Processes associated with Convection Initiation in the North American Mesoscale Forecast System, Version 3 (NAMv3)

In support of the Next Generation Global Prediction System (NGGPS) project, this study explores processes leading to convection initiation in the North American Mesoscale Forecast System Version 3 (NAMv3). Two severe weather outbreaks—occurring in the Southeast United States on 28 April 2014 and the central Great Plains on 6 May 2015—are simulated using the NAMv3 CONUS (4-km) and Fire Weather (1.33-km) nests, each with 5-minute output. Points of convection initiation are identified, and patterns leading to convection initiation are recognized. In the 30 minutes preceding convection initiation at a grid point, upward motion at low levels of the atmosphere enables a parcel to rise to its level of free convection, above which it is accelerated by the buoyancy force. Typically, the model develops a moist absolutely unstable layer (MAUL) at the top of the updraft. When strong updrafts are collocated with large vertical gradients of potential temperature and moisture, noisy vertical profiles of temperature, moisture, and hydrometeor concentration develop beneath the rising MAUL. The noisy profiles found in this study are qualitatively similar to those which resulted in NAMv3 failures during simulations of Hurricane Joaquin in 2015. Using the CM1 cloud model to reproduce these noisy profiles, it is determined that the noise can be mitigated by including sufficient vertical mixing in the model. Left unchecked, the noisy profiles can impact convective storm features such as cold pools, precipitation, and the initiation of spurious convection.