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The Impact of Ecosystem Functional Type Changes on the La Plata Basin Climate

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doi: 10.1007/s00376-012-2149-x

  • In this paper, the effects of land cover changes on the climate of the La Plata Basin in southern South America are investigated using the Weather and Research Forecasting (WRF) Model configured on a 30/10-km two-way interactive nested grid. To assess the regional climate changes resulting from land surface changes, the standard land cover types are replaced by time-varying Ecosystem Functional Types (EFTs), which is a newly devised land-cover classification that characterizes the spatial and interannual variability of surface vegetation dynamics. These variations indicate that natural and anthropogenic activities have caused changes in the surface physical parameters of the basin, such as albedo and roughness length, that contributed to regional climate changes. EFTs are obtained from functional attributes of vegetation computed from properties of the Normalized Difference Vegetation Index (NDVI) to represent patches of the land surface with homogeneous energy and gas exchanges with the atmosphere. Four simulations are conducted, each experimental period ranging from September to November in two contrasting years, 1988 and 1998. The influence of an identical EFT change on the surface heat fluxes, 2-m temperature and humidity, 10-m winds, convective instabilities and large-scale moisture fluxes and precipitation are explored for 1988 (a dry year) and 1998 (a wet year). Results show that the surface and atmospheric climate has a larger response to the same EFT changes in a dry year for 2-m temperature and 10-m wind; the response is larger in a wet year for 2-m water vapor mixing ratio, convective available potential energy, vertically integrated moisture fluxes and surface precipitation. For EFTs with high productivity and a weak seasonal cycle, the near-surface temperature during the spring of 1988 and 1998 increased by as much as 1C in the central and western portions of La Plata Basin. Additionally, for higher productivity EFTs, precipitation differences were generally positive in both dry and wet years, although the patterns are not uniform and exhibit certain patchiness with drier conditions.
    摘要: In this paper, the effects of land cover changes on the climate of the La Plata Basin in southern South America are investigated using the Weather and Research Forecasting (WRF) Model configured on a 30/10-km two-way interactive nested grid. To assess the regional climate changes resulting from land surface changes, the standard land cover types are replaced by time-varying Ecosystem Functional Types (EFTs), which is a newly devised land-cover classification that characterizes the spatial and interannual variability of surface vegetation dynamics. These variations indicate that natural and anthropogenic activities have caused changes in the surface physical parameters of the basin, such as albedo and roughness length, that contributed to regional climate changes. EFTs are obtained from functional attributes of vegetation computed from properties of the Normalized Difference Vegetation Index (NDVI) to represent patches of the land surface with homogeneous energy and gas exchanges with the atmosphere. Four simulations are conducted, each experimental period ranging from September to November in two contrasting years, 1988 and 1998. The influence of an identical EFT change on the surface heat fluxes, 2-m temperature and humidity, 10-m winds, convective instabilities and large-scale moisture fluxes and precipitation are explored for 1988 (a dry year) and 1998 (a wet year). Results show that the surface and atmospheric climate has a larger response to the same EFT changes in a dry year for 2-m temperature and 10-m wind; the response is larger in a wet year for 2-m water vapor mixing ratio, convective available potential energy, vertically integrated moisture fluxes and surface precipitation. For EFTs with high productivity and a weak seasonal cycle, the near-surface temperature during the spring of 1988 and 1998 increased by as much as 1C in the central and western portions of La Plata Basin. Additionally, for higher productivity EFTs, precipitation differences were generally positive in both dry and wet years, although the patterns are not uniform and exhibit certain patchiness with drier conditions.
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Manuscript revised: 09 November 2012
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The Impact of Ecosystem Functional Type Changes on the La Plata Basin Climate

    Corresponding author: Seung-Jae LEE, sjlee@atmos.umd.edu
  • 1. Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, U. S. A.; 
  • 2. Department of Botany, University of Granada, Granada, Spain
Fund Project:  The authors thank the reviewers for their comments that improved the manuscript. This research was supported by NSF Grant ATM0646856, NASA Grant NNX08AE50G, and the Inter American Institute for Global Change Research (IAI) through the Cooperative Research Network (CRN)-2094.

