A numerical modeling investigation of entrainment in cumulus congestus clouds and its effects upon precipitation

Despite over 70 years of extensive research, the factors controlling how clouds entrain environmental air are still not well understood, a problem contributing to the poor quantitative precipitation forecast skill of numerical weather prediction models and the highly sensitive precipitation biases in climate models. Inspired by a recent field investigation of convection over England in 2013, the research presented in this dissertation seeks to improve the current understanding of entrainment by investigating mechanisms that may influence the properties of entrained air and potentially alter the rates of convective rainfall. An idealized, simulated cumulus congestus was used to analyze how successive thermals diluted differently when entraining air locally modified by their predecessors. Directly-calculated entrainment rates were found to be strongest at the rear of each successive thermal, but the entrainment rates alone cannot predict the erosion rate of the high liquid water content cores. A novel analysis of sampling entrained and detrained air within each successive thermal demonstrates how humid, positively buoyant air was preferentially entrained by later thermals that rose in the wakes of their predecessors, limiting their dilution. Extending this approach, a series of idealized simulations were performed where a number of cumulus congestus clouds were allowed to develop as a linear population with different cloud separation distances. A strong dependency of the resulting convective rainfall on the distance between neighboring clouds revealed the importance of interacting cold pools in forcing subsequent generations of convection. Substantial moistening of the locally entrained air into the later generations served to protect the clouds from dilution and enhanced the warm rain process