Preprints
https://doi.org/10.5194/acp-2021-472
https://doi.org/10.5194/acp-2021-472

  14 Jun 2021

14 Jun 2021

Review status: this preprint is currently under review for the journal ACP.

Supersaturation, buoyancy, and moist convective dynamics

Wojciech W. Grabowski and Hugh Morrison Wojciech W. Grabowski and Hugh Morrison
  • Mesoscale and Microscale Meteorology Laboratory, National Center for Atmospheric Research, Boulder, CO 80307, USA

Abstract. Motivated by recent discussions concerning differences of convective dynamics in polluted and pristine environments, the so-called convective invigoration in particular, this paper provides an analysis of factors affecting convective updraft buoyancy, such as the in-cloud supersaturation, condensate and precipitation loading, and entrainment. We use the deep convective period from simulations of daytime convection development over land discussed in our previous publications. An entraining parcel framework in used in the theoretical analysis. We show that for the specific case considered here finite (positive) supersaturation noticeably reduces pseudo-adiabatic parcel buoyancy and cumulative CAPE in the lower troposphere. This comes from keeping a small fraction of the water vapor in a supersaturated state and thus reducing the latent heating. Such a lower-tropospheric impact is comparable to the effects of the condensate loading and entrainment in the idealized parcel framework. For the entire tropospheric depth, loading and entrainment have a much more significant impact on the total CAPE. For instance, an increase in the fractional entrainment rate from 0.05 km−1 to 0.3 km−1 reduces the theoretical level of neutral buoyancy from the upper to the middle troposphere and CAPE by a factor of 4. For the cloud model results, we compare ensemble simulations applying either a bulk microphysics scheme with saturation adjustment or a more comprehensive double-moment scheme with supersaturation prediction. The diagnosed bulk fractional entrainment rate, independent of the microphysics scheme applied in the simulations, is either 0.13 or 0.20 km−1 depending on whether we consider profiles of the upper end of the percentile range or of the mean in-cloud equivalent potential temperature. We compare deep convective updrafts, buoyancies, and supersaturations from all ensembles. In agreement with the parcel analysis, the saturation adjustment scheme provides noticeably stronger updrafts in the lower troposphere. For the simulations predicting supersaturation, there are small differences between pristine and polluted conditions below the freezing level that are difficult to explain by standard analysis of the in-cloud buoyancy components. By applying the piggybacking technique, we show that the lower-tropospheric buoyancy differences between pristine and polluted simulations come from a combination of temperature (i.e., latent heating) and condensate loading differences that work together to make polluted buoyancies and updraft velocities slightly larger when compared to their pristine analogues. Overall, the effects are rather small and contradict previous claims of a significant invigoration of deep convection in polluted environments.

Wojciech W. Grabowski and Hugh Morrison

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-472', Anonymous Referee #1, 16 Jul 2021
  • RC2: 'Comment on acp-2021-472', Anonymous Referee #2, 17 Jul 2021

Wojciech W. Grabowski and Hugh Morrison

Wojciech W. Grabowski and Hugh Morrison

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Short summary
The manuscript provide a discussion of key elements of moist convective dynamics: cloud buoyancy, latent heating, precipitation, and entrainment. The motivation comes from recent discussions concerning differences of convective dynamics in polluted and pristine environments.
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