Abstract: In this paper, the effects of land cover changes on the climate of the La Plata Basin in southern South America are investigated using the Weather and Research Forecasting (WRF) Model configured on a 30/10-km two-way interactive nested grid. To assess the regional climate changes resulting from land surface changes, the standard land cover types are replaced by time-varying Ecosystem Functional Types (EFTs), which is a newly devised land-cover classification that characterizes the spatial and interannual variability of surface vegetation dynamics. These variations indicate that natural and anthropogenic activities have caused changes in the surface physical parameters of the basin, such as albedo and roughness length, that contributed to regional climate changes. EFTs are obtained from functional attributes of vegetation computed from properties of the Normalized Difference Vegetation Index (NDVI) to represent patches of the land surface with homogeneous energy and gas exchanges with the atmosphere. Four simulations are conducted, each experimental period ranging from September to November in two contrasting years, 1988 and 1998. The influence of an identical EFT change on the surface heat fluxes, 2-m temperature and humidity, 10-m winds, convective instabilities and large-scale moisture fluxes and precipitation are explored for 1988 (a dry year) and 1998 (a wet year). Results show that the surface and atmospheric climate has a larger response to the same EFT changes in a dry year for 2-m temperature and 10-m wind; the response is larger in a wet year for 2-m water vapor mixing ratio, convective available potential energy, vertically integrated moisture fluxes and surface precipitation. For EFTs with high productivity and a weak seasonal cycle, the near-surface temperature during the spring of 1988 and 1998 increased by as much as 1C in the central and western portions of La Plata Basin. Additionally, for higher productivity EFTs, precipitation differences were generally positive in both dry and wet years, although the patterns are not uniform and exhibit certain patchiness with drier conditions.

摘要: In this paper, the effects of land cover changes on the climate of the La Plata Basin in southern South America are investigated using the Weather and Research Forecasting (WRF) Model configured on a 30/10-km two-way interactive nested grid. To assess the regional climate changes resulting from land surface changes, the standard land cover types are replaced by time-varying Ecosystem Functional Types (EFTs), which is a newly devised land-cover classification that characterizes the spatial and interannual variability of surface vegetation dynamics. These variations indicate that natural and anthropogenic activities have caused changes in the surface physical parameters of the basin, such as albedo and roughness length, that contributed to regional climate changes. EFTs are obtained from functional attributes of vegetation computed from properties of the Normalized Difference Vegetation Index (NDVI) to represent patches of the land surface with homogeneous energy and gas exchanges with the atmosphere. Four simulations are conducted, each experimental period ranging from September to November in two contrasting years, 1988 and 1998. The influence of an identical EFT change on the surface heat fluxes, 2-m temperature and humidity, 10-m winds, convective instabilities and large-scale moisture fluxes and precipitation are explored for 1988 (a dry year) and 1998 (a wet year). Results show that the surface and atmospheric climate has a larger response to the same EFT changes in a dry year for 2-m temperature and 10-m wind; the response is larger in a wet year for 2-m water vapor mixing ratio, convective available potential energy, vertically integrated moisture fluxes and surface precipitation. For EFTs with high productivity and a weak seasonal cycle, the near-surface temperature during the spring of 1988 and 1998 increased by as much as 1C in the central and western portions of La Plata Basin. Additionally, for higher productivity EFTs, precipitation differences were generally positive in both dry and wet years, although the patterns are not uniform and exhibit certain patchiness with drier conditions.

1 Introduction
  • The La Plata Basin (LPB) is the fifth largest basin in the world and second largest basin in South America, after the Amazon Basin. The LPB is shared by surrounding five countries and occupied by approximately 50% of their populations and it is very important for regional economies, ecological communities, and hydroelectric power energy sources (see (Mechoso et al., 2001) for more information). Previous observational and analytical studies have shown that LPB has suffered from significant changes in land surface characteristics during recent decades. Significant deforestation has occurred in eastern Paraguay and large areas of the Paraná basin (Tucci and Clarke, 1998). (Tucci and Clarke, 1998) note large changes in the annual discharge of major La Plata tributaries after several decades of major deforestation. Other regions have experienced afforestation for commercial uses, or conversions between pastures, croplands and forests.

    A few recent numerical regional modeling studies have shown that land-cover and land-use changes in southern South America may have contributed to alterations in the regional climate. (Beltr -Przekurat et al., 2012) showed that the conversion from grass to agriculture led to cooler and wetter near-surface atmospheric conditions, while conversion of wooded grasslands or forest to agriculture produced warmer temperatures. (Lee and Berbery, 2012) performed ensemble simulations for an extreme land-cover change scenario (all croplands within LPB) with the Weather and Research Forecasting (WRF) system based on a two-way interactive nested grid. According to their results, an extensive agricultural practice could imply a reduction of surface temperature and total precipitation for the northern part of LPB, where croplands replace forests and savanna. The opposite behavior is noted in the southern part of LPB, where crops would replace grasslands, resulting in a slight increase of surface temperature and increased precipitation. They note that the results are not strictly local, and advective processes tend to modify the downstream circulation and precipitation patterns over the South Atlantic Ocean.

