Clim Dyn (2007) 28:759–780 DOI 10.1007/s00382-006-0203-z
Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system. Part 1: recent climate simulation (1971–2000)
Eun-Soon Im Æ Won-Tae Kwon Æ Joong-Bae Ahn Æ Filippo Giorgi
Received: 21 January 2006 / Accepted: 6 October 2006 / Published online: 8 December 2006 Springer-Verlag 2006
Abstract We present an analysis of a high resolution of simulated daily mean temperature agrees well with multi-decadal simulation of recent climate (1971–2000) the observed seasonal and spatial patterns. In the over the Korean Peninsula with a regional climate summer season, however, daily variability is underes- model (RegCM3) using a one-way double-nested sys- timated. (3) The RegCM3 simulation adequately cap- tem. Mean climate state as well as frequency and tures the seasonal evolution of precipitation associated intensity of extreme climate events are investigated at to the East Asia monsoon. In particular, the simulated various temporal and spatial scales, with focus on winter precipitation is remarkably good, clearly show- surface air temperature and precipitation. The mother ing typical precipitation patterns that occur on the intermediate resolution model domain encompasses northwestern areas of Japan during the winter mon- the eastern regions of Asia at 60 km grid spacing while soon. Although summer precipitation is underesti- the high resolution nested domain covers the Korean mated, area-averaged time series of precipitation over Peninsula at 20 km grid spacing. The simulation spans Korea show that the RegCM3 agrees better with the 30-year period of January 1971 through December observations than ECHO-G both in terms of seasonal 2000, and initial and lateral boundary conditions for evolution and precipitation amounts. (4) Heavy rainfall the mother domain are provided from ECHO-G fields phenomena exceeding 300 mm/day are simulated only based on the IPCC SRES B2 scenario. The model at the high resolution of the double nested domain. (5) shows a good performance in reproducing the clima- The model shows a tendency to overestimate the tological and regional characteristics of surface vari- number of precipitation days and to underestimate the ables, although some persistent biases are present. precipitation intensities. (6) A CSEOF analysis reveals Main results are as follows: (1) The RegCM3 success- that the model captures the strength of the annual fully simulates the fine-scale structure of the tempera- cycle and the surface warming trend throughout the ture field due to topographic forcing but it shows a simulated period. systematic cold bias mostly due to an underestimate of maximum temperature. (2) The frequency distribution
E.-S. Im (&) W.-T. Kwon 1 Introduction Climate Research Lab, METRI, KMA, Seoul, South Korea The climate of Korea has experienced a gradual e-mail: [email protected] warming throughout the twentieth century (Oh et al. J.-B. Ahn 2004) in agreement with the warming observed at the Department of Atmospheric Sciences, global scale (Intergovernmental Panel on Climate Pusan National University, Pusan, South Korea Change, IPCC 2001). Average temperature over South F. Giorgi Korea has increased by about 1.5 C during the twen- Abdus Salam ICTP, Trieste, Italy tieth century (more than twice the corresponding
123 760 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system global warming) mainly due to the greenhouse effect Im et al. 2006a). In the double nested approach and rapid urbanization (Kwon 2005). The Korean (Christensen et al. 1998) an intermediate resolution peninsula appears thus to be particularly responsive to ‘‘mother’’ domain simulation is first completed using possible anthropogenically induced climatic changes. lateral boundary forcing fields from an AOGCM. Fields As the global mean surface temperature is projected to from this mother domain simulation are then provided further increase in the twenty-first century, it is rea- as lateral boundary conditions to high resolution nested sonable to expect that Korea will be strongly vulnera- RCM simulations over a sub-domain of interest. In our ble to climate change. In fact, also associated with double-nested model, the mother domain encompasses global warming are changes in the frequency and the eastern regions of Asia and adjacent oceans at intensity of extreme climatic events (Bell et al. 2004), 60 km grid spacing and the nested domain