Research Article Spatial Downscaling of TRMM...

Hindawi Publishing CorporationAdvances in MeteorologyVolume 2013, Article ID 237126, 9 pageshttp://dx.doi.org/10.1155/2013/237126

Research ArticleSpatial Downscaling of TRMM Precipitation UsingGeostatistics and Fine Scale Environmental Variables

No-Wook Park

Department of Geoinformatic Engineering, Inha University, Incheon 402-751, Republic of Korea

Correspondence should be addressed to No-Wook Park; [email protected]

Received 6 September 2013; Revised 19 November 2013; Accepted 27 November 2013

Academic Editor: Chung-Ru Ho

Copyright © 2013 No-Wook Park. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A geostatistical downscaling scheme is presented and can generate fine scale precipitation information from coarse scale TropicalRainfall Measuring Mission (TRMM) data by incorporating auxiliary fine scale environmental variables. Within the geostatisticalframework, the TRMM precipitation data are first decomposed into trend and residual components. Quantitative relationshipsbetween coarse scale TRMM data and environmental variables are then estimated via regression analysis and used to derive trendcomponents at a fine scale. Next, the residual components, which are the differences between the trend components and the originalTRMM data, are then downscaled at a target fine scale via area-to-point kriging. The trend and residual components are finallyadded to generate fine scale precipitation estimates. Stochastic simulation is also applied to the residual components in order togeneratemultiple alternative realizations and to compute uncertaintymeasures. From an experiment using a digital elevationmodel(DEM) and normalized difference vegetation index (NDVI), the geostatistical downscaling scheme generated the downscalingresults that reflected detailed characteristics with better predictive performance, when compared with downscaling without theenvironmental variables. Multiple realizations and uncertainty measures from simulation also provided useful information forinterpretations and further environmental modeling.

1. IntroductionPrecipitation information has been regarded as one of theimportant information sources for understanding hydro-logical, ecological, and environmental systems [1–3]. Thisinformation can be obtained from either rain gauge stationdata or remote sensing data. Although precise precipitationinformation can be obtained from rain gauge data, fewrain gauge data are usually available, which may hinder thegeneration of reliable maps. Remote sensing data that canprovide periodic and exhaustive information can be effec-tively used to map precipitation information. For example,the Global Precipitation Climatology Project (GPCP) [4]and the Tropical Rainfall Measuring Mission (TRMM) [5]provide precipitation data at regional and global scales [3].As an international satellite mission, the Global PrecipitationMeasurement (GPM) mission, scheduled to launch in 2014,will also provide next-generation observations of rain andsnow worldwide [6]. Although these missions or projects canprovide time-series precipitation information, their spatialresolutions are too coarse to be applied to local analysis (e.g.,the finest resolution from TRMM data is 0.25 degree).

In hydrological, ecological, and environmentalmodeling,various data sets acquired at the different scales are generallyused as inputs in addition to precipitation data. Thus, scaleconversion or change of support is usually required for theconsistent analysis. When coarse scale precipitation dataand other fine scale data sets are available; for example,downscaling of the coarse scale precipitation data is requiredbefore subsequent modeling is conducted. Downscaling canbe regarded as the spatial prediction of unknown valuesat a finer scale from coarse scale data. In relation to thiskind of change of support, several statistical downscalingschemes have been proposed and applied to various researchfields [7–11]. The spatial downscaling of coarse scale datacan be categorized into two cases according to data avail-ability. In the first case, in which ground measurements aresimultaneously available, calibration of the coarse scale dataand integration for downscaling are enabled by using theground measurement data. In the second case, in which nogroundmeasurements are available, direct downscaling of thecoarse scale data is only feasible by using spatial correlationinformation from the available data. If auxiliary environment

2 Advances in Meteorology

variables that are related to the primary attribute of interestare acquired at a relatively finer scale, these data can improvethe quality of the downscaling result.

Regarding downscaling of coarse scale precipitation data,auxiliary environmental variables such as topography andvegetation have been used for downscaling with regressionanalysis [12–17]. Regression analysis can account for thestatistical relationships between precipitation and environ-mental variables, but residuals that cannot be explainedby the auxiliary variables usually remain after regressionanalysis. If the auxiliary variables fail to convey sufficientinformation on precipitation patterns, the residuals cannotbe discarded, resulting in unreliability in the downscalingresults. Conventional regression analysis follows the assump-tion that residuals are spatially independent or uncorrelated.If reasonable spatial correlation structures are observed inthe residuals, this correlation information can improve thequality of the downscaling. Recently, Jia et al. [3] proposeda statistical scheme for the downscaling of TRMM data withfine scale elevation and normalized difference vegetationindex (NDVI).They first conductedmultiple regression anal-ysis at various spatial scales then interpolated the residuals tothe fine scale grid, and finally generated downscaling resultsby adding the trend component by regression analysis at anoptimal spatial scale to the residuals at a fine scale. Duringthe interpolation of the residuals, however, they neither con-sidered the spatial patterns nor examined the reproduction oforiginal TRMM values.

