both increased with altitude, and the largest detrainment events were almost all associated with air that was at their level of neutral buoyancy, findings that are consistent with previous studies.

A New Approach for Simultaneous Estimation of Entrainment and Detrainment Rates in Non‐Precipitating Shallow Cumulus

“…The mass fractions of entrained air ( m e ) and detrained air ( m d ) gradually increase with increasing altitude, and this distribution is similar to the results of Norgren et al. (2016). The distribution of m e is consistent with previous studies (Ackerman, 1963; Cotton, 1975; Houze, 1993; M. ‐L.…”

“…Similar to the results of Norgren et al. (2016), m d is much smaller than m e . Generally, m d is smaller than 50%, while m e varies from ∼0% to ∼80%.…”

An Observational Comparison of Level of Neutral Buoyancy and Level of Maximum Detrainment in Tropical Deep Convective Clouds

“…In this regard, observed detrainment levels are found to be strongly dependent on the updraft strength and the conditions in which that deep convection initiates and develops (e.g., Hartmann & Larson, 2002). Furthermore, estimates for the observed levels of maximum detrainment (as well as the net detrainment mass flux) are expected to vary significantly with convection type, size, and age (e.g., Barnes et al., 1996; Norgren et al., 2016). Previous studies (e.g., Takahashi & Luo, 2012) have argued that differences in the detrained outflow heights and the level of neutral buoyancy can act as a simple proxy for effective bulk convective entrainment.…”

An Observational Study of Entrainment Rate in Deep Convection

“…Liquid water content (LWC) must be larger than 0.001 g·m −3 and cloud droplet number concentration (N) must be greater than 10 cm −3 [22]. Given that the bulk-plume approach is only appropriate for developing clouds, the percentage of updraft within a cloud must exceed 80% [8] and the cloud penetration time must be longer than 5 s [11] to eliminate the clouds that are too small. There are a total of 156 clouds satisfying these criteria.…”

“…Besides the bulk-plume approach, Jensen and Del Genio [10] estimated λ in convective clouds using the vertical variation of potential temperature in cloud obtained from surface-based remote sensing data. Based on the conservation of cloud mass, total water content and moist static energy, Norgren et al [11] estimated the ratio of entrained mass to the total cloud mass and found that this ratio increased with height.…”