Impact of land surface conditions on 2004 North American monsoon in GCM experiments

Abstract

[1] In this study, two sets of six-member ensemble simulations were performed for the boreal summer of 2004 using the Finite Volume General Circulation model to investigate the sensitivity of the North American monsoon (NAM) system to land surface conditions and further to identify the mechanisms by which land surface processes control the NAM precipitation. The control simulation uses a fully interactive land surface model, whereas the sensitivity experiment uses prescribed land surface fields from the Global Land Data Assimilation System. [2] The response of the monsoon precipitation to land surface changes varies over different regions modulated by two different soil moisture-precipitation feedbacks. The vast northern NAM region, including most of Arizona and New Mexico, as well as the northwestern Mexico shows that soil moisture has a positive feedback with precipitation primarily due to local recycling mechanisms. The reduction of soil moisture decreases latent heat flux and increases sensible heat flux and consequently increases the Bowen ratio and surface temperature, leading to a deep (warm and dry) boundary layer, which suppresses convection and hence reduces precipitation. Over the west coast of Mexico near Sinaloa, a negative soil moisture-precipitation relationship is noted to be associated with a large-scale mechanism. The reduced soil moisture changes surface fluxes and hence boundary layer instability and ultimately low-level circulation. As a result, the changes in surface pressure and large scale wind field increase moisture flux convergence and consequently moisture content, leading to increased atmospheric instability and in turn enhancing convection and accordingly precipitation. These results further reinforce the important role of land surface conditions on surface process, boundary structure, atmospheric circulation, and rainfall during the NAM development.

Influence of initial soil moisture and vegetation conditions on monsoon precipitation events in northwest México

“…For instance, a positive soil moisture-precipitation feedback has been identified using regional climate models (e.g., Small, 2001;Xu et al, 2004;Vivoni et al, 2009). Using a global model, Feng et al (2013) found that a positive soil moisture-precipitation feedback was limited to the northern (more arid) sections of the NAM region, while a negative feedback was apparent in more southerly (tropical) areas. While discrepancies in the direction of the feedback mechanism might be due to variations in approach, prior studies suggest that the mechanisms underlying the soil moisture-precipitation feedback in the NAM region are not well understood, in particular at the scale of storm events.…”

Quantifying the strength of land–atmosphere coupling in the 2004 North American monsoon

“…The model performance has been validated in our previous study based on observations and reanalysis (Feng et al, ). Overall, the control simulation with the fully coupled CLM is capable of reproducing the observed mean precipitation, as well as surface temperature in the 2004 NAM, which provides us confidence in the analyses of control and sensitivity experiment results in the following section.…”

A possible link between wildfire aerosol and North American Monsoon precipitation in Arizona–New Mexico

“…This raises the issue of the scale at which wildfire aerosol effects may be relevant, and as a corollary, is it possible for them to assume greater importance at the margins of the NAM? At the AZNM scale, and given the weak rainfall signal, subtle local‐scale impacts on stability (Feng et al , ) and cloud microphysics could potentially have significant impacts on rainfall delivery. If indeed wildfire aerosol plays a role in NAM precipitation processes, this analysis suggests a negative feedback but does not propose a specific mechanism (SDM, aerosol microphysics effect or both) or temporal/spatial scale at which aerosol forcing is relevant, nor does it implicate particular aerosol types. Further research is required to explore these uncertainties.…”

“…At the synoptic scale, NAM is characterized by displacements of the Pacific and Bermuda Highs, formation of an upper‐level anticyclone and thermal low, and low‐level jets from the Great Plains, Caribbean and Gulf of California responsible for moisture transport (Feng et al , and references therein). Pacific sea surface temperatures (SSTs) and their associated atmospheric teleconnections (Carleton, , ; Andrade and Sellers, ; Carleton et al , ; Higgins and Shi, ; Castro et al , ; Liang et al , ; Ciancarelli et al , ) have been shown to affect the NAM, although diagnosis of the effects of anomalous SST sufficiently definitive to improve seasonal forecasting has proven elusive.…”

“…Gutzler and Preston () observed that excessive (deficient) snow in the west‐central US leads to deficient (abundant) summer rain in New Mexico, a relation expanded upon by Gutzler () and recently confirmed by simulations in the study by Notaro and Zarrin (). It should be noted that in a simulation of the 2004 NAM, Feng et al () suggest that soil moisture, rather than having a negative feedback, may also have a positive feedback on precipitation in the AZNM region after monsoon onset owing to local recycling mechanisms. Recent studies have attempted to provide more detail on the influence of land surface boundary conditions on intra‐seasonal variability of NAM.…”