Geostatistics has been known as an effective tool for scaleconversion and data integration [18, 19]. Spatial downscalingor disaggregation based on geostatistics has been applied tovarious fields such as land-cover mapping [20], populationdensity estimation [21], and image sharpening [22]. Despiteits great potential for spatial downscaling, geostatistics hasnot, to the author’s knowledge, been applied to the downscal-ing of coarse scale TRMMdata with fine scale environmentalvariables.

In this paper, a geostatistical downscaling scheme ispresented that incorporatesmultiple fine scale environmentalvariables into downscaling of coarse scale TRMM data inthe absence of any ground measurement data. Geostatisticaldownscaling and data integration are implemented by usingarea-to-point residual kriging with regression. Elevation andNDVI at a fine scale, which are related to precipitation patt-erns, are used as auxiliary environmental variables for down-scaling of TRMM data. Regression analysis is first conductedto derive statistical relationships between TRMM data andauxiliary environmental variables at an original coarse scale.Then residuals at the coarse scale are downscaled to a finerscale via area-to-point residual kriging by accounting for thespatial correlation information of the input residuals [23].Thefinal downscaling results are obtained by adding the down-scaled residuals to the trend components estimated byregression analysis.This approach can reproduce the originalTRMM precipitation values when the downscaling results ata fine scale are upscaled or aggregated to the coarse scale.To quantify the uncertainty of downscaling, stochastic sim-ulation based on block sequential simulation is also imple-mented. A downscaling experiment using the data sets from

South Korea is carried out to examine the potential anddemonstrate the applicability of the presented geostatisticalscheme for downscaling.

2. Study Area and Data SetsFor the downscaling experiment, the TRMM3B43 data acqu-ired in October 2005 over South Korea were experimentallyused. The original TRMM data that provide precipitationrates (mm/h) at a 0.25∘ scale were geocoded to TransverseMercator (TM) coordinates with a spatial resolution of 25 km,and converted to monthly cumulative precipitation at a cmscale (Figure 1(a)). Both digital elevation model (DEM) andNDVI data at 1 km resolution were used as fine scale auxiliaryvariables, for topography and a proxy for vegetation, respec-tively. The DEM was generated from digital topographicmaps and MODIS monthly NDVI data were used (Figures1(b) and 1(c)). DEM and NDVI values at land areas wereonly considered and sea areas were masked out. Independentprecipitation data observed at 69 meteorological observationstations in October 2005 were used to quantitatively evaluatethe predictive performance of the presented downscalingscheme. In this experiment, the target scale was set to 1 km,which is the same as that of DEM and NDVI.

3. Geostatistical Downscaling

The geostatistical downscaling scheme presented in thispaper is summarized in Figure 2. The main key idea of thisapproach is that TRMMprecipitation values are decomposedinto deterministic trends and stochastic residuals. The trendcomponents are estimated using auxiliary environmentalvariables such as DEM and NDVI. More specifically, statis-tical relationships between original precipitation values andmultiple environmental variables at a coarse scale are firstderived via multiple regression analysis. Under the assu-mption that attribute values at a coarse scale are linear ave-rages of their constituent fine scale point values, these relati-onships are applied to environmental variables at a fine scaleand finally trend components are estimated at a fine scale.

Suppose that a study area of interest consists of𝐾 TRMMprecipitation blocks acquired at a coarse scale {𝑧(V

𝑘), 𝑘 =

1, . . . , 𝐾}, where V𝑘= V(u

𝑘) is the 𝑘th data with its centroid

u𝑘. In this study, the TRMM data at a coarse scale are

regarded as block data. Other data sources are 𝑀 auxiliaryfine scale environmental variables {𝑦𝑘

𝑖

(u𝑛), 𝑖 = 1, . . . ,𝑀, 𝑛 =

1, . . . , 𝑁} within each 𝑘th block data.𝑁 denotes the numberof discretizing points within each block and its determinationdepends on the predefined finer scale value.

