\cite{Wang_2010}
- Avissar and Schmidt (1998) found that a weak geostrophic wind of 2.5 m s−1 is enough to reduce the effect of land surface inhomogeneities on the CBL while a 5 m s−1 geostrophic wind could eliminate all impacts ofsurface inhomogeneities. However, Raasch and Harbusch (2001) pointed out that the surface effect of heterogeneity on the CBL is still significant for a geostrophic wind of 7.5 m s−1. Some other studies showed that relatively high background wind does not necessarily inhibit the mesoscale circulations, but usually advects them away, and that coherent advection of the mesoscale rolls by this wind is an important effect (Weaver and Avissar, 2001; Baidya Roy and Avissar, 2002; Weaver, 2004). The possible reason for this disagreement may arise from the different heterogeneity alignments with respect to wind directions used in those studies.
- Land surface heterogeneities with different spatial scales
also have different effects on the CBL; land surface heterogeneities at scales of tens of kilometres can induce mesoscale circulations (Gopalakrishnan et al., 2000; Baidya Roy et al., 2003; Kustas and Albertson, 2003; Kang et al., 2007; Taylor et al., 2007; Garcia-Carreras et al., 2010) while heterogeneities with scales (λ) on the order of the boundary-layer height (Zi) mainly influence various CBL properties depending on values of λ/Zi (Prabha et al., 2007; and the references therein). Baidya Roy and Avissar (2000) showed that meso-γ-scale (2 ∼ 20 km) surface heterogeneity produces organized circulations with the same horizontal length-scale as that of the heterogeneity. However, if the length-scale of the surface heterogeneity exceeds 5 ∼ 10 km, random turbulent thermals also develop and affect their structure.
\cite{Garcia_Carreras_2011}
- Ensembles of simulations with a 2D large‐eddy model were performed using various heterogeneous land surfaces. Rainfall was found to be 4–6 times higher over warmer surface anomalies, associated with cropland, compared to a homogeneous surface, but rainfall was reduced to half or less over the forest.
- Subsidence over the forest acts to suppress cloud initiation, thus considerably limiting rainfall amounts. The cropland boundaries, on the other hand, are regions where CAPE is maximized (GC11) leading to increased cloud depth and organization. This mechanism leads to a persis- tent increase in rainfall over the cropland boundaries.
\cite{Taylor_2011} Sahel region
- For finer spatial resolution, we exploit land surface temperature (LST) data, available from geostationary satellite every15 min under clear sky at a resolution of3km (ref. 12). Sahelian soil wetting and drying cycles create strong anomalies in LST (henceforth LSTA) on a daily timescale. We use variability in daytime mean LSTA as a proxy for surface fluxes with negative anomalies indicative ofincreased evaporation and reduced sensible heat flux.
- We tracked convective cloud areas using commonly adopted thresholds of brightness temperature and areal extent (see Methods). We defined an MCS initiation event when a cold cloud first appeared, before expansion into an MCS.
- The probability of initiation (PI) for different soil-moisture conditions at this scale (Fig. 1a) provides no clear evidence for a link between soil moisture and MCS initiation. However, a strong relationship emerges when one considers mesoscale anomalies in soil moisture (Fig. 1b).
- To assess the relevance of finer-scale soil-moisture patterns
in the initiation process, we examined the spatial variability of LSTA within each 40km grid box by computing the standard deviation (σLSTA) ofthe 3km pixels within. The value of PI increases strongly with soil-moisture heterogeneity (Fig. 1c; χ2 = 119, df = 9,P < 0.0001). Comparing the number of initiations in the lowest decile with the highest decile, PI increases by a factor of 2.5. This relationship is robust to the methods used to compute the LSTA, but is sensitive to the choice of grid box size (Supplementary Fig. S1).
- On the other hand, when convective inhibition is large (dpLFC >300 hPa), the sensitivity to σLSTA is stronger, with a threefold increase in PI between the lowest and highest deciles. This strong sensitivity under unfavourable large-scale atmospheric conditions implies that mesoscale soil-moisture patterns trigger MCS which would not otherwise occur
- To assess the preferred surface length scales, we performed a wavelet analysis on along-wind LSTA transects for every initiation. The average of these wavelets reveals strong variability on wavelengths of20–75km centred 0–10km downwind of initiation (Fig. 2b).
