How Convective Mass Flux Responds to Environmental Humidity

TL;DR

The paper analyzes how tropical convective mass flux responds to lower-tropospheric humidity using five high-resolution, convection-resolving simulations from the DYAMOND project, complemented by a minimal parcel framework. It documents three key humidity-driven behaviors: a transition to deep inflow with increasing humidity, a quasi-exponential pickup of mid-level mass flux, and a muted change in mean convective buoyancy. Through a stochastic-entrainment parcel model, the authors show that increased humidity enhances parcel survival and increases dilution, which collectively raise mass flux and can reduce buoyancy, without relying on buoyancy increases. These findings have direct implications for improving convection parameterizations, suggesting that variable entrainment and buoyancy sorting—captured via stochastic processes and dilution—are essential to reproducing observed humidity-convection relationships in the tropics.

Abstract

Our goal in this study is to characterize the relationship between lower tropospheric environmental humidity and convective mass flux in the tropics. To do so, we have created gridded convective mass flux datasets from five global storm-resolving models (GSRMs). We have three principal findings. First, in humid environments, mass flux increases with height from the surface through the depth of the lower free troposphere, forming a ``deep-inflow". In dry environments, mass flux does not increase with height in the lower free troposphere. Second, mid-tropospheric mass flux increases nonlinearly with increasing lower tropospheric humidity, resembling a widely reported pickup in tropical precipitation. Third, increased lower tropospheric humidity is associated with reduced deep convective updraft buoyancy. To interpret these findings, we employ a simple three-equation parcel model with stochastic entrainment. The parcel model suggests that the response of convective mass flux to lower tropospheric humidity is governed by two effects: (1) survival, in which a greater share of entraining parcels ascend rather than detrain with greater humidity; and (2) dilution, in which the average entrainment rate among surviving parcels increases with environmental humidity. Together, survival and dilution account for the three mass flux responses to humidity.

Figures (17)

• Figure 1: (a) Convective mass flux simulated by the NASA GEOS GSRM for February 2020. (b) Precipitation simulated by the NASA GEOS GSRM for February 2020.
• Figure 2: (a) Probability distribution function of $LTRH$ for the GSRMs and reanalysis. (b) Precipitation conditionally averaged over $LTRH$for the GSRMs and for IMERG precipitation (matched to reanalysis $LTRH$).
• Figure 3: (a) GSRM convective mass flux conditionally averaged over $LTRH$. Each curve color denotes an $LTRH$ bin. (b) Normalized convective mass flux ($M/M_{850\ hPa}$) in the GSRMs, conditionally averaged over $LTRH$.
• Figure 4: (a) GSRM convective mass flux at 600 hPa, conditionally averaged over $LTRH$. (b) As in (a), except with mass flux plotted on a logarithmic scale.
• Figure 5: (a) $\Delta \theta_{e,bl}$: The difference between boundary-layer $\theta_{e}$ and free-troposphere $\theta_{e}^*$ in the GSRMs, conditionally averaged over $LTRH$. (b) $\Delta \theta_{e,ft}$: The difference between free-troposphere $\theta_{e}$ and free-troposphere $\theta_{e}^*$ in the GSRMs, conditionally averaged over $LTRH$.