If the environmental variables show linear relationshipswith the TRMM precipitation values, the TRMM precipi-tation data at both coarse and fine scales can be expressedin terms of the environmental variables via multiple linearregression analysis as

𝑧 (V𝑘) = 𝑎 +

𝑀

∑

𝑖=1

𝑏𝑖⋅ 𝑦𝑖(V𝑘) + 𝑅 (V

𝑘) ,

𝑧𝑘

(u𝑛) = 𝑎 +

𝑀

∑

𝑖=1

𝑏𝑖⋅ 𝑦𝑘

𝑖

(u𝑛) + 𝑅𝑘

(u𝑛) ,

(1)

Advances in Meteorology 3

150<

125–150

100–125

75–100

50–75

25–50

0–25

(a)

800–900

700–800

600–700

500–600

400–500

300–400

200–300

100–200

>900

<100

(b)

0.8–1.0

0.6–0.8

0.4–0.6

0.2–0.4

0.0–0.2

−0.2–0.0

(c)Figure 1: Data sets used for the downscaling experiment: (a) TRMM precipitation data (unit: mm), (b) DEM (unit: m), and (c) NDVI. Therectangles denote TRMM blocks and the size of each block is 25 km by 25 km.

where are 𝑎 and 𝑏𝑖are regression coefficients for the intercept

and slope of the 𝑖th variable, respectively. 𝑧𝑘(u𝑛) is the

downscaled precipitation value at a target finer scale withinthe 𝑘th block data. 𝑅(V

𝑘) and 𝑅𝑘(u

𝑛) denote the residual

components at coarse and fine scales, respectively, whichcannot be accounted for by environmental variables.

If the original TRMM data, auxiliary environmentalvariables, and the residual components at a coarse scale canbe expressed by the average values of𝑁 fine scale data withineach block, respectively, (1) can be reformulated as (2)

𝑧 (V𝑘) = 𝑎 +

𝑀

∑

𝑖=1

𝑏𝑖[1

𝑁

𝑁

∑

𝑛=1

𝑦𝑘

𝑖

(u𝑛)] +1

𝑁

𝑁

∑

𝑛=1

𝑅𝑘

(u𝑛)

=1

𝑁

𝑁

∑

𝑛=1

[𝑎 +

𝑀

∑

𝑖=1

𝑏𝑖𝑦𝑘

𝑖

(u𝑛) + 𝑅𝑘

(u𝑛)]

=1

𝑁

𝑁

∑

𝑛=1

𝑧𝑘

(u𝑛) .

(2)

To obtain the downscaled precipitation value (𝑧𝑘(u𝑛)),

the residual component value (𝑅𝑘(u𝑛)) at a fine scale needs

to be determined from (2), because the regression coeffi-cients have already been determined from (1). To predictresidual components at a fine scale, area-to-point simplekriging proposed in Kyriakidis [23] is applied to the residualcomponents available at a coarse scale. Area-to-point simplekriging predicts the residual component values at a fine scaleby a linear combination of neighboring attribute values at acoarse scale [23, 24], as shown in (3):

𝑅𝑘

(u𝑛) =

𝐾

∑

𝑘=1

𝜆𝑘(u𝑛) 𝑅 (V𝑘) , (3)


TRMMprecipitation

at a coarse scaleat a coarse scale

at a coarse scale

at a coarse scale

DEM DEMat a fine scale

at a fine scale

at a fine scale

at a fine scale at a fine scale

RegressionNDVI NDVI

Upscaling

Residual component

Residual component

at a fine scale

Residual component

at a coarse scaleTrend component Trend component

Precipitation

Precipitation

Residualsimulation

realizations

at a fine scalerealizations

Area-to-point residual kriging

Figure 2: Work flows for geostatistical downscaling of coarse scale TRMM precipitation data.

where 𝜆𝑘(u𝑛) is a simple kriging weight assigned to the

neighboring block residual component 𝑅(V𝑘) at a prediction

location. Since the mean value of the residual component iszero, a constant mean value required for simple kriging is setto zero and does not appear in (3).