    The quality of numerical weather and climate predictions and simulations is dependent on accurate depictions of the surface conditions and realistic representations by satellite-derived surface information. (Oleson et al., 1997) conducted stand-alone model runs using two satellite-derived land cover maps and a global land-cover dataset commonly used in GCMs to show that the partitioning of net radiation into sensible and latent heat fluxes was different for the two satellite-derived datasets. Several studies used different regional models to show the importance of land cover for model simulations. (Kurkowski et al., 2003) implemented satellite-derived land cover data in the Eta Model (Black, 1994) and showed that use of the near-real-time vegetation fraction data from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiomenter (AVHRR) data improves the forecasts of both the 2-m temperature and dewpoint temperature for much of the growing season. (Yucel, 2006) employed Moderate Resolution Imaging Spectroradiometer (MODIS) land cover and albedo in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5) (Grell et al., 1994) at two contrasting U.S. regions and showed remarkable improvements in near-surface temperature and humidity in both study areas. (Ge et al., 2008) incorporated the MODIS data of the leaf area index and vegetation fractional cover in the Regional Atmospheric Modeling System (RAMS) (Pielke et al., 1992) and showed that the spatial, seasonal, and diurnal characteristics of the model land surface temperature were improved by MODIS data. The land-cover data used in all those studies are based on "structural" attributes of vegetation. For example, the United States Geological Survey (USGS) data set employs 24 types (Loveland and Belward, 1997), the International Geosphere-Biosphere Programme (IGBP) data set identifies 17 land cover types (Belward1999; Scepan1999), and ECOCLIMAP (Champeaux et al., 2005) identifies 15 land cover types.

    (Alcaraz-Segura et al., 2013) proposed utilization of the Ecosystem Functional Types (EFTs, Paruelo et al.,2001) as a new land-cover classification exclusively based on "functional" attributes of vegetation. EFTs, derived from descriptors of the seasonal dynamics of Normalized Difference Vegetation Index (NDVI), are surface patches with homogeneous exchanges of mass and energy. (Lee et al., 2013) implemented EFT data as lower boundary conditions in the WRF system and showed that their incorporation, instead of the USGS land cover dataset, could improve the accuracy in climate simulations of 2-m air temperature and surface precipitation over the LPB during austral spring 1998. This paper follows (Lee et al., 2013) to further explore the impact of land-use and land-cover changes on the climate of LPB using EFTs as lower boundary conditions. We specifically examine the sensitivity of 2-m air temperature and surface precipitation to the interannual variability of EFTs. We perform four numerical experiments consisting of September to November simulations with different EFTs configurations; specifically, a low net primary production year (1988) is contrasted with a high net primary production year (1998). To understand the feedbacks between surface conditions and the hydrologic cycle over the LPB, we examine the impact of the EFT differences between 1988 and 1998 on the surface heat fluxes, near-surface atmospheric variables, local thermodynamic instabilities, large-scale moisture flux convergence, and lastly surface precipitation.

    The numerical model configuration and EFT maps are presented in section 2. Section 3 presents the results of model simulations, surface and atmospheric prognostic and diagnostic variables analysis, and qualitative comparisons with observation. Finally, section 4 summarizes this paper and presents the conclusions.

    Figure 1.  Fig. 1.The WRF model domain configuration used in this study. Mother and nested domains have horizontal resolutions of 30 km and 10 km, respectively. Contour intervals for topography are indicated at the bottom in km. The LPB boundary is in red.

2 Methodology and data
  • The Advanced Research WRF model version 3.1.1 was used for the numerical simulations in this study. All of the simulations were performed on a two-way nested grid configuration with an outer grid of 625×377 points at a 30-km grid spacing to cover southern South America (including the LPB). An inner grid with 768×552 points at a 10-km grid spacing was nested to cover most of LPB and the northern and central parts of Argentina (see Fig. 1). Both grids have 27 vertical levels, and the intervals geometrically increase with height.

    The simulations covered a three-month period from September to November in 1988 and 1998, and the time step for the model integration was 180 s for the mother grid. Model physical parameterizations follow (Lee, 2010), who found the best results using the Dudhia shortwave scheme (Dudhia, 1989), the WRF Single Moment six-class microphysics (Hong and Lim, 2006), the Betts-Miller-Janjic’ cumulus scheme (JanjiÀ1,994, 2000), the Mellor-Yamada-Janjic’ boundary layer scheme (JanjiÀ 1990, 1996, 2002), the Monin-Obukhov-Janjic’ surface layer scheme (JanjiÀ 1996, 2002), and the Noah land surface model (Chen and Dudhia, 2001).