covers the and discernable evidence of increased rainfall intensity, Korean Peninsula at 20 km grid spacing. In a previous shifts of climatic seasons and lengthening of the paper (Im et al. 2006a) we examined the basic model growing season has already been observed over Korea performance in a ‘‘perfect boundary condition’’ (Giorgi (Kwon et al. 2005). It is thus important that credible and Mearns 1999) experiment using NCEP/NCAR scenarios of climate change over Korea are developed reanalysis boundary conditions for the period 2001– in order to evaluate related impacts and adaptation/ 2003. The model was validated against a dense obser- mitigation measures. vational network over the Korean territory and showed The primary tools used to generate climate change a realistic representation of Korean climate, however scenarios are coupled atmosphere ocean general cir- with some persistent biases, such as a cold bias during culation models (AOGCMs), and several generations winter (Im et al. 2006a). of AOGCMs have been used to produce such scenarios In this and the companion paper by Im et al. (2006b) (Kittle et al. 1998; Cubasch et al. 2001; Giorgi et al. we use the double-nested model system to investigate 2001; Giorgi and Bi 2005). AOGCMs represent many possible changes in regional surface climate due to broad features of current climate reasonably well global warming and to produce fine-scale regional cli- (McAvaney et al. 2001) and can reproduce the ob- mate information for impact assessment studies over served large-scale changes in climate over the recent the Korean peninsula. Toward this goal, we carried out past (Mitchell et al. 2001). They can therefore be used two 30-year long experiments, one for present day with some confidence to produce projections of the conditions (covering the period 1971–2000) and one for global climate response to anthropogenic activities. near future climate conditions (covering the period However, in areas where complex coastal and moun- 2021–2050) under forcing from the B2 IPCC emission tainous features have a significant effect on weather scenario, which lies towards the low end of the IPCC and climate, scenarios based AOGCMs, whose reso- scenario range. Global fields used to drive the mother lution is still of the order of 100–200 km, generally fail domain simulation are obtained from a corresponding to capture the local detail needed for impact assess- scenario experiment with the ECHO-G AOGCM. ments at the national and regional level (Mearns et al. In this paper, we focus on the analysis of the present 2001). Also, at such coarse resolutions, extreme events day climate simulation (1971–2000), which is key for such as drought or heavy rainfall are either not cap- assessing and understanding the future climate change tured or substantially underestimated. Korea is a re- scenario. Im et al. (2006b) then focus on the future gion where these limitations of AOGCMs are scenario simulation. Our analysis is primarily centered particularly important, since the Korean peninsula is on large scale circulations affecting the region and small (the area of South Korea is 99,585 km2) and different statistics of surface air temperature and pre- mountainous, and since the climate of Korea is char- cipitation, the two variables most used in impact acterized by the occurrence of extreme precipitation assessment studies (Mearns et al. 2001). Among the episodes (Park et al. 2003; Yun et al. 2001). statistics analyzed are monthly, seasonal, and annual One method for overcoming the resolution limita- means and the distribution of daily temperature and tions of AOGCMs and adding regional detail to global precipitation events, including extremes. In addition, projections is to use one-way nested regional climate we investigate whether the modeling system is capable models (RCMs; Giorgi and Mearns 1999). Because of to reproduce observed trends during the 1971–2000 the reasons presented previously, this technique can be period using a cyclostationary empirical orthogonal especially useful for the Korean peninsula, and there- function (CSEOF) technique. The model results are fore we developed a one-way double-nested RCM evaluated by comparison with large scale reanalysis system for the Korean region based on the regional data as well as station observations covering the South model RegCM3 (Giorgi et al. 1993a, b; Pal et al. 2006; Korea territory.