The simple kriging weight is computed by solving thefollowing simple block kriging system:

𝐾

∑

𝑘=1

𝜆𝑘(u𝑛) 𝐶 (V𝑘, V𝑙) = 𝐶 (V

𝑘, u𝑛) 𝑘 = 1, . . . , 𝐾, (4)

where 𝐶(V𝑘, V𝑙) and 𝐶(V

𝑘, u𝑛) refer to block-to-block covari-

ance and block-to-point covariance, respectively.Unlike conventional punctual kriging algorithms, both

block-to-block and block-to-point covariances are requiredto compute the simple kriging weights. These two covariancevalues can be computed by averaging point covariance valuessuch as the conventional block kriging system [24, 25]. Theblock-to-block covariance can be computed by averaging thecovariance values between any two points discretizing twoblocks.The covariance values between the point location anda set of points discretizing the block are averaged to obtain theblock-to-point covariance. Thus, a point-support covariance(equivalently, variogram) is required to obtain the above twocovariance values. However, only residuals at a coarse scaleare available, so it is not feasible to directly obtain the point-support variogram. In relation to this issue, Goovaerts [24]proposed an iterative deconvolution procedure to estimatethe point-support variogram from block or areal data irre-spective of their shapes. Variogram deconvolution iteratively

finds an optimal point-support variogram model that mini-mizes the difference between the regularized variogram andthe variogram of the coarse scale data [24]. More specifically,after defining an initial point-support variogram model, thetheoretically regularized model is computed and comparedwith the variogram of the coarse scale data. By computing thedifference statistic from those two models, the parameters ofthe point-support variogrammodel are adjusted to minimizethat difference statistic. This procedure is iteratively repeateduntil the stop criteria are satisfied [24, pp.113–115].

After computing the point-support variogram, the simplekriging weight is computed using (4) and then residualcomponent values at a fine scale are predicted using (3). Thefinal downscaling results are obtained by adding the predictedresidual component at a fine scale to the trend componentthat can be directly computed by applying regression relation-ships to the auxiliary environmental variable at a fine scale in(1).

An interesting property of area-to-point kriging is itsability for coherent predictions, whichmeans that the averageof the predicted values at the fine scale points discretizinga given block reproduces the original block data value [23].Since the residual component at a fine scale is generatedby preserving this coherence property, the final downscalingresults could reproduce the original TRMM precipitationvalues when upscaled.

The downscaling results by area-to-point residual krigingwith regression are optimal in the least-squares sense. As dis-cussed in Boucher andKyriakidis [20]; however, downscalingshould be regarded as a kind of under-determined inversionprocess, since there are many possible attribute values at


a fine scale that lead to the same original values at a coarsescale when upscaled. Thus, stochastic simulation, which cangenerate multiple alternative realizations of unknown truthat a fine scale, is also applied for downscaling of residualcomponents at a coarse scale. Block sequential simulationproposed by Liu and Journel [26] is applied in this study.By adopting the concept of direct sequential simulation,simulation does not require any data transformation unlikesequential Gaussian simulation. Originally, this simulationalgorithm was developed to combine kriging using bothblock and point data with direct sequential simulation [19,26]. As only block data (i.e., TRMM precipitation data) areavailable in this downscaling study, block sequential simu-lation with block data is implemented to generate multipledownscaling realizations.The same point-support variogrammodel estimated from variogram deconvolution is also usedfor simulation. Like the prediction by area-to-point kriging,residuals at a coarse scale are first simulated at a fine scale andfinal simulation results are obtained by adding the simulatedresidual components to the trend components.

Simulation aims at generating a set of alternative realiza-tions with the reproduction of spatial patterns, not a singledownscaling result like kriging. By comparing the differencesbetween multiple realizations, the uncertainty attached tothe prediction can be quantified. In this study, conditionalvariance values are computed and used as an uncertaintymeasure.

4. Results and Discussion

4.1. Regression Analysis Results. First, regression analysis wasapplied to derive statistical relationships between precipi-tation and the two environmental variables at the original25 km resolution. For this analysis, DEM and NDVI wereupscaled to 25 km resolution data via linear averaging. Thelinear correlation coefficients between precipitation and envi-ronmental variables were 0.56 and 0.26 for DEM and NDVI,respectively. In addition to the two variables, the interactionof DEM with NDVI was included in the multiple regressionanalysis, since forest areas showing high NDVI values tend tobe located in high altitude zones (the linear correlation coef-ficient between DEM and NDVI was 0.65). From backwardstepwise elimination, DEM and the interaction of DEM withNDVI were finally chosen as statistically significant variablesand the regression relation between precipitation and thosetwo variables was modeled as

𝑧∗

(V𝑘) = 17.508 + 0.513 DEM (V

𝑘)

− 0.592 DEM (V𝑘) ⋅NDVI (V

𝑘) ,

(5)

where 𝑧∗(V𝑘), DEM(V

𝑘), and NDVI(V

𝑘) are the estimated

mean (trend component) from regression analysis, DEM, andNDVI values at a block location, respectively. This regressionrelationshipwas also applied to the same variables to generatethe trend components at 1 km resolution.

