Diurnal equilibrium convection and land surface–atmosphere interactions in an idealized cloud‐resolving model

Abstract

The influence of soil moisture and atmospheric stability on mid-latitude diurnal convection and land-atmosphere exchange is investigated in an idealized cloudresolving modelling framework using a full set of parametrization schemes. In each member of a series of month-long experiments, the model attains a state where deep, precipitating convection is triggered every day. This state is referred to as equilibrium diurnal convection. The triggering occurs via different mechanisms depending on the atmosphere-soil setting. In our framework latent heat fluxes comprise the primary control over the precipitation amounts. We find that evaporation is regulated by the availability of energy on the one hand and the availability of soil moisture and the near-surface saturation deficit of the atmosphere on the other. Increased cloud cover over wet soils reduces net short-wave radiation but increases net long-wave radiation, leading to a near-compensation of the two effects on available energy. Increased boundary layer moisture is removed by deep convection, thus increasing the near-surface saturation deficit and preventing a negative feedback of boundary layer moisture content on the latent heat fluxes. We also find that there is a spatial correlation between soil moisture and precipitation anomalies, suggesting that the soil moisture precipitation feedback acts on a scale of 10-50 km. Copyright c 2012 Royal Meteorological Society Key Words: convection; cloud-resolving modelling; land-surface atmosphere interactions

Annual Climatology of the Diurnal Cycle on the Canadian Prairies

“…In addition, our climate dataset has no information on soil moisture, except indirectly from lagged correlations with precipitation (Betts et al, 2014b), nor have we analyzed the upper air data. This means we cannot address the issues of soil moisture and BL-atmosphere coupling that have been a research focus in recent years (Koster and Suarez, 2001;Findell and Eltahir, 2003;Koster et al, 2004;Dirmeyer, 2006;van Heerwaarden et al, 2009;Ferguson and Wood, 2011;Ferguson et al, 2012;Koster and Mahanama, 2012;Schlemmer et al, 2012;Santanello et al, 2013). Modeling studies highlight the critical role of the cloud and radiation fields on land-surface processes (e.g., Betts, 2004Betts, , 2007, but the wide variation between models Guo et al, 2006;Koster et al, 2006;Flato et al, 2013;Taylor et al, 2013) may reflect in part model errors in the cloud fields.…”

A simplified model of precipitation enhancement over a heterogeneous surface

“…We tested the wet soil advantage sounding of Findell and Eltahir (2003) where, in contrast to the dry soil advantage sounding, convection triggering requires a strong moistening of the boundary layer. We also tested the sounding of Schlemmer et al (2012), indicated as ID, which represents an idealization of the typical atmospheric state prone to convection in Europe (see Fig. 2b).…”

“…Overview of the performed simulations. The first column indicates the experiment name, whereas the second column indicates the sounding used for initialization: DA for dry soil advantage, after Findell and Eltahir (2003), and ID for idealized, after Schlemmer et al (2012). Third and fourth columns indicate the value of soil moisture over the dry and wet patches, respectively, in percentage of the saturation value.…”

“…This is illustrated with the ID_ cases (see Table 3), where the idealized sounding of Schlemmer et al (2012) is used to initialize the atmosphere (see Sect. 2).…”

A simplified model of precipitation enhancement over a heterogeneous surface

“…First, we decrease the initial value of φ wet to 70 % of the saturation value. Second, we change the initial atmospheric profile to the one used in Schlemmer et al (2012) which represents an idealization of the typical atmospheric state prone to convection in Europe. Results of these perturbed experiments will be briefly described throughout the paper when needed.…”