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 761
In Sect. 2 we first present a brief description of the simulation include the comprehensive radiative model system, experiment design and observation data. transfer package of the NCAR Community Climate The results for the AOGCM, mother and nested do- Model, version CCM3 (Kiehl et al. 1996), the non- main simulations are then validated and intercompared local boundary layer scheme of Holtslag et al. (1990), in Sect. 3 and conclusion are presented in Sect. 4. the BATS land surface scheme (Dickinson et al. 1993), the mass flux cumulus cloud scheme of Grell (1993) and the resolvable scale precipitation scheme 2 Models, experiment design and validation strategy of Pal et al. (2000). The same model physics schemes are used in the mother and nested domain simula- 2.1 Atmosphere–ocean global climate model tions and we adopt a linear orography blending method similar to Hong and Juang (1998) at the The global climate model which provides the initial lateral boundaries in order to minimize the discern- and lateral boundary fields for the mother domain ible systematic error due to scale mismatch between simulation is the ECMWF Hamburg Atmosphere driving and model fields. Model Version 4 (ECHAM4) coupled with the Ham- burg Ocean Primitive Equation-Global model (HOPE- 2.3 Experiment design G) of the Max Planck Institute for Meteorology (MPI) (hereafter referred to as ECHO-G; Min et al. 2005, Figure 1 shows the mother and nested model domains 2006). The horizontal and vertical resolution of EC- used in this study, which are the same as in Im et al. HAM4 is spectral T30 grid (approximately ~3.75 ) and (2006a). The mother domain covers the eastern regions 19 hybrid sigma-pressure levels (highest level at of Asia (including the Korean Peninsula) at 60 km grid 10 hPa), respectively. The horizontal resolution of spacing, while the nested domain encompasses the HOPE-G corresponds to a Gaussian T42 grid (~2.8 ) South Korean Peninsula at 20 km grid spacing. As and the model employs 20 vertical levels. HOPE-G shown in the nested domain, the Korean Peninsula is a also includes a dynamic-thermodynamic sea-ice model mountainous region with the most prominent ranges with snow cover. The simulation of ECHO-G used in reaching elevations of over 1,000 m. Approximately this study is based on the Special Report Emission 70% of the South Korean territory consists of hills and Scenario (SRES) B2 GHG emission scenario devel- mountains (Choi et al. 2003). These geographical oped by IPCC (2000). A total of 19 well-mixed gases characteristics cause significant fine-scale variations in including CO2,N2O, CH4 and industrial halocarbons weather and climate. Comparison of the two domains are considered. GHG concentrations are reconstructed shows that the mountain ranges in the nested domain from observations for the period 1860–1990, and are are much more realistic than in the mother domain, taken from the B2 scenario for the period 1990–2100. which emphasizes the necessity of the double-nesting
In this scenario the CO2 concentration increases to system. We carefully selected the domain area through 606 ppmv by 2100 compared to 1990. The ECHO-G various sensitivity experiments (Im et al. 2006a). In simulation covers the period 1860–2100 and is de- both the mother and nested domain simulations the scribed in detail by Min et al. (2005, 2006) and Oh et al. model employs 18 vertical sigma levels. (2004). The regional model can be run with initial and lat- eral boundary conditions from either global analysis 2.2 Regional climate model data or the output of a GCM. In our experiments the initial and time-dependent meteorological lateral The regional climate model used in this study is the boundary conditions are interpolated at 6-hourly Abdus Salam International Centre for Theoretical intervals from a transient ECHO-G B2 scenario sim- Physics (ICTP) Regional Climate Model (RegCM) in ulation which covered the period 1860–2100. For the its latest version RegCM3. This is an upgraded ver- present day climate conditions discussed here the sion of the model originally developed by Giorgi RegCM simulation covers the 30-year period of 1971– et al. (1993a, b) and then improved as discussed by 2000. Sea surface temperature (SST) over the ocean Giorgi and Mearns (1999) and Pal et al. (2006). The areas, initial soil moisture and GHG concentrations are dynamic core of the RegCM3 is equivalent to the obtained from the corresponding ECHO-G fields. We hydrostatic version of the NCAR/Pennsylvania State analyze the simulation results focusing on the summer University mesoscale model MM5 (Grell et al. 1994). (June, July, August; or JJA) and winter (December, The physics parameterizations employed in this January, February; or DJF) seasons.