The coefficient of determination (𝑅2) was 0.37, whichmeans that about 37% of the variation of precipitation couldbe accounted for by these two terms. This value of 𝑅2implies that a simple application of multiple regression for

700

600

500

400

300

300

200

200

100

100

0

0 50 150 250

Vario

gram

Distance (km)

Experimental variogram of block residualsVariogram model of block residualsDeconvolution model of residuals

Figure 3: Variogram deconvolution results for residual compo-nents.

downscaling could not generate reliable downscaling resultsand that the residuals should be considered for downscaling.

4.2. Variogram Deconvolution Results. After generating resi-dual components from regression analysis at original 25 kmresolution, variogram deconvolution was applied to estimatethe unknown point-support variogram of the residuals andimplemented using SpaceStat software (BioMedware). Byconsidering the target resolution (i.e., 1 km), each block ofTRMM data at 25 km resolution was discretized by 25 by 25points at 1 km resolution.

The variogram deconvolution result is shown in Figure 3.The variogram of residual components at 25 km resolutionhad a bounded correlation structure with a range of about266 km. The deconvoluted point-support variogram, which,once regularized, was the most similar to the variogrammodel of residuals at 25 km resolution, had a greater sillvalue than that of the residual components at 25 km res-olution. It also showed a reasonable correlation structurewith two spherical models showing short- and long-rangecorrelation structures, respectively. This bounded variogrammodel implies that the residual components were spatiallycorrelated unlike the assumption of spatial independenceof residuals in regression. Such spatial correlation of theresidual components was accounted for in the estimation andsimulation of residuals.

4.3. Area-to-Point Residual Kriging Results. The residualcomponents at 1 km resolution were estimated by area-to-point kriging using SpaceStat software and then added to thetrend components. To investigate the effects of using auxiliaryenvironmental variables on downscaling results, the originalTRMM data were directly downscaled without DEM andNDVI by applying area-to-point kriging to the original blockvalues, not to the residuals. The point-support variogrammodel of the original TRMM data was also estimated byvariogram deconvolution.

The two downscaling results generated by area-to-pointkriging are shown in Figure 4. Overall patterns of both down-scaling results were very similar to those of the coarse scale


125–150

100–125

75–100

50–75

25–50

0–25

150<

(a)

125–150

100–125

75–100

50–75

25–50

0–25

150<

(b)

Figure 4: Downscaling results by area-to-point residual kriging: (a) downscaling without DEM and NDVI and (b) downscaling with DEMand NDVI.

120

100

80

60

40

20

00 12010080604020

Agg

rega

ted

valu

e

TRMM precipitation value

(a)

120

100

80

60

40

20

00 12010080604020

Agg

rega

ted

valu

e

TRMM precipitation value

(b)

Figure 5: Scatterplots of the original TRMM precipitation values versus averages of area-to-point kriging estimates within each block: (a)downscaling without DEM and NDVI and (b) downscaling with DEM and NDVI.

TRMM data in Figure 1(a) (e.g., high precipitation values inKangwon province areas). As expected, however, the directdownscaling results of the original TRMMdatawithoutDEMandNDVI showed very smoothly varying patterns, comparedwith that of using DEM and NDVI. In addition, local detailsthat reflect both topographic and vegetation patterns wereobserved in the downscaling result with DEM and NDVI.When both downscaling results were averaged within theoriginal 25 km resolution block, the coherence property waswell preserved (Figure 5). The downscaling result withoutDEM and NDVI perfectly satisfied the coherence property.In the downscaling result with DEM andNDVI, the averagedvalues at some blocks were slightly different from the originalblock value. This small discrepancy was observed at some

blocks where only small portions were occupied by landsor islands, and elevation and NDVI values at a finer scalewere only available at those land areas. Average values werecomputed from the land areas within such blocks and seaareas were not considered, which resulted in the slightdiscrepancy. However, this difference was negligible and thecoherency property was well satisfied.

To quantitatively evaluate the effects of using environ-mental data for downscaling, precipitation values at 69meteorological observation stations were compared withthose of two downscaling results.The predictive performancewas quantified using the mean absolute error (MAE) andthe root mean square error (RMSE), and validation resultsare presented in Table 1. As expected, the incorporation


125–150

100–125

75–100

50–75

25–50

0–25

150<

(a)

125–150

100–125

75–100

50–75

25–50

0–25

150<

(b)

Figure 6: Simulation results: (a) 1st realization and (b) 10th realization.

