Role of land surface processes in monsoon development: East Asia and West Africa

Abstract

[1] Evidence is presented that exchanges of water and energy between the vegetation and the atmosphere play an important role in east Asian and West African monsoon development and are among the most important mechanisms governing the development of the monsoon. The results were obtained by conducting simulations for five months of 1987 using a general circulation model (GCM) coupled with two different land surface parameterizations, with and without explicit vegetation representations, referred to as the GCM/vegetation and the GCM/soil, respectively. The two land surface models produced similar results at the planetary scale but substantial differences at regional scales, especially in the monsoon regions and some of the large continental areas. In the simulation with GCM/soil, the east Asian summer monsoon moisture transport and precipitation were too strong in the premonsoon season, and an important east Asian monsoon feature, the abrupt monsoon northward jump, was unclear. In the GCM/ vegetation simulation, the abrupt northward jump and other monsoon evolution processes were simulated, such as the large-scale turning of the low-level airflow during the early monsoon stage in both regions. With improved initial soil moisture and vegetation maps, the intensity and spatial distribution of the summer precipitation were also improved. The two land surface representations produced different longitudinal and latitudinal sensible heat gradients at the surface that, in turn, influenced the low-level temperature and pressure gradients, wind flow (through geostrophic balance), and moisture transport. It is suggested that the great east-west thermal gradient may contribute to the abrupt northward jump and the latitudinal heating gradient may contribute to the clockwise and counterclockwise turning of the low-level wind. The results showed that under unstable atmospheric conditions, not only low-frequency mean forcings from the land surface, such as monthly mean albedo, but also the perturbation processes of vegetation were important to the monsoon evolution, affecting its intensity, the spatial distribution of precipitation, and associated circulation at the continental scale.

A review on regional dynamical downscaling in intraseasonal to seasonal simulation/prediction and major factors that affect downscaling ability

“…They have been listed as one of the major sources to generate small scale features in RCMs (Denis et al, 2002). Although at intraseasonal and seasonal scales, oceanic forcing may be the main source of climate variability over many regions, in places where land-atmosphere coupling is strong (Koster et al, 2006;Xue et al, 2004Xue et al, , 2006Xue et al, , 2010b, soil moisture and vegetation biophysical processes could make significant contributions to dynamic downscaling. Extreme climate events, such as droughts and flooding, are an important focus in RCM land/atmosphere interaction studies (e.g., Seth and Giorgi, 1998;Bosilovich and Sun, 1999;Zhang et al, 2003;Gao et al, 2011;Liu et al, 2013;Stefanon et al, 2013).…”

“…Because PBL schemes and radiation parameterizations are so different in different atmospheric models, it is necessary to design a specific approach to implement land models into an atmospheric model (e.g. Xue et al, 2001Xue et al, , 2004. Normally, the process includes modifications of both land surface and PBL schemes.…”

Intercomparison and analyses of the climatology of the West African Monsoon in the West African Monsoon Modeling and Evaluation project (WAMME) first model intercomparison experiment

“…Although numerous diagnostic studies have been conducted to investigate the WAM, there are relatively few general circulation model (GCM) studies to explore the WAM seasonal predictability and mechanisms associated with WAM variability (e.g., Rowell et al 1995;Douville et al 2001;Xue et al 2004). Some key research issues remain with regard to our understanding of WAM variability and important associated features, such as the African Easterly Jet (AEJ) and the impacts of aerosol, oceanic, and land processes.…”

“…SSIB-1 (Xue et al 1991) None UCLA MRF AGCM (Kanamitsu et al 2002;Xue et al 2004) T62L28 (*2°, 28 vertical levels) Chou (1992), Chou and Suarez (1999), Hou et al (1996) SAS (Pan andWu 1995, Hong andPan 1996) SSIB-1 (Xue et al 1991) None References cited in Table 1 Kanamitsu et al 2002b;Xue et al 2004), the UCLA GCM (Mechoso et al 2000;Xue et al 2009), and the COLA (Center for Ocean-Land-Atmosphere Interactions, Kinter III et al 1997) GCM have similar land surface schemes. More information on the physical components of participating models, including land surface models, can be found in Table 1.…”

Impact of soil moisture initialisation and lateral boundary conditions on regional climate model simulations of the West African Monsoon

“…Rowell et al (1995) and Douville (2002) show that allowing SM to evolve freely during the wet season further improves the model's response to SSTs (compared to forcing it to follow a SM climatology). According to Xue et al (2004), the intraseasonal variability of WAM rainfall in GCM simulations could also be refined through the improvement of initial SM conditions and the SM-precipitation coupling, that is the degree to which precipitation-induced SM anomalies can feedback to the atmosphere and affect subsequent precipitation (Koster et al 2006;Notaro 2008).…”