123 762 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
Fig. 1 Model domain and topography (m) for the mother (60 km grid spacing) and nested (20 km grid spacing) simulations
2.4 Verification strategy product obtained by assimilating quality-controlled observations from different sources. Therefore, pre- Various reanalysis and observation datasets are used to cipitation in the two datasets can be different. Tem- validate the performance of the RegCM3 double-nes- perature results are compared with the NCEP/NCAR ted system. First of all, 57 climate stations with data for Reanalysis data. Both reanalysis datasets cover the 25- the full 30-year period of 1971–2000 throughout South year period of 1979–2003. Korea are used, including daily mean, maximum and minimum temperature and precipitation. This data- set allows a validation of the fine-scale structure of the 3 Results nested model simulation and the relatively high model resolution justifies the comparison between station and 3.1 ECHO-G and mother domain simulation model grid point data. The topography and location of the 57 stations are shown in Fig. 2. Note that the In this section we present an analysis of the 30-year dataset includes both low elevation and high elevation climatology of the mother domain simulation as driven stations. Here, two mountain systems are most rele- by the ECHO-G fields. This is important to assess vant, the Taebaek Mountains, extending from north to whether the driving fields in the mother domain are of south along the eastern coastal regions of Korea, and adequate quality for use in the double nesting method. the Sobaek Mountains located in the south-central Figure 3 first compares NCEP/NCAR reanalysis regions of the peninsula. and simulated (mother domain) average large scale For the validation of results from the ECHO-G and circulation at 850 mb in the winter and summer sea- mother domain simulations we use global reanalysis sons. Results for ECHO-G are not shown, as they are datasets. Precipitation results are compared with the generally similar to those from the mother domain Climate Prediction Center (CPC) Merged Analysis of simulation. The climate of East Asia is greatly influ- Precipitation (CMAP) (Xie and Arkin 1997) and the enced by the seasonal evolution of the monsoon cir- NCEP/NCAR Reanalysis dataset (Kalnay et al. 1996), culation. During the winter season, the Siberian high which include monthly data at a 2.5 resolution. The determines the development of the winter monsoon. CMAP dataset was produced by merging gauge The wind field at 850 mb shows a predominant north- observations, estimates inferred from a variety of sa- westerly lower tropospheric flow over East Asia asso- tellite observations, and precipitation forecasts from ciated with anticyclonic circulation induced by the the NCEP/NCAR reanalysis. On the other hand, the Siberian high (Jhun and Lee 2004). This circulation NCEP/NCAR reanalysis precipitation is a model carries cold air from the polar regions into East Asia
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 763
during the winter season. The model captures the ob- served wave pattern associated with the Siberian high. During the summer (Fig. 3), the western branch of the Pacific sub-tropical high penetrates inland, pro- ducing southwesterly monsoon flow that sweeps East Asia. The model shows a generally good performance in reproducing the migration of the southwesterly monsoon flow across the region, as well as the westerly flow that affects the northern regions of Asia and in particular the Korean peninsula. The main model deficiency appears to be an eastern displacement of the monsoon front and excessively strong westerlies over northeast China and the Korean peninsula. However, the simulation of the monsoon progression is of good quality compared to earlier simulations in terms of amplitude and phase. (e.g. Hirakuchi and Giorgi 1995; Kato et al. 2001; Ji and Vernekar 1997). This is an indication of the relatively good quality of the ECHO- G large scale fields, which is then transmitted to the mother domain simulation. It should be stressed that global models have traditionally had substantial diffi- culties in simulating the East Asia monsoon (Gao et al. 2006). Fig. 2 Topography (m) and location of climate stations used for the assessment of the model performance Figure 4 compares the average surface air temper- ature in the NCEP/NCAR reanalysis (25 years) and