Table 1: Predictive performance evaluation results.

Downscalingwithout DEMand NDVI

usingarea-to-point

kriging

Downscalingwith DEMand NDVI

usingarea-to-point

residualkriging

Downscalingwith DEMand NDVI

usingordinarykriging

of residualsMAE 19.98 13.03 17.73Relative improvementof MAE — 34.78% 11.30%

RMSE 27.49 20.03 24.81Relative improvementof RMSE — 27.14% 9.75%

of DEM and NDVI showed smaller prediction errors andimproved MAE and RMSE by 34.78% and 27.14%, respec-tively, compared to the downscaling without the auxiliaryvariables. In addition, point ordinary kriging of residualsat the centroid points of original TRMM blocks was alsoapplied to highlight the advantage of area-to-point residualkriging.The variogrammodel of residuals at 25 km resolutionwas used for implementation of point ordinary kriging. Thepredictive performance of point ordinary kriging of residualsshowed improvedMAE and RMSE (11.30% and 9.75%, resp.),compared to the downscaling without DEM and NDVI viaarea-to-point kriging (Table 1). However, this result comesmainly from using the auxiliary variables such as DEM andNDVI, regardless of the kriging algorithms applied to thedownscaling of residuals. Point ordinary kriging of residualsshowed poorer predictive performance than area-to-pointresidual kriging. In addition, the downscaling result by pointordinary kriging of residuals could not reproduce the originalTRMM precipitation values when upscaled.

These quantitative evaluation results confirmed that usingboth fine scale auxiliary environmental variables and spatialcorrelation information of residuals within the presentedgeostatistical scheme can generate reliable downscalingresults with better predictive performance, compared to thecase without fine scale environmental variables. Regardingdownscaling of residuals, the presented geostatistical schemebased on area-to-point residual kriging could account forthe support difference between the target scale and theinput data scale, and outperformed the simple application ofconventional interpolation methods by treating coarse scaleblock data as point data.

4.4. Simulation Results. Block sequential simulation wasimplemented using SGeMS software [19] and 50 alternativerealizations of precipitation at 1 km resolution were gener-ated. Two of the 50 realizations are given in Figure 6. Theoverall patterns in the simulation results were similar to thosein the kriging result, but local variations were much strongerin the simulation results, whichwas the typical property of thesimulation. When averaged within original 25 km resolutionblocks, all realizations well satisfied the coherence property(results are not shown here).

Two summary statistics can be computed from the 50realizations, the E-type (average) estimate and conditionalvariance for the overall pattern and the uncertainty measure,respectively, and are given in Figure 7. The E-type estimateand the kriging result in Figure 4(b) showed very similarspatial patterns. Unlike kriging variance that cannot includethe effects of surrounding data values [18], conditional vari-ance computed from the 50 realizations, which reflect thedifferences among the 50 alternative realizations, can be usedas a quantitative measure of uncertainty. The lower variancevalues, which imply few differences among the 50 realizationsand, therefore, lower uncertainty, were mainly observed inthe southern parts where negative residuals were computedand smaller precipitation values were predicted.


125–150

100–125

75–100

50–75

25–50

0–25

150<

(a)

100–120

40–60

20–40

80–100

60–80

0–20

120<

(b)Figure 7: Summary statistics values computed from 50 simulations: (a) E-type estimate and (b) conditional variance.

It should be noted that the ultimate goal of stochasticsimulation is to generate alternative realizations that can beused for quantification of uncertainty attached to the spatialprediction or downscaling [18, 19], rather than either togenerate the E-type estimate as shown in Figure 7(a). Moreimportantly, all realizations are equi-probable or equallyimportant [27], and a single best realization showing thebest predictive performance should not be looked for, asdiscussed in Boucher and Kyriakidis [20]. A set of alternativerealizations can be used for modeling the uncertainty aboutthe unknown true precipitation at a fine scale. For example,each downscaled precipitation realization can be fed intohydrological models to generate numerous model outputs.By analyzing these model outputs, the impacts of uncertaintyabout precipitation information on model outputs (i.e., erroror uncertainty propagation) can be investigated. Althoughmultiple alternative realizations and summary statistics weregenerated via simulation in this experiment, this kind of erroror uncertainty propagation problem should be regarded as animportant application using simulation.

