\cite{Khanna_2014}
- that sensible heat flux heterogeneities can be a major factor in governing the regional atmospheric response when deforestation results in patches of about 1 km size. Deforestation at this scale triggers thermally generated mesoscale cir- culations that have been shown to cause an increase in shallow cloudiness [Cutrim et al., 1995; Negri et al., 2004; Wang et al., 2009] and precipitation [Chagnon et al., 2004; Chagnon, 2005] during the dry season (June to September) in Rondônia.
- First, it is found that the mesoscale circulation is contained within the deforested region. This is unlike thermally generated mesoscale circulations which are expected to be strong at the forest-clearing interface and have a downward branch over the forest [Avissar and Schmidt, 1998].Second, the simulated mesoscale circulation is present at night as well, although with varying intensity (compare Figure 7 with Figure 3). This again contrasts with the typical thermally
generated mesoscale circulation, which subsides at night. Third, the area aver- aged sensible heat fluxes between 1200 LT and 1300 LT are lower in DEF by 37Wm−2 as compared to FOR (Figure 8).
\cite{Mishra_2018}
- There is a strong relationship between precipitation
extremes and cloud-top temperature (Arkin and Meisner, 1987; Kuligowski, 2002; Mishra et al., 2010; Lau and Wu, 2011; Mishra, 2013). Lau and Wu (2011) investigated the climatological characteristics of tropical rain and cloud sys- tems over the Tropics using brightness temperature (TB) data from the Visible and Infrared Scanner (VIRS) on the Tropical Rainfall Measuring Mission (TRMM) and pre- cipitation data from the microwave imager and precipitation radar (PR) on the TRMM. It was found that the top 10% of heavy precipitation could be associated with convective clouds (TB ≤ 220 K) and light precipitation was closely related to low clouds (TB ≥ 260 K).
Precipitation change
\cite{Chu_1994}
- In the case of deep convective clouds, the sensors measure radiation from the top of clouds. In the case of clear sky, the satellite radiometer monitors longwave radiation emit- ted mainly from the earth surface. In the Tropics, the satellite-derived OLR mainly reflects cloud-top tem- peratures, with lowOLR values corresponding to cold and high clouds, which generally denote enhanced convection. Thus, an inverse relationship generally holds between OLR and convection.
- Results from two statistical analyses as presented
in Figs. 3-5 are mutually supportive in suggesting a slight increase in tropical convection (or rainfall) over much of Amazonia concurrent with deforestation
- One reason for this difference may be due to the fact that deforestation has not yet occurred on a basinwide scale. As the deforested area in- creases, the effects could be expected to gradually increase. Another reason for the difference between model simulations and observations may be due to limitations of the models: namely, that the GCM failed to accurately simulate the extent and duration of convective cloud cover in the Tropics (Shuttleworth et al. 1990).
\cite{Baidya_Roy_2002}
- Differential heating of the planetary boundary layer
(PBL) due to heterogeneity in the underlying Earth surface gives rise to atmospheric circulations over a wide range of spatial and temporal scales. At the mesoscale, sea breeze and lake breeze produced by the thermal gradient between adjacent land and water bodies are interesting examples of this type of circulation. Significant natural and human-induced heterogeneity, existing within the land surface in the form of patches, whose radiative and thermal properties differ from those of their surroundings, can also produce horizontal temperature and pressure gradi- ents strong enough to generate and sustain organized me- soscale circulations of a similar nature [Segal and Arritt, 1992].
- The complexity of the landscape heterogeneity in Rondoˆnia (described in detail by Calvet et al., [1997] showing fine
structures, only a few kilometers in width, embedded within ⬃100 km wide swaths), combined with the relatively sharp gradi- ent between the forested and the deforested patches, makes this region ideal for development of coherent mesoscale circulations.
- Hence the PBL over the pasture is higher than that over the forest by a few hundred meters, as indicated by the height of the inversion capping the convective boundary layer (CBL) in Figure 6. The PBL heights are well within the range observed by Fisch et al. [2000] during the course of RBLE-3 (for example 1471 ⫾ 479 m over pas- tures and 902 ⫾ 307 m over forests at 1400 LT). The mixed layer over the forest is shallower and hence denser than that over the pasture. These density gradients lead to horizontal pressure gradients between the forest and the pasture areas.