    Initial atmospheric fields and atmospheric lateral boundary conditions were taken from the National Centers for Environmental Prediction (NCEP) / National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al., 1996). The lateral boundary update interval is six hours, and five grid points were used for the lateral boundary nudging. Implementation of spectral nudging of large-scale dynamics is known to reduce biases in precipitation locations and improve monsoon circulations (e.g., Miguez-Macho et al.2005; Cha et al.2006). This study adopts upper-air spectral nudging techniques, and the nudging is conducted at every model time step on the coarse grid. The spectral nudging uses cut-off wave numbers of six and five in x and y directions of the domain, respectively.

  • Seasonal curves of NDVI can be used to derive 64 unique EFTs based on three descriptors of carbon gain dynamics: annual mean (surrogate of primary production), seasonal coefficient of variation (indicator of seasonality), and date of maximum NDVI (descriptor of phenology) see (Paruelo et al., 2001); (Alcaraz-Segura et al., 2013) for further details]. The EFT patterns can be obtained on a yearly basis to give a representation of the time varying land surface conditions. Use of EFT data for lower boundary conditions may offer a more realistic representation of the land surface for regional climate investigations.

    Figure 2.  Three-month (SON) average observed precipitation for spring (a) 1988 and (b) 1998 (Units: mm d-1).

    (Alcaraz-Segura et al., 2011) derived a consistent set of biophysical properties of the land surface based on EFTs, and examined their changes over LPB. They found that ecosystems in temperate South America have NDVI maxima in summer and autumn. EFTs with summer maxima tend to show medium-to-low productivity and high seasonality, while EFTs with autumn and spring maxima show most of the possible combinations of productivity and seasonality. EFTs with NDVI maxima during winter tend to exhibit either very high or very low productivity under very low seasonality values. When the interannual variability of vegetation properties was analyzed, high variability was found for surface roughness length, stomatal resistance, and minimum leaf area index, while low variability was observed for emissivity and radiation stress. Rooting depth, background albedo, green vegetation fraction, and maximum leaf area index exhibited intermediate variability. The present study uses the EFT maps of two very different years (1988 and 1998) in the WRF system for southern South America.

  • The 1988 and 1998 spring season periods were chosen for model sensitivity tests because climate factors produced strong differences in the EFTs distribution between 1988 and 1999. In 1988, the dominant EFTs showed high seasonality and medium to low productivity; in 1998, EFTs with high productivity and low seasonality dominated temperate South America, particularly LPB. Figure 2 shows three-month mean precipitation values from rain gauge data during September to November in 1988 and 1998. In general, the precipitation patterns are similar, with the largest precipitation in the southeastern part of the LPB. However, the regional details and precipitation magnitude were different. Compared with 1988, 1998 had an overall increase in precipitation, and the maximum precipitation region was somewhat shifted toward northeast. The shift leads to a reduction in precipitation over the southern LPB and enhances precipitation in the northern LPB.

    The two periods showed very different characteristics in spatial distribution and the magnitude of EFT productivities. Figure 3 shows the EFT maps for the two years over coarse and fine model grids. Orange colored regions indicate higher carbon productivity by vegetation, while dark brown colored regions show lower carbon productivity. The Andes Mountains exhibit very little changes in EFTs, while LPB exhibits dramatic changes in the EFTs. Overall, 1998 had higher productivity than 1988, implying that carbon production was more vigorous in 1998. Hereafter, the EFT conditions in 1988 and 1998 are called "LowEFT" and "HighEFT", respectively.

    Based on Figs. 2 and 3, pairs of simulations were conducted for September-November 1988 (a dry year) and 1998 (a wet year). Within a given pair, one member employed LowEFT as the surface boundary condition, while the other used HighEFT. This experimental design has the objective to examine the general impact of EFT changes on the climate of LPB and specifically explore how land surface forcing may affect the model representation of the contrasting two periods. The model validation is presented in Appendix A.

    Figure 3.  Ecosystem functional type data (Lee et al., 2013) used for the coarse (upper) and the fine (bottom) model grids. Left (right) panels are for 1988 LowEFT (1998 HighEFT) with 30-km and 10-km resolutions. Orange (dark brown) colored regions denote higher (lower) carbon productivity of vegetation. Each of the 64 EFTs was assigned a code based on two letters and a number, i.e., 1: Aa1; 2: Aa2; 3: Aa3; 4: Aa4; 5: Ab1; ...; 63: Dd3; 64: Dd4. The first letter of the code (capital) corresponds to the NDVI mean level, ranging from A to D for low to high (increasing) productivity. The second letter (small) shows the seasonal coefficient of variation, ranging from a to d for high to low (decreasing) seasonality. The numbers indicate the season of maximum NDVI (1-4: spring, summer, autumn and winter).

