The Impact of Observed Vegetation Changes on Land–Atmosphere Feedbacks During Drought

Abstract

Moderate Resolution Imaging Spectroradiometer (MODIS)-derived vegetation fraction data were used to update the boundary conditions of the advanced research Weather Research and Forecasting (WRF) Model to assess the influence of realistic vegetation cover on climate simulations in southeast Australia for the period 2000-08. Results show that modeled air temperature was improved when MODIS data were incorporated, while precipitation changes little with only a small decrease in the bias. Air temperature changes in different seasons reflect the variability of vegetation cover well, while precipitation changes have a more complicated relationship to changes in vegetation fraction. Both MODIS and climatology-based simulation experiments capture the overall precipitation changes, indicating that precipitation is dominated by the large-scale circulation, with local vegetation changes contributing variations around these.Simulated feedbacks between vegetation fraction, soil moisture, and drought over southeast Australia were also investigated. Results indicate that vegetation fraction changes lag precipitation reductions by 6-8 months in nonarid regions. With the onset of the 2002 drought, a potential fast physical mechanism was found to play a positive role in the soil moisture-precipitation feedback, while a slow biological mechanism provides a negative feedback in the soil moisture-precipitation interaction on a longer time scale. That is, in the short term, a reduction in soil moisture leads to a reduction in the convective potential and, hence, precipitation, further reducing the soil moisture. If low levels of soil moisture persist long enough, reductions in vegetation cover and vigor occur, reducing the evapotranspiration and thus reducing the soil moisture decreases and dampening the fast physical feedback. Importantly, it was observed that these feedbacks are both space and time dependent.

Land surface albedo and vegetation feedbacks enhanced the millennium drought in south-east Australia

“…The findings confirm that the damped surface fluxes during the development of the 2006 drought result in less area exhibiting the feedbacks compared to 2002, particularly the slow biological feedback. It also concurs with the finding in Meng et al (2014a) that the fast feedback is less likely to occur if the slow feedback is present (relatively few orange areas), as it acts to reduce the soil moisture changes.…”

“…Figure 14 shows when the fast physical and slow biological mechanisms are active during the 2006 drought and is comparable to Fig. 12 in Meng et al (2014a), which shows the same thing for the 2002 drought. The findings confirm that the damped surface fluxes during the development of the 2006 drought result in less area exhibiting the feedbacks compared to 2002, particularly the slow biological feedback.…”

“…A linear unmixing methodology was used in the derivation of vegetation fraction (Guerschman et al, 2009). Further details on the processing and analysis of this observation record are provided in Meng et al (2014a). Vegetation fraction anomalies are shown in Fig.…”

“…To account for the different timescales associated with albedo and vegetation changes, the methodology in Meng et al (2014a) can be used to identify the presence of fast physical feedbacks associated with albedo and soil moisture changes as discussed above, and slower vegetation-related changes that impact the strength of the fast feedbacks. Figure 14 shows when the fast physical and slow biological mechanisms are active during the 2006 drought and is comparable to Fig.…”

“…While some studies have used observations to quantify the soil moisture-precipitation feedback (Findell and Eltahir, 1999;Taylor et al, 2011Taylor et al, , 2012Catalano et al, 2016), more common is the use of model experiments to isolate and allow quantification of this behaviour (Schar et al, 1999;Koster et al, 2006;Seneviratne et al, 2013;Hirsch et al, 2014). Other slowly varying surface variables that have been found to provide feedbacks to precipitation include albedo (Charney et al, 1975;Lofgren, 1995;Zaitchik et al, 2007;Teuling and Seneviratne, 2008;Meng et al, 2014b) and vegetation (Pielke et al, 1998;Zeng and Neelin, 2000;Wang et al, 2006;Meng et al, 2014a). These feedbacks act on different timescales and can subdue or reinforce the feedback from the soil moisture field.…”