- The fact that the convergence starts over Highway BR-364 and not on smaller deforested patches indicates that a minimum length scale of heterogeneity is required to trigger organized circulations [Avissar and Schmidt, 1998; Go- palakrishnan et al., 2000; Baidya Roy and Avissar, 2000].
- In the REAL case, as discussed later, mesoscale eddies enhance the mixing of moisture in the CBL, thereby reducing the vertical moisture gradient therein. Hence in this case, the turbulent latent heat flux is lower by up to 100 Wm⫺2
compared to that for the MEAN case. Thus surface
heterogeneity and the resulting mesoscale circulations signifiantly affect turbulent transport of moisture in the PBL.
- A major problem with these circulations is that they
cannot be observed with conventional meteorological net- works. Occasionally, they are masked by strong synoptic and orography-induced wind and can be seen only as a perturba- tion from the mean flow. The clouds often produced by these circulations are small, shallow cumulus type with a very short life span and difficult to observe in satellite images. Also, since the circulations can be advected by large-scale winds, the clouds are not always anchored to the surface heterogeneity pattern; hence the circulations are not readily apparent. Thus, dedicated field experiments are required to observe these me- soscale processes.
\cite{Negri_2004}
- This revealed that mesoscale circulations enhanced cloudiness and localized rainfall as well as dry season enhancement of shallow clouds under weak synoptic forcing. Obser- vations and modeling of landscape heterogeneity re- sulting from differential land use indicated a direct thermal circulation over areas ofthe Midwest (Weaver and Avissar 2001).
- The objectives of this study are to demonstrate that,
observationally, differences in cloud cover and rain- fall exist between regions of forest and pasture. We speculate that, in the dry season, increased surface heating over deforested regions creates a direct thermal circulation, which increases the occurrence of shallow cumulus clouds, deep convective cloudiness, and rainfall. Similar (and more pronounced) effects are noted over a small, naturally unforested region of savanna, suggesting that size or scale may play a role in modulating the resultant circulation.
- We are struck by the similarity of this region and its surroundings to the spatial extent of the Florida peninsula and its juxtaposition between two larges bodies of water. We believe that the cir- culations that arise are similar to peninsula-forced sea-breeze circulations. Deforestation is evident along roads and waterways, and the fish-bone pattern mod- eled by Wang et al. (2000) is clearly evident.
- Increased surface heating is evident over the deforested and savanna areas, on the order of 5–10 K, as well as over the urban area of Porto Velho (9⬚S, 63.5⬚W). The increased heating is due to the lower specific heat and shorter roughness length of the deforested area. This
Cloud modeling
\cite{Weaver_2001}
Deforestation caused changes
\cite{Lawton2001}
- Because grasses and crops usu- ally have shallower roots than do forest trees, the volume of water available for transpiration and latent heat transfer is greater for forests than for agricultural land developed from them (15, 16).
- Deforestation and conversion ofland to pasture or cropland generally increase surface al- bedo (12), reduce aerodynamic roughness length and mechanically turbulent mixing in the boundary layer (13), reduce evapotranspiration, and increase the ratio of convective sensible heat transfers to latent heat transfers from the surface to the atmosphere (13).
- Late morning dry season cumulus was much less abundant over the long-deforested parts of Costa Rica’s Rio San Juan basin than over the nearby forested region in Nicaragua. The less thoroughly deforested areas of Costa Rica near the river showed intermediate degrees ofcumu- lus development. Landsat images of dry season days with cumulus cloud field development in the region also showed that cumulus clouds were commonly absent or poorly developed over deforested areas (Fig. 1B).
- Our simulations suggest that conversion of
forest to pasture has a significant impact on cloud formation. The differences between sim- ulated forest and pasture surface air tempera- tures and sensible and latent heat transfers (Fig. 2, A through C) were similar to those observed between Amazonian forest and adjacent pasture. The greater evaporative flux over forest lowered the lifting condensation level in comparison to that over pasture and increased the convective available potential energy in air parcels. The dry season atmosphere measured by the soundings we used was such that mixing in the boundary layer became vigorous enough to initiate cloud formation in both forested and pasture scenarios by mid-morning.