5. Conclusions

This paper has demonstrated the application of geostatisticalkriging and simulation to integrate auxiliary fine scale envi-ronmental variables for downscaling of precipitation valuesderived from coarse scale TRMM data. The environmentalvariables contribute to the determination of trend compo-nents via regression analysis. The residual components thathave a reasonable spatial correlation structure are accountedfor during downscaling.

An experiment on downscaling of TRMM precipitationdata with DEM and NDVI produced reliable downscalingresults that not only preserved overall patterns in the originalTRMM data, but also presented local details from DEM andNDVI. Improved predictive performance was also obtained,compared to the downscaling without the auxiliary environ-mental variables. Stochastic simulation providedmuch richer

information for uncertainty modeling by generating multiplealternative realizations, unlike the single downscaling resultby kriging.

The geostatistical downscaling scheme presented in thispaper can be applied to areas in which ground measurementdata are very sparse or even unavailable, especially in devel-oping countries. Fine scale DEM and NDVI can be obtainedfrom the Shuttle Radar Topography Mission (SRTM) DEMand MODIS data, respectively, and hence used effectively fordownscaling of precipitation data in such areas. In additionto downscaling of TRMM precipitation data, the presentedscheme can also be extended and generalized to downscalingof other coarse scale remote sensing data, such as coarse scalesoil moisture data from the Soil Moisture and Ocean Salinity(SMOS) mission.

The applicability of the presented geostatistical down-scaling scheme has been tested on only one TRMM data inthis study. The time-series data sets are usually considered inany precipitation patterns analyses.Therefore,more extensiveexperiments using time-series TRMM precipitation data setswill be required to confirm the major findings of this study.The impacts of variations in correlation strengths betweenprecipitation data and auxiliary variables on downscalingperformance should also be included in future work.

Acknowledgments

This research was supported by the Basic Science ResearchProgram through theNational Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT, and FuturePlanning (NRF-2012R1A1A1005024).

References

[1] P. Goovaerts, “Geostatistical approaches for incorporating ele-vation into the spatial interpolation of rainfall,” Journal of Hyd-rology, vol. 228, no. 1-2, pp. 113–129, 2000.


[2] P. C. Kyriakidis, J. Kim, and N. L. Miller, “Geostatistical map-ping of precipitation from rain gauge data using atmosphericand terrain characteristics,” Journal of Applied Meteorology, vol.40, no. 11, pp. 1855–1877, 2001.

[3] S. Jia, W. Zhu, A. Lu, and T. Yan, “A statistical spatial down-scaling algorithm of TRMM precipitation based on NDVI andDEM in the Qaidam Basin of China,” Remote Sensing of Envi-ronment, vol. 115, no. 12, pp. 3069–3079, 2011.

[4] G. J. Huffman, R. F. Adler, D. T. Bolvin, and G. Gu, “Improvingthe global precipitation record: GPCP version 2.1,” GeophysicalResearch Letters, vol. 36, no. 17, Article ID L17808, 2009.

[5] C. Kummerow, W. Barnes, T. Kozu, J. Shiue, and J. Simpson,“TheTropical RainfallMeasuringMission (TRMM) sensor pac-kage,” Journal of Atmospheric and Oceanic Technology, vol. 15,no. 3, pp. 809–817, 1998.

[6] A. Y. Hou and G. Skofronick-Jackson, “GPMmission overviewand US science status,” in Proceedings of the IEEE InternationalGeoscience and Remote Sensing Symposium (IGARSS ’13), Mel-bourne, Australia, 2013.

[7] D. Harris and E. Foufoula-Georgiou, “Subgrid variability andstochastic downscaling of modeled clouds: effects on radiativetransfer computations for rainfall retrieval,” Journal of Geophys-ical Research D, vol. 106, no. 10, pp. 10349–10362, 2001.

[8] A. J. Tatem, H. G. Lewis, P. M. Atkinson, and M. S. Nixon,“Super-resolution land cover pattern prediction using a Hop-field neural network,” Remote Sensing of Environment, vol. 79,no. 1, pp. 1–14, 2002.

[9] G. Kim and A. P. Barros, “Downscaling of remotely sensed soilmoisture with a modified fractal interpolation method usingcontraction mapping and ancillary data,” Remote Sensing ofEnvironment, vol. 83, no. 3, pp. 400–413, 2002.

