The Simulated Impact of the Snow-Albedo and Soil Moisture Feedbacks on Convective Precipitation within the Rocky Mountains under Climate Warming

Moist diurnal convection is an important part of midsummer mountain climates and can be particularly susceptible to changes in land surface characteristics such as snow cover and soil moisture. As the climate warms, snow cover is depleted quicker in regions of high terrain. Enhanced snowmelt during the late spring acts to further increase warming through the snow albedo feedback. It also exposes the bare soil earlier in the season and leads to reduced soil moisture much later during the summer months. This helps to drive more vigorous sensible heat fluxes and increased planetary boundary layer heights, both of which can intensify deep convection. Drier soils and reduced snow cover can also modify local heating and cooling rates. Changes in local circulations, driven by these increased thermal contrasts between the ground and adjacent free atmosphere, can influence precipitation through enhanced convergence or subsidence. This study seeks to assess the role each feedback plays in modifying diurnal mountain precipitation.

Regional climate simulations using the Weather Research and Forecasting (WRF) model are performed at 4 km grid spacing with a domain centered on the Rocky Mountains of Colorado. The simulations are conducted from April through July across a period of 7 years extending from 2006 to 2013 in order to represent both the ‘transitional’ and late summer periods when the snow albedo and soil moisture-precipitation feedbacks are at their strongest respectively. These simulations include both a control (CTRL) run, forced by reanalysis, and a pseudo global warming (PGW) run in which a fixed mean warming perturbation derived from climate models is applied to the boundary conditions. Both scenarios are forced by output from a series of 4 km WRF regional climate runs that cover a continental-scale domain. An additional pair of PGW runs is performed where the snow water equivalent (SWE) and then both the SWE and soil moisture are forced to have the same values as the CTRL state, thereby isolating the effects of each feedback relative to the overall warming pattern.

A variety of factors are taken into account to diagnose changes in diurnal convection including: precipitation accumulation on multiple time scales, vertically integrated cloud liquid water path, cloud fraction, and vertical updraft strength. The causes of changes in diurnal convection are related to changes in the thermodynamic environment and mesoscale circulations. Environmental attributes considered include: convective available potential energy, convective inhibition and precipitable water, along with radiative effects such as changes in cloud cover and surface fluxes. Mesoscale circulations considered include those relevant to convective initiation and maintenance: localized diurnal mountain breezes and the mountain-plains solenoidal circulation are investigated for any patterns indicative of increased diurnal forcing.