- Reduced evapotranspiration after deforesta-
tion in tropical lowlands decreases the moisture content of the air mass flowing up the slopes of adjacent mountains. This increases the lifting condensation level and thus the elevation ofthe base of the cloud deck. The model results thus suggest that deforestation in the lowland tropics of the trade wind zone tends to shift the cloud forest environment upward in adjacent down- wind mountains.
- Negri et al. [2002] analyze a larger area encompassing the whole Amazon Basin and examine the effect of geography (rives, lakes, coasts) and topography on the diurnal cycle. Rainfall is seen to avoid the Amazon river in the afternoon while an early morning maximum is seen over the large rivers.
\cite{Silva_Dias_2002}
- The wet season results show that the heat flux into the soil generally reaches up to 10 W m?2 and 50 W m?2 in the forest and the pasture, respectively, except for some peaks which are related to stronger solar forcing, such as those due to sunspecks in the forest. Measurements of temperature profiles in the soil (at depths of 2, 10, 20 and 40 cm at both sites) indicated a smaller diurnal range in the forest. The surface soil layer in the forest was always drier than in the deeper layers, because the soil is very sandy (more than 80%), which causes a fast drainage of the water after rainfall. The computed soil heat capacity for the layer 10 to 40 cm, considering the soil moisture for period between day 48 to 69 (Julian day), varied between 1.95 and 2.23 M J m?3 K?1 for the forest, and between 2.22 and 2.52 M J m?3 K?1 for the pasture. These values increase rapidly after rainfall, and decrease immediately thereafter. They are, on average, 13% higher in the pasture during the period considered. Finally, the computed apparent soil thermal diffusivity and conductivity show a behavior similar to the one presented by the soil heat capacity [Alvala´et al., 2002].
- The energy budget (Table 2) shows the dominant
role of net radiation (Rn) in latent heat flux at both pasture and forest, whose mean Bowen ratio (rate between sensible heat flux, H and latent heat flux, LE) is 0.32 and 0.20, respectively. However, the evapotranspiration of the forest was about 0.6 mm day?1 larger than that of the pasture, explained by the higher Rn and smaller H, which contribute with about half of this difference in the evapotranspiration.
- the penetration of radiation inside the canopy, determined by three five-level profiles (westward, south- ward and eastward of the tower) of net radiometers, PAR sensors and pyranometers, showed a rapid decrease in radiation intensity. Less than 40% of the PAR and incoming solar radiation above the forest reaches the 30 meter level (between 5 and 10 m below the canopy top) and only about 8% of the PAR and 14% of the solar radiation reaches the 15 meter level; 3. the radiation reaching the forest floor, measured by
eight net radiometers, eleven PAR sensors and 12 pyranometers regularly distributed around the tower, at one meter above the ground, and integrated over five dry days between 0700 AM and 0500 PM (local time), was only 4.2% ± 2.0%, 1.6% ± 1.4% and 3.4% ± 2.2% of the radiation observed above the canopy, for net radiation, PAR and incoming solar radiation, respectively.
- In a westerly regime shallow cumulus start more randomly while during the easterly regime they show a preference for higher altitude and forest. Rutledge et al. [2001] Their results also suggest that topography or land elevation appear to play an equal or greater role than deforestation in rain production over the Amazon and that effects of deforestation on rain production are more substantial over elevated terrain.
- Deforestation seems to be of secondary importance
during the wet season as compared to the dry season. However, there are differences between forest and pasture in the radiation budget and in the surface fluxes. There is a complex interaction between cloudiness and surface fluxes during the wet season leading to different times of initiation of first clouds. Mixed layer heights grow faster over forest and reach about the same maximum values in the afternoon as over pasture in the wet season. Shallow clouds form first over forest and, on the average, 30 min later over pasture. Deforestation may represent a reduced sink for ozone as compared to forest. 4. There are indications that mesoscale convective systems are initiated over high terrain. Deforestation may be enhancing rainfall.
