Carbon dioxide exchange in an idealized valley

Abstract

On a global scale the budget of carbon dioxide (CO2) bears a substantial uncertainty, which is commonly understood to be mainly due to land-surface exchange processes. In this paper it is examined to what extent mountainous model topography can amplify these land-surface exchange processes. The underlying hypothesis is that, on the meso-scale, topography adds additional atmospheric mechanisms that drive the exchange of CO2 at the surface.This model sensitivity study represents an advancement in model development through the implementation of a pioneering combination of the Weather Research and Forecasting (WRF) model, coupled with the Community Land Model (CLM4) to account for photosynthesis, and the semi-empirical TPGPP-LAI model to address ecosystem respiration. An idealized sine-shaped valley is investigated to examine the effect of complex topography on the CO2 budget compared to flat terrain. The systematic variation of meteorological initial conditions and plant functional types create an ensemble that unveils the fundamental factors which dominate the differences of CO2 fluxes between simulations with topography compared to a flat reference surface.During daytime, when there is photosynthesis, the vegetation temperature of the hypothetical sub-grid scale valley is on average 0.7 K colder and its vapor pressure deficit is 0.8 hPa lower which results in stronger net ecosystem exchange. During nighttime the valley is 0.6 K warmer than the plain, which increases ecosystem respiration compared to the plain. Over 24 h the valley is 0.1 K colder, but it acts as a sink of CO2 as the two opposing processes, during daytime and at night, result in 0.2 gC m−2 d−1 stronger net ecosystem exchange in the valley. During daytime differences are caused implicitly by meso-scale circulations through a secondary effect, as slope-winds shape the vertical temperature and humidity profiles, whereas at night the larger surface area of the valley is the prime cause for the differences. In contrast, the effect of differences in ambient CO2 on the ecosystem exchange is negligible.