\cite{Knox_2011} Amazon precipitation between forests and non-forests
- The event-based areal fractions of precipitation coverage (precipitation fraction) are calculated; referenced to forested, nonforested, and forest-edge land cover; and compared. As results are generally con- sistent with previous findings, the novel conclusions here extend that precipitation frequency in the south- western Amazon (i) decreases over regions of nonforests far removed (10-plus km) from forest borders but (ii) increases within several kilometers of the forest edges, particularly over the nonforest side of the transition.
- Low-level forest-to-pasture momen- tum fluxes and high-level pasture-to-forest momentum fluxes have been observed in the southwestern Amazon (Souza et al. 2000). Assuming that deforested patches are indeed warmer, the high sensible heat flux influences vigorous boundary layer development (Pielke 2001). The combination of boundary layer development and convergent lifting winds provides the mechanical energy necessary to lift air parcels to the level of free convection.
- There is some general agreement that land-cover length
scales on the order of tens of kilometers, or perhaps the Rossby radius, are most optimal for promoting solenoidal circulations (Pielke 2001; Dalu et al. 1996). Heterogeneity scales larger than 100 km cannot produce significant gradients in surface temperatures, whereas scales smaller than several kilometers diffusion processes tend to homogenize thermal properties, thereby destroying the thermal gradients that drive circulation (Baldi et al.
- It has already been observed that dry-season after-
noon shallow cumulus clouds show a positive bias over deforested areas of the region (Cutrim et al. 1995; Rabin et al. 1990; Chagnon et al. 2004). Wang et al. (2009) found the opposite was true for high cold clouds, which were more prevalent over intact forests. Correspondingly, both convective available potential energy and convective inhibition were typically weaker over nonforests and stronger over intact forests, suggesting that nonforested land covers are associated with relatively rapid boundary layer development but have a lesser ability to generate deep convection.
- The TRMM 2A25 is a swath-based near– instantaneous measurement of rainfall converted from backscattered reflectivity by the TRMM precipitation radar (Kummerow et al. 1998). The estimated rainfall is not without its uncertainties, considering, for example, its known biases over the region of interest (Rozante et al. 2010), patterned bias in sampling frequency, and its limited ability to measure the diurnal cycle or rainfall at fine scales (Negri et al. 2002).
- There was concern that multiple environmental cova-
riates to precipitation were present in the area of interest, thus making it difficult to confidently discern how the linkage between land cover and precipitation exists in this area. Two obvious factors are topographic elevation and weather patterns.
- Experiment 1 suggested that precipitation fractions were somewhat balanced, perhaps marginally bi- ased toward the nonforested side of the transect (see upper left panel). Experiment 2 suggested that pre- cipitation fractions were less over the interior nonforest regions, compared with all other landscapes (see upper right panel). Experiment 3 showed that the transition bands had the greatest precipitation fractions (see lower left panel), while the interior forested regions had greater precipitation fractions than the interior nonforests.
- Nonforests (grasslands, savannas, croplands, among
others) in the southwestern Amazon are typically asso- ciated with higher surface temperatures, rapidly de- veloping atmospheric boundary layers, and subsequently
lower lower convective inhibition (Wang et al. 2009). At certain heterogeneity scales, this convective focus can create convergence zones and solenoidal circulations (Atkinson 1981; Pielke 1984, 2001; Renno´
and Ingersoll 1996).
- Two observations are possible. The subsidence- based decrease in precipitation frequency on the intact side poses a drying threat to forest ecosystem stability. Alternatively, the enhanced precipitation frequency on the nonforest side could conceivably stimulate forest regrowth by minimizing drought stress. These competing theories are dependent on many factors, including the holistic community responses to precipitation changes regarding growth rates, mortality rates, coupled effects with soil and nutrient composition, and endogenous dis- turbance. There is evidence, though, that the Amazon rain forest shows a basinwide general sensitivity to drought stress (Phillips et al. 2009).
- However, there is no reason these results could not extrapolate to other regions of the world. The key for this to occur depends on the fact that precipitation is convectively driven and that there exists mesoscale land-cover heterogeneity (several to tens of kilometers) with associated differential heating rates. It is also clear that certain ambient conditions constrain the likelihood of stimulating land-surface-influenced precipitation. It was shown analytically through a stochastic analysis that the mesoscale circulations driven by heterogeneous land surface heating rates are strongest when ambient winds are at their lowest (Wang et al. 1996). Wet-season pre- cipitation in Rondoˆ nia features frontal-based precipi- tation and widespread mesoscale convective complexes that often spawn convective cells triggered from the downdrafts of other cells (Lima and Wilson 2008)