- In the present case of very weak large-scale forcing a preference for deep cloud development over the mountains is suggested and deforestation may be acting as an enhancing factor for total rainfall. Clearly, there is a limit to the potential increase in rainfall in association with deforestation since, eventually, drier air may hinder further convective development.
\cite{D_Almeida_2007} Review paper
- Many macroscale modeling studies have simulated a complete deforestation in Amazonia, typically predicting reductions in precipitation, evapotranspiration, moisture convergence and (possibly) runoff, along with increments in surface temperature. However, this outcome is not strictly consistent with findings from various mesoscale model studies, which have continually suggested an increase in convection and potential rainfall along the borders between forested and deforested areas. In a simi- lar manner, apparently conflicting results have also been encountered by observational studies pursued at different scales. Enhanced overland flow has been observed over disturbed catchments in Amazonia, while significant trends on river discharge records collected close to the mouth of the basin have not been reliably observed yet.
- On the basis of the size of Amazonia and on the importance of its veg- etation to climate, the overall hydrological impact of deforestation seems to depend on both extent and spatial heterogeneity of the disturbance, as a result of the distinct land–atmosphere interactions induced by each particular scenario.
- Various observational studies (reviewed by Segal et al.,
1988) detected mesoscale anomalous circulations induced by air-temperature contrasts over regions of extreme land- surface gradients in different parts of the globe. In Ama- zonia, such circulations are expected to be observed espe- cially during the dry season, when contrasts in soil mois- ture conditions and therefore on the convective boundary layer (CBL) depth over forests and pastures are greater (Fisch et al., 2004).
- It has been estimated that, at the mesoscale, a landscape with a relatively large discontinuity tends to produce more precipitation than a homogeneous domain, inducing a negative feedback that ultimately tends to elimi- nate the discontinuity (Avissar and Liu, 1996). In some cases, the thermal circulation induced may become as intense as a sea-breeze circulation, for example, over domains with extended areas of unstressed dense vegetation bordering areas of bare soil (Segal et al., 1988). The horizontal scale of such landscape heterogeneities is another factor that may affect the establishment of pre- cipitation (Pielke et al., 1991), while the optimum scale for triggering convection seems to depend on the air- humidity level (Avissar and Schmidt, 1998).
- Thus, taking into account current data resolution, abundance and quality, one cannot be entirely sure whether deforestation is affecting the water cycle in
Amazonia, since the inherent effects could be occurring at subgrid, undetectable scales (Marengo, 1995).
- On the basis of these findings, it has been proposed
that deforestation in Amazonia seems to induce contrasting effects, depending on the spatial scale associated with the observed or simulated disturbance (D’Almeida et al., 2006). The primary cause for such a dependency is not strictly conceptual, but also operational. It relates to the fact that coarse resolution models cannot resolve small- scale phenomena with the same degree of detail as more refined models do. The same principle applies to obser- vations, which may represent any particular phenomenon differently, depending on the grid resolution, or on the distribution of gauging stations available. Secondly, the considerable size of the Amazon basin together with the land–atmosphere interactions occurring within, cause opposing factors to be dominant at different scales, and, therefore, a contrast naturally emerges.
- Such anomalous circulations are presently being generated on the mesoscale, but, since they may evolve to higher scales (Baidya Roy et al., 2003), they must in fact be adequately represented by AGCMs through their parameterization schemes (Bonell, 1998). However, despite the intense research on this topic (Avissar, 1992; Henderson-Sellers and Pitman, 1992; Koster and Suarez, 1992; Dickinson, 1996; Liu et al., 1999, among others), a consistent representation of these processes has not been widely adopted by the macroscale modeling community yet. The parameteriza- tions employed by the current generation of AGCMs tend to rely only on the quantification of turbulence effects, neglecting the influence of the heat fluxes associated with anomalous mesoscale circulations (Baidya Roy and Avis- sar, 2002).
\cite{Spracklen_2018}
- As the scale of deforestation increases from ∼1 km to 10s km, thermally driven circulations, may diminish (58), and changes in surface roughness may become more important (88, 89). Over
areas of pasture larger than 10 km, reductions in surface roughness can combine with increases in sensible heat fluxes to induce mesoscale circulations that cause redistribution of precipitation (Figure 3) (88).