3 Results
  • Figure 4 shows surface albedo and roughness length fields for LowEFT and their differences (HighEFT minus LowEFT) over the LPB. Figures 4a and b indicate that in cases of LowEFT, the central eastern region of LPB has relatively small surface albedo (<15%) and large roughness length (>35 cm). Compared with LowEFT, the HighEFT field generally has smaller surface albedo (Fig. 4c) and larger surface roughness length (Fig. 4d) over the LPB. In particular, the southern part of LPB below 27°S (Uruguay and its neighbor regions) has the largest decrease in albedo and the largest increase in roughness length (Figs. 4c and d).

    Figure 5 illustrates the model-simulated three-month mean diurnal cycle of the surface energy balance over LPB. The simulated surface energy budget over LPB in spring 1988 and 1998 shows that the sensible heat flux is an important energy component, while the latent heat flux and the ground heat flux are approximately two times smaller (Figs. 5a and b). Figure 5b shows that the latent (sensible) heat flux is larger (smaller) in 1998 because the year 1998 has more moisture in the surface. The increase in sensible heat fluxes due to EFT changes from LowEFT to HighEFT (Figs 5c and d) is mainly associated with decreased surface albedo as shown in Fig. 4c. Figs. 5c and d indicate that the EFT changes have different effects on partitioning of net radiation into sensible and latent heat fluxes. For 1988, the major impact of EFT changes is to increase sensible heat fluxes, but the main effect in 1998 is an increase in latent heat fluxes. This observation indicates that EFT changes over the LPB increase sensible heat fluxes in a dry year and latent heat fluxes in a wet year.

    Figure 5.  Three-month (SON 1988 and 1998) mean diurnal cycle of surface fluxes (W m-2) averaged over La Plata Basin. Solid and dotted lines denote LowEFT and HighEFT, respectively, for (a) 1988 and (b) 1998. (c) and (d) show the differences HighEFT - LowEFT for 1988 and 1998, respectively. Black=Net radiation; Red=Turbulent sensible heat flux; Green=Turbulent latent heat flux; Blue=Conductive ground heat flux. All times are expressed in Zulu (Z) time (=UTC time) on the x-axis.

  • The horizontal distributions of sensible and latent heat fluxes in LPB play important roles in determining near-surface temperature and humidity. Figure 6 shows the three-month averaged 2-m temperature (℃) fields for 1988 and 1998 using LowEFT and the corresponding differences (HighEFT - LowEFT) for each year. The spatial gradient of near-surface temperature is similar between the two years, and the main difference occurs over the northern LPB, where 1998 shows colder spring temperature than 1988 (Figs. 6a and b). Figures 6c and d show that higher values of EFTs over LPB increase near-surface temperature in both 1988 and 1998. This observation is directly related to the increased sensible heat fluxes caused by the reduced surface albedo as shown in Figs. 5c and d. The largest increase is primarily located around Paraguay and Uruguay, where the temperature change reached +0.6℃to +0.8℃. There are weak cooling regions near the northeastern boundary of LPB and west of Uruguay. The two numbers in the bottom panels of the figures denote mean and standard deviation of the corresponding fields. The values indicate that the net impact of EFT changes from LowEFT to HighEFT on the near-surface temperature over the LPB is positive, and the corresponding variability is somewhat larger during a dry year.

    Figure 7 presents the three-month averaged 2-m water vapor mixing ratio fields (g kg-1) for 1988 and 1998 using LowEFT and the corresponding differences (HighEFT - LowEFT) for each year. The data indicate that 1998 has larger 2-m humidity over Paraguay (Figs. 7a and b). Because EFT changes in LPB increase latent heat fluxes (see Fig. 5), the basin-averaged 2-m water-vapor maxing ratio shows overall net increases in LPB (Figs. 7c and d). The largest increases are noted in western LPB for both 1988 and 1998. The largest decreases are found in the northeastern part of LPB for 1988 and southeastern LPB for 1998 (Figs. 7c and d). The net effect of more productive EFTs (HighEFTs) generally increases the near-surface humidity and its variability.