Fig. 3 Observed (NCEP/ Dec Jan Feb DJF NCAR reanalysis) (upper (a) panels a and c), and simulated (lower panels b and d) 30-year averaging 850 hPa winds for the winter and summer season. Units are in meter/s
NCEP/NCAR
(b)
RegCM3
(c) Jun Jul Aug JJA NCEP/NCAR
(d) RegCM3
123 764 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
(a) NCEP/NCAR (b)RegCM (c) ECHO-G
Fig. 4 a Observed (NCEP/NCAR reanalysis) and simulated b RegCM3 and c ECHO-G 30-year averaging surface air temperature for the winter and summer seasonal mean. Units are in degrees the ECHO-G and RegCM3 mother domain simula- especially over the Korean peninsula. Both the general tions (30 years) for the winter and summer seasons. spatial patterns and the winter monsoon precipitation Overall, the model results agree well with the broad patterns over northwestern Japan are reproduced spatial patterns of observed seasonal surface air tem- clearly in the RegCM3 mother domain run. During the perature. In addition, the RegCM3 simulation shows summer, the mother domain simulation produces some fine-scale structure missed by the reanalysis and higher precipitation amounts and a more intense rain- ECHO-G data due to their coarse resolution. Note that belt than ECHO-G, a result in the direction of a better the ECHO-G shows an almost zonal temperature agreement with the reanalysis data. However, some pattern that does not properly reflect the detailed substantial differences between CMAP and RegCM3 geography of the Korean peninsula in the winter sea- simulated summer precipitation are found. In particu- son. Overall, the RegCM shows a cold bias of a few lar, the CMAP data show a pronounced precipitation degrees, particularly in the cold season. This feature maximum over the Korean peninsula while the pre- was also found in the perfect boundary condition cipitation band in the RegCM3 simulation extends experiment of Im et al. (2006a) and hence it appears to farther to the north. Surprisingly, we find considerable be a persistent bias of the RegCM over this region. discrepancies between the CMAP and NCEP precipi- More discussion of this bias is presented later. tation fields in the summer season. In fact, the model The simulated precipitation field successfully simulated precipitation is in agreement with the NCEP reproduces observed large scale features and captures precipitation than the CMAP one. the seasonal evolution of the precipitation pattern To quantitatively assess whether the regional cli- associated with the East Asia monsoon system (Fig. 5). mate model indeed shows an improvement compared However, the precipitation simulated by ECHO-G to the global climate model ECHO-G, we provide shows a smaller than observed range of seasonal vari- additional objective measures of performance, such as ation over East Asia. In the winter season, the ECHO- the spatial pattern correlation and bias, between model G overestimates precipitation over eastern China and results and reanalysis data (Table 1). The pattern Korea and misses typical precipitation patterns that correlation measures the agreement between the spa- occur on the northwestern side of Japan during the tial patterns of simulation and observations while the winter monsoon (Kato et al. 2001). Conversely, the bias is a direct measure of how the model average summer precipitation simulated by ECHO-G is climatology deviates from observed. Here, we calculate underestimated in response to a relatively weak mon- the scores at the two different model resolutions to soon rain-belt. In the RegCM3 mother domain simu- estimate both the broad patterns and the fine-scale lation, winter precipitation agrees with observations, features.
