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Tropical convective transition diagnostics and the dynamics of convective deep inflow


Moist convection drives much of the circulation and precipitation in Earth's atmosphere, especially in the tropics, and thus has far-reaching consequences for human activities. The parameterization of moist convection in general circulation models (GCMs) remains a major source of error in these models, highlighting the importance of understanding the physical processes by which deep convection interacts with larger scales. A physical pathway contributing to the uncertainty of parameterizations concerns how environment air enters convective updrafts, i.e., entrainment. This dissertation addresses several aspects of the physics of convection, including causal pathways by which the environmental water vapor impacts convection and the implications of this for model parameterizations, related process-oriented diagnostics for comparing convective processes in GCMs to observations, and theoretical foundations for a different view of the entrainment process.

First, it has been noted that as the atmospheric column water vapor (CWV) exceeds a critical threshold, precipitation sharply picks up. This ``pickup'' has been demonstrated to be consistent with the lower-free-tropospheric humidity impacting updraft buoyancy---thus convection---through entrainment. By performing a set of parameter perturbation experiments in the NCAR Community Earth System Model (CESM), we establish the causal relationship of the observed transition to deep convection---only when the convective scheme includes substantial entrainment can the model reproduce the pickup. The change in the precipitation--CWV relation also leads to a moist troposphere compared with when there is no entrainment. The reevaporation of convective precipitation is found to have only minor effects on the pickup.

Second, from observations and reanalyses, we compile and expand the set of statistics characterizing the transition to deep convection at fast timescales---termed ``convective transition statistics.'' Given that the spatial autocorrelation scales of tropospheric temperature and humidity are greater than that of precipitation, the precipitation--CWV relationship is robust to spatial resolution up to ~1� and time-averaging up to ~6 hours. The critical CWV at which precipitation starts to pick up increases with bulk tropospheric temperature, while the corresponding critical column relative humidity (CRH) decreases. This is consistent with prior entraining plume calculations. The CWV value relative to critical appears to be an effective predictor of conditional instability (hence precipitation) with only minor geographic variations in the tropics. The distribution of precipitation intensity drops rapidly for low CWV, and develops into a robust long-tail distribution for CWV around and above critical. The robustness of convective transition statistics---especially to the spatial-temporal resolution---suggests that these are suitable for model diagnostic purposes.

Third, we compared the convective transition statistics calculated using high-frequency (1-6 hourly) output from a set of GCMs to the observed. Comparing statistics among models that primarily differ in representations of moist convection suggests that convective transition statistics can substantially distinguish differences in convective representation and its interaction with the large-scale flow, while models that differ only in spatial–temporal resolution, microphysics, or ocean–atmosphere coupling result in similar statistics. Most of the models simulate some version of the observed sharp pickup of precipitation as well as that convective onset occurs at higher CWV but at lower column RH as temperature increases. However, departures from observations in various aspects of the precipitation--CWV relationship are also noted in many models.

Lastly, a puzzle regarding entrainment profiles in the vertical is addressed. Observations and large-eddy simulations has pointed to a ``deep-inflow'' updraft mass flux structure, in which mass enters the updraft through a deep layer in the lower troposphere. Looking for a simple explanation for the observed deep inflow, we investigate the nonlocal response of vertical velocity field to buoyancy under the anelastic framework. We find that the vertical structure of response is determined by the horizontal length scales contributing to the buoyancy structure. For a wide range of conditions relevant for isolated cumulonimbus and organized systems, the nonlocal dynamics entailing interaction between the buoyant layer and the surface results in the deep inflow. Furthermore, the largest, most heavily-precipitating contributions to convection are suggested here to be a simplifying factor for their representations in convective parameterizations.

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