    Figure 8 shows the three-month averaged 10-m wind fields for 1988 and 1998 using LowEFT and corresponding differences (HighEFT - LowEFT) for each year. Both years (1988 and 1998) have larger wind speed in eastern LPB, and easterly component winds are dominant (Figs. 8a and b). EFT in 1988 increases and produces overall decreases in the 10-m wind speed (Fig. 8c) due to increases in roughness length (see Fig. 5c). Such decreases are additionally observed for 1998, but the maximum values appear at different locations (Figs. 8c and d). The net impact of EFT changes on the near-surface wind is negative, and the corresponding variability is larger in a dry year.

  • The changes in the surface fluxes and surface energy balance are expected to influence atmospheric instability. The convective available potential energy (CAPE) and convective inhibition (CIN) are useful terms to analyze the local thermodynamic processes that contribute to the development of convective precipitation. CAPE (CIN) is the amount of positively (negatively) buoyant energy, and both terms are defined as positive signs in this paper. CAPE (CIN) can be understood as a thermodynamic force facilitating (inhibiting) local convection and precipitation (Bluestein1993; Barlow1998). To simplify the analysis, the maximum values from the vertical profiles of CAPE and CIN were used and termed as MCAPE and MCIN, respectively (Refer to Lee and Berbery (2012), for a more detailed description).

    Figure 9 shows the three-month averaged MCAPE over LPB for 1988 and 1998 using LowEFT and the corresponding differences (HighEFT - LowEFT) for each year. The two years have comparatively large MCAPE over Paraguay for the same LowEFT conditions (Figs. 9a and b). When the lower boundary conditions are determined by HighEFT (as obtained from HighEFT - LowEFT), 1988 shows small changes in the magnitude of MCAPE with a weak signal (Fig. 9c). The EFT changes in LPB for 1998 produces more striking changes in MCAPE compared with 1988 (Fig. 9d). In 1998, the largest increase in MCAPE occurs over northern Paraguay, and the largest decrease occurs in southern LPB, especially between Paraguay and Uruguay (Fig. 9d). Regions of the maximum increase (decrease) in MCAPE tend to correspond to regions of maximum increase (decrease) in 2-m water vapor mixing ratio, especially for 1998 (Fig. 7). These data indicate that higher EFTs tend to increase the atmospheric instability. Similarly, the corresponding variability is much larger in a wet year.

    Figure 7.  The same as Fig. 6 but for 2-m water vapor mixing ratio (g kg-1).

    Figure 10 is similar to Fig. 9 but for MCIN. The two years, 1988 and 1998, have comparatively large MCIN around Paraguay for the same LowEFT condition, with 1998 showing larger values over northern Paraguay (Figs. 10a and b). Compared with MCAPE (Figs. 9c and d), the MCIN difference field (Figs. 10c and d) has a less organized structure. Figures 9c, d and 10c, d indicate that the northwestern part of LPB has a favorable condition for local convective precipitation due to the increased MCAPE and the decreased MCIN. The net impact of EFT changes (HighEFT - LowEFT) on MCIN is very small for both a dry year and a wet year, and the variability of MCIN corresponding to the EFT change is almost the same between a dry year and a wet year (Figs. 10c and ,d). The effect of EFT changes on the MCIN field does not show much dependency on the two contrasting years (Figs. 10c and d), in contrast with MCAPE (Figs. 9c and d) that shows highly different features between a dry year and a wet year. Therefore, the MCAPE field is expected to play a more decisive role than MCIN in determining the local convective precipitation.

  • The change in precipitation over LPB cannot be attributed only to changes in the MCAPE and MCIN fields. The change in the surface fluxes and near-surface atmospheric variables can affect not only the local instability but also regional moisture fluxes by modifying the surrounding atmospheric circulation.

    A major reason for variability in precipitation over LPB is moisture that Low-Level Jets (LLJs) transport from the Amazon basin (e.g., Berbery et al., 2000). Figure 11 presents vertically integrated moisture fluxes for 1988 and 1998 using LowEFT and the corresponding differences (HighEFT minus LowEFT). The convergence/divergence of the moisture fluxes is shaded using the color scale to the right of the figure. The changes in moisture flux and convergence/divergence over LPB can be associated with the increased near-surface wind due to the reduction in roughness length as shown in Figs. 4d and 8c, d. Figures 11a and b illustrate the well-known moisture supply that the northerly LLJ provides to LPB during the warm season. A region of southward low-level moisture flux into the LPB occurs throughout the relatively low-altitude lands formed between the Andes Mountains and the Brazilian Highlands. The wet year of 1998 has larger moisture transport from the north, and the region of largest moisture flux convergence is observed around Paraguay (Fig. 11b).

