Land Atmosphere Coupling Strength and its Impact on Precipitation during North American Summer

Precipitation is one of the most significant and unsolved modeling challenges in Earth system science. A key contributor to local convective precipitation is land-atmosphere coupling, when it is strong. Two indices derived from atmospheric thermodynamic vertical profiles: convective triggering potential (CTP) and low -level humidity (HI-low), have become preferred measures of land-atmospheric coupling strength. 20-years of literature has illustrated a correlation between early-morning CTP-HI and afternoon precipitation on classic convective boundary layer (CBL) days. The CTP-HI framework has been used to characterize a region's precipitation tendency as a function of soil moisture. In some instances, this was accomplished using output from high-resolution atmospheric reanalysis and mesoscale models for which diurnal model output for afternoon clouds and precipitation was available but omitted from analysis. Why some authors limit their analysis to early morning CTP-HI when they have access to full model data is unclear. When the CTP-HI framework was introduced, only the 2.5-degree NCEP-NCAR atmospheric reanalysis existed and data storage and computing was still prohibitively expensive for high resolution climate modeling. Today is a dramatically different research environment with readily accessible supercomputing in the cloud, convection-permitting modeling at regional and global scales, and cheap data storage.

The main objective of this study is to evaluate the viability of the CTP-HI framework in forecasting precipitation given the wealth of observational and model data constraints now available. For the period from 2002-2022, we quantify the number of days with a precipitation advantage over wet soils as well as a precipitation advantage over dry soils at each 1.0-degree gridpoint in CONUS. Locations for which the CTP-HI framework is consistently ineffective, and misleading are highlighted and discussed in the context of select multiday weather sequences. To calculate CTP-HI, we have used temperature and water vapor mixing ratio data from NASA’s Atmospheric Infra-Red Sounder (AIRS). Comparison precipitation and cloud fraction data have been taken from insitu METeorological Aerodrome Reports (METAR) and fifth generation ECMWF atmospheric reanalysis (ERA5). Our study will highlight the common shortcomings of the CTP-HI framework and contribute to a more clear-eyed view of the framework’s representativeness.