[10] Y.H.Kaheil,M.K.Gill,M.McKee, L. A. Bastidas, andE. Rosero,“Downscaling and assimilation of surface soil moisture usingground truth measurements,” IEEE Transactions on Geoscienceand Remote Sensing, vol. 46, no. 5, pp. 1375–1384, 2008.

[11] D. Liu and R. Pu, “Downscaling thermal infrared radiance forsubpixel land surface temperature retrieval,” Sensors, vol. 8, no.4, pp. 2695–2706, 2008.

[12] G.Wotling, C. Bouvier, J. Danloux, and J.-M. Fritsch, “Regional-ization of extreme precipitation distribution using the principalcomponents of the topographical environment,” Journal ofHydrology, vol. 233, no. 1–4, pp. 86–101, 2000.

[13] D. K. Nykanen and D. Harris, “Orographic influences on themultiscale statistical properties of precipitation,” Journal ofGeophysical Research D, vol. 108, no. 8, 2003.

[14] R. B. Smith and I. Barstad, “A linear theory of orographic preci-pitation,” Journal of the Atmospheric Sciences, vol. 61, no. 12, pp.1377–1391, 2004.

[15] M. G. Badas, R. Deidda, and E. Piga, “Orographic influences inrainfall downscaling,” Advances in Geosciences, vol. 2, pp. 285–292, 2005.

[16] H. Guan, J. L. Wilson, and H. Xie, “A cluster-optimizing regre-ssion-based approach for precipitation spatial downscaling inmountainous terrain,” Journal of Hydrology, vol. 375, no. 3-4, pp.578–588, 2009.

[17] W. W. Immerzeel, M. M. Rutten, and P. Droogers, “Spatialdownscaling of TRMM precipitation using vegetative responseon the Iberian Peninsula,” Remote Sensing of Environment, vol.113, no. 2, pp. 362–370, 2009.

[18] P. Goovaerts,Geostatistics for Natural Resources Evalution, App-liedGeostatistics, OxfordUniversity Press, NewYork, NY,USA,1997.

[19] N. Remy, A. Boucher, and J. Wu, Applied Geostatistics withSGeMS: A User’s Guide, Cambridge University Press, Cambri-dge, UK, 2009.

[20] A. Boucher and P. C. Kyriakidis, “Super-resolution land covermapping with indicator geostatistics,” Remote Sensing of Envi-ronment, vol. 104, no. 3, pp. 264–282, 2006.

[21] X. H. Liu, P. C. Kyriakidis, and M. F. Goodchild, “Population-density estimation using regression and area-to-point residualkriging,” International Journal of Geographical Information Sci-ence, vol. 22, no. 4, pp. 431–447, 2008.

[22] E. Pardo-Iguzquiza, P. M. Atkinson, and M. Chica-Olmo,“DSCOKRI: a library of computer programs for downscalingcokriging in support of remote sensing applications,”Computers& Geosciences, vol. 36, no. 7, pp. 881–894, 2010.

[23] P. C. Kyriakidis, “A geostatistical framework for area-to-pointspatial interpolation,” Geographical Analysis, vol. 36, no. 3, pp.259–289, 2004.

[24] P. Goovaerts, “Kriging and semivariogram deconvolution in thepresence of irregular geographical units,” Mathematical Geo-sciences, vol. 40, no. 1, pp. 101–128, 2008.

[25] A. G. Journel and C. J. Huijbregts, Mining Geostatistics, Aca-demic Press, New York, NY, USA, 1978.

[26] Y. Liu andA.G. Journel, “A package for geostatistical integrationof coarse and fine scale data,” Computers & Geosciences, vol. 35,no. 3, pp. 527–547, 2009.

[27] A. G. Journel, “Modelling uncertainty and spatial dependence:stochastic imaging,” International Journal of Geographical Infor-mation Systems, vol. 10, no. 5, pp. 517–522, 1996.

Submit your manuscripts athttp://www.hindawi.com

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

ClimatologyJournal of

EcologyInternational Journal of


EarthquakesJournal of


Hindawi Publishing Corporationhttp://www.hindawi.com

Applied &EnvironmentalSoil Science

Volume 2014

Mining


Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of


Journal of Computational Environmental SciencesHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal ofPetroleum Engineering


GeochemistryHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Atmospheric SciencesInternational Journal of


OceanographyHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Advances in


MineralogyInternational Journal of


MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Paleontology JournalHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

ScientificaHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014


Geological ResearchJournal of


Geology Advances in

Research Article Spatial Downscaling of TRMM...

Documents

Transcript of Research Article Spatial Downscaling of TRMM...