    EFT changes (HighEFT minus LowEFT) over the LPB produce changes in moisture flux and the associated convergence/divergence fields. Greater organization in the effects of EFT changes on moisture flux produces a counterclockwise circulation near central Paraguay in 1998 (Fig. 11d). These mesoscale circulations generate a moisture convergence (divergence) zone over northern (southern) Paraguay. The net impact of EFT changes on the large-scale moisture transport is almost neutral, while the corresponding variability is larger in a wet year. Figures 11e and f show the 1000 hPa-850 hPa layer moisture fluxes and their convergence. Although the net effect of EFT changes with the moisture flux convergence, and the corresponding variability is similar to the one vertically integrated from 1000 hPa to 300 hPa, the intensity of moisture flux convergence is strong because atmospheric humidity is larger at lower levels. Figures 11e and f show that EFT changes on the lower tropospheric moisture convergence produce overall increases (decreases) in precipitation in the northwestern (southern) parts of LPB.

    Figure 9.  The same as Fig. 6 but for maximum CAPE (J kg-1).

  • As a reasonable approximation, surface precipitation over the LPB can be understood as a combined product of local thermodynamic effects and larger-scale dynamical effects discussed in sections 3.3 and 3.4, respectively. Figures 12a and b show the three-month average precipitation field for 1988 and 1998 using LowEFT. Precipitation differences between the two years are striking, and precipitation increases due to the EFT changes greatly over Paraguay in the wet year (Figs. 12a and b). Such an increase of precipitation over Paraguay is consistent with the large MCAPE (Fig. 9b) and the large moisture flux convergence (Fig. 11b) over Paraguay in the wet year.

    Figures 12c and d present the corresponding differences (HighEFT minus LowEFT) for each year. The increase in EFT values is related to increases in precipitation for both years, and the corresponding variability is larger in a wet year. The net increase in precipitation produced by higher EFT boundary conditions is larger for the wet year (1998) than the dry year (1988). Figure 12d shows that EFT changes tended to greatly increase precipitation in northern LPB above latitude 26°S and decrease precipitation in southern Paraguay. Figures 11c, d and 12c, d show an overall agreement between total precipitation and vertically integrated moisture flux convergence. The regions of precipitation increase (decrease) are collocated with vertically integrated moisture flux convergence (divergence) for both dry and wet years.

    Figure 11.  . Fig. 11. (a)-(d) Same as Fig. 6 but for vertically integrated moisture fluxes [kg (m s)-1] and their convergence (mm d-1). (e)-(f) Same as (c)-(d) but for moisture fluxes integrated for lower troposphere from 1000 hPa to 850 hPa.

    Figures 12e and f are similar to Figs. 12c and d, but for the convective component of total precipitation. The net effect of the EFT changes on the convective precipitation is small for a dry year, and the corresponding variability is larger in a wet year than a dry year. The horizontal distributions of the convective and total precipitation look somewhat, similar except for the magnitude. The northern part of LPB has dominant increases in the convective precipitation for the wet year (Fig. 12f), which seems to be related with the MCAPE field in Fig. 9d.

  • To assess the realism of the simulations, Fig. 13 presents the observed changes in precipitation and 2-m temperature between 1988 and 1998. Figure 13a shows that greater precipitation occurred in the northern LPB and less precipitation in the southern LPB compared to 1988. The region with the maximum increase (decrease) in precipitation is located east of southern Paraguay (around Uruguay). Figure 13b shows a colder near-surface temperature over the northwestern LPB and a slightly warmer near-surface temperature over the northeastern and southeastern parts of LPB in 1998.

    The simulated changes in precipitation from 1988 to 1998 are similar to the observed changes. For example, the signals of changes in observed precipitation (Fig. 13a) are found in the simulated MCAPE (Fig. 9d), vertically integrated moisture flux convergence (Figs. 11e and f) and convective precipitation fields (Fig. 12f). EFT changes in a wet year may induce a large increase of CAPE, especially in the northern LPB, and may facilitate the development of mesoscale convective systems responsible for much of the region's precipitation.

    The simulated changes in 2-m temperature (Fig. 6c and d) from 1988 to 1998 are similar to the observed fields (Figs. 13b) over southern LPB, especially around Uruguay. This observation is explained because the largest changes in the surface albedo and roughness length primarily occurred over that region (see Figs. 4c and d), so the effect of EFT changes is more evident in those regions. However, other regions such as Paraguay show opposite features between the simulated and observed 2-m temperature. This observation implies that those regions are strongly exposed to factors other than land cover changes.

