Despite the key role of precipitation in climate and climate impacts, it remains one of the most poorly modeled climate variables. In addition to well-known biases in the simulated climatology of tropical precipitation, there are also biases in tropical precipitation sensitivity to climate perturbations. For example, even if a model has its convection zone in the proper mean location vis a vis the observations, it does not necessarily follow that the sensitivity of the convection to variations in temperature, wind, or inflow water vapor is correct. Under the previous grant, we have developed a number of tools that we propose can contribute to identifying and addressing the biases in convective processes. In particular, we outline new diagnostic methods to understand the transition to strong convection, presenting preliminary examples using satellite observations of precipitation and column water vapor. We propose to apply these diagnostics to better constrain the temperature and moisture dependences of the onset of precipitation required for climate model convective parameterizations, and to contrast these measures in observations to current model simulations. We propose to complement the satellite observations with in situ sounding data for vertical structure in strongly convecting regions, beginning with the high temporal coverage Atmospheric Radiation Measurement (ARM) Program site at Nauru Island. We will focus first on quantifying the convective threshold and its dependences on thermodynamic variables. The water vapor-temperature dependence of the onset of convection can be important to mechanisms by which large-scale processes, such as the wave dynamics occurring in teleconnections, or inflow air masses from dry regions into the convective margins, interact with the small-scale convective processes. We propose to examine how variations in inflow from the dry Southeastern Pacific into the South Pacific Convergence Zone (SPCZ) interact with the threshold for the the onset of convection in models compared to observations. This will help quantify the contribution of this interaction to the mean and sensitivity biases of the convectivemargin in this region. We propose to analyze similar impacts of the variations in the moisture relative to the convective threshold in ENSO variations. We will aim to formulate our diagnostics of observed and modeled convective processes in terms that can be directly useful to modeling groups.