Observing the Tropical Atmosphere in Moisture Space

Abstract

Measurements from the Barbados Cloud Observatory are analyzed to identify the processes influencing the distribution of moist static energy and the large-scale organization of tropical convection. Five years of water vapor and cloud profiles from a Raman lidar and cloud radar are composed to construct the structure of the observed atmosphere in moisture space. The large-scale structure of the atmosphere is similar to that now familiar from idealized studies of convective self-aggregation, with shallow clouds prevailing over a moist marine layer in regions of low-rank humidity, and deep convection in a nearly saturated atmosphere in regions of high-rank humidity. With supplementary reanalysis datasets the overall circulation pattern is reconstructed in moisture space, and shows evidence of a substantial lower-tropospheric component to the circulation. This shallow component of the circulation helps support the differentiation between the moist and dry columns, similar to what is found in simulations of convective self-aggregation. Radiative calculations show that clear-sky radiative differences can explain a substantial part of this circulation, with further contributions expected from cloud radiative effects. The shallow component appears to be important for maintaining the low gross moist stability of the convecting column. A positive feedback between a shallow circulation driven by differential radiative cooling and the low-level moisture gradients that help support it is hypothesized to play an important role in conditioning the atmosphere for deep convection. The analysis suggests that the radiatively driven shallow circulations identified by modeling studies as contributing to the self-aggregation of convection in radiative–convective equilibrium similarly play a role in shaping the intertropical convergence zone and, hence, the large-scale structure of the tropical atmosphere.

The Two Diurnal Modes of Tropical Upward Motion

“…While the nocturnal increase of latent heating (Figure h) linked to boundary layer rooted convection is likely critical to the diurnal change in ω in the lower‐midtroposphere (Ciesielski et al, ; Ruppert & Hohenegger, ), here we test the notion that SW ∗ directly drives the circulation change in the middle‐upper troposphere. This effect can be approximated from the weak temperature gradient approximation (Sobel et al, ) through where s is dry static energy, and [ ] ′ denotes a deviation from the daily mean (Ruppert & Hohenegger, ; Schulz & Stevens, ). ω SW is shown in Figure d for night and day.…”

“…where s is dry static energy, and [ ] ′ denotes a deviation from the daily mean (Ruppert & Hohenegger, 2018;Schulz & Stevens, 2018). SW is shown in Figure 3d for night and day.…”

Sensitivity of Convective Self‐Aggregation to Domain Size

“…Although CSA is often considered within an idealized modeling framework, a few recent studies have focused on the relevance of CSA to observations of organized convection in the tropical atmosphere (e.g., Holloway et al, ; Schulz & Stevens, ; Tobin et al, ). Holloway et al () showed that the dry region is too dry compared to observations in a ∼1,500 × 1,500‐km domain but is more humid and closer to observations in long‐channel simulations (∼12,000 × 200 km).…”

Multi-layer Cloud Conditions in Trade Wind Shallow Cumulus – Confronting Models with Airborne Observations

“…As shallow cumulus convection is not expected to trigger at the same time and place in a model and reality, a statistical approach is adopted here, in which the airborne observations are compared to their model counterpart for different LWP regimes. The analysis in 80 LWP space is similar to the studies in moisture space that first have been published by Schulz and Stevens (2018) for groundbased observations and by Naumann and Kiemle (2019) for airborne observations. In the LWP space it is possible to study microphysical cloud processes like the transition from non-precipitating to precipitating clouds.…”