4 Summary and concluding remarks
  • The climate of the La Plata Basin in South America has been subject to land-cover and land-use changes with important consequences for the environment. In this study, the WRF modeling system and EFT data were used to explore the role of changing land cover conditions from 1988 to 1998 in the regional climate of LPB. EFT maps over the LPB indicate that the surface albedo in 1988 was larger than 1998, while the surface roughness length was lower in 1988 than 1998. This observation is consistent with more plentiful vegetation during 1998, as measured by NDVI. To examine the dependency of the land surface forcing on two different years, simulations were conducted exchanging the corresponding lower boundary conditions. Four simulations were produced: one for the austral spring of 1988 with its corresponding lower boundary conditions (as estimated from the EFTs), and a second one for the austral spring of 1998 and its corresponding lower boundary conditions. Two other simulations for the same years exchanged the lower boundary conditions to evaluate the sensitivity to the land surface states for a dry year (1988) and a wet year (1998).

    Replacing the 1988 EFT data (LowEFT) with the 1998 EFT data (HighEFT) altered the simulated spatial patterns of the surface heat fluxes, near-surface atmospheric variables, local thermodynamic and larger-scale dynamic forcings, and surface precipitation. Table 1 summarizes the impact of EFT changes on all prognostic variables. The range of surface and atmospheric responses to the given EFT changes is larger in a dry year for 2-m temperature and 10-m wind. However, the signals in a wet year are larger for the 2-m water vapor mixing ratio, convective available potential energy, vertically integrated moisture fluxes and surface precipitation. When EFTs with high productivity and a weak seasonal cycle are used, the near-surface temperature for the 1988 and 1998 springs tends to increase by as much as 1℃ in the central and western portions of the La Plata Basin. High productivity induces higher precipitation, revealing a positive feedback, during a dry or wet year (although the patterns are not uniform and exhibit certain patchiness with drier conditions). The simulated differences between the precipitation patterns of 1988 and 1998 when using EFTs as lower boundary conditions are consistent with the observed precipitation differences. The simulated 2-m temperature changes reflect observed changes over the southern LPB.

    The most significant changes occurred over southern LPB (26°S), where simulations were most sensitive to EFT changes. Considering the HighEFT, lower boundary conditions led to an increase in the 2-m temperature, especially during a dry year, and an overall decrease in the spring precipitation total, especially during a wet year. This result agrees with previous modeling studies (e.g., Narisma and Pitman, 2003; Marshall et al., 2004; Pongratz et al., 2006; McAlpine et al., 2007) that indicate decreasing regional precipitation and increasing temperature due to historical land cover changes.

    It is suggested that ecosystem functional type changes over the LPB can be important climate elements in the La Plata Basin that are worth monitoring on a regular basis. Their impacts on regional climate need to be further assessed with longer-term ensemble model simulations to determine the statistical significance of the preliminary results presented in this study.

  • The model's performance was evaluated in terms of 2-m temperature and precipitation. The dataset for the first model was produced by the University of East Anglia Climate Research Unit (CRU) with version 3.0 (Mitchell2005; Brohan2006; University of East Anglia Climate Research Unit, 2010). The second model employed a dataset of gridded observed precipitation (land only) provided by the NCEP's Climate Prediction Center (Shi et al.,2000; Silva et al.,2007). This product is based on daily rain gauge observations interpolated to a 1°×1°grid covering South America. Figure A1 presents the 3-month (SON, 1988, 1998) averaged 2-m temperature from CRU, the model simulation, and their difference. The overall patterns of observed and simulated temperatures show high resemblance in structure and northeast-southwest gradients in a dry (1988) and a wet (1998) years, although the model tends to slightly overestimate the observation by 1°C-2°C over the northern basin. Figure A2 presents the SON 1988 and SON 1998 3-month averages for the two estimates of observed precipitation and the model's simulated precipitation. Performance against observed precipitation is worse compared to model performance for 2-m temperature fields. The model simulations capture the observed trend between 1988 and 1998, but the magnitude and locations are somewhat unmatched. This difficulty in realistic precipitation reproduction is a common characteristic of many other models as reported in the literature (e.g., Misra et al.,2002; Seth et al.,2003; Solman et al.,2008; Menàdez et al., 2010; Lee and Berbery, 2012). Note that, due to sparse distribution of rain-gauge measurement stations, the observed precipitation dataset may have unreliable and misleading values particularly over mountainous regions between Bolivia and Paraguay. The evaluation needs to be qualitative rather than quantitative, and the model result is acceptable considering the numerous studies published previously, as mentioned above.

    Figure A1.  Three-month averaged 2-m temperature (°C) from surface observations (CRU) for (a) SON 1988 and (b) SON 1998. (c) and (d) are the same as (a) and (b) but for model simulation results.

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