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 765
(a) CMAP (b) NCEP/NCAR (c)RegCM (d) ECHO-G
Fig. 5 a CMAP, b NCEP/NCAR reanalysis, simulated c RegCM3 and d ECHO-G 30-year averaging precipitation for the winter and summer seasonal mean. Units are in millimeters/day
Table 1 Spatial pattern Corerlation Bias correlation and bias between model results and observation RegCM3 ECHO-G RegCM3 ECHO-G
2.5 Tem. (NCEP) DJF 0.98 0.98 –1.5 1.39 JJA 0.85 0.92 –0.88 –1.14 Pre. (NCEP) DJF 0.75 0.87 –0.18 0.71 JJA 0.65 0.58 1.59 –1.16 Pre. (CMAP) DJF 0.86 0.92 –0.29 0.60 JJA 0.52 0.49 1.10 –1.65 60 km Tem. (station) DJF 0.88 0.80 –1.71 4.32 JJA 0.63 0.28 –0.96 –1.2 Pre. (station) DJF 0.57 0.53 0.14 0.78 JJA –0.05 –0.08 –2.47 –5.04
First, the seasonal mean temperature and precipi- The improvement deriving from the regional climate tation fields from the RegCM3 mother domain and model is further measured by the comparison with the ECHO-G are compared with NCEP and CMAP Korean station observations (land only: 125 E–130 E, (precipitation only) reanalysis data at a 2.5 resolution 34 N–38.5 N). The ECHO-G results are interpolated, over the mother domain interior after both model re- and the observations are aggregated, onto the mother sults are bilinearly interpolated onto the reanalysis domain 60 km grid. The RegCM3 pattern correlations grid. For temperature we find generally high correla- are higher than ECHO-G correlations except for tion coefficients for both model simulations. The spa- summer precipitation. Because of better resolved tial pattern correlations of ECHO-G are slightly higher topographic forcing, the increased resolution of the than those of RegCM3 except for summer precipita- mother domain (compared to ECHO-G) evidently tion. In fact, the topographic signal is almost absent leads to increased spatial agreement with the fine-scale from the reanalysis data at a 2.5 resolution due to the observation fields. For summer precipitation, we do not coarse resolution of the data. Conversely, the bias find a significant pattern agreement between either of scores illustrate a pronounced deficiency of the the model results and observations. However, the bias ECHO-G simulation, with a severe underestimate of of RegCM3 is much lower than that of ECHO-G. summer precipitation and overestimate of winter pre- To provide a quantitative measure of model per- cipitation. Both these scores are improved in the formance related to the seasonal cycle, we calculated RegCM simulation. the monthly variation of area-averaged temperature
123 766 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system and precipitation over East Asia (110 E–142 E, 27 N– In summary, Figs. 3, 4, 5, 6, 7 and 8 indicate that the 45 N) and the Korean peninsula (123 E–131 E, 32 N– mother domain simulation adequately simulates the 43 N), including data over ocean areas (Figs. 6, 7). The basic features of East Asia climate, including the RegCM3 systematic cold bias varying from –2.3 Cin Korean peninsula, and is thus adequate to provide late winter to –0.3 C in the fall is evident over both the realistic forcing fields to the nested domain simulation. East Asia and Korea regions. By comparison, the The Korean peninsula is directly affected by the ECHO-G simulatin shows a warm bias of up to 2.7 C monsoon activity and is characterized by a marked in the winter and a cold bias of up to –1.3 C in the spatial variability of precipitation as influenced by the summer (Fig. 6). topography of the region. In the next section we turn Figure 7 compares area-averaged observed and our attention to the analysis of the fine scale nested simulated precipitation over East Asia and the Korean domain experiment. peninsula. As already mentioned, ECHO-G severely underestimates precipitation in summer and overesti- 3.2 Nested domain simulation mates it in winter, thereby markedly underestimating the seasonal precipitation cycle. The mother domain In the analysis of our nested domain simulation we simulation substantially improves upon the ECHO-G compare the model results with observations derived results, with a much better simulated seasonal precip- from 57 climate stations in South Korea with no itation cycle, especially during the summer months. missing data during the 30-year period of 1971–2000. In The main model deficiencies are an overestimate of assessing the quality of the simulated results for the early to mid summer precipitation over East Asia and nested domain, we use as reference the mother domain an underestimate of later summer precipitation over results over the nested domain area. Use of the double Korea. nesting approach is indeed justified when the nested In order to examine the model performance related model results are superior (according to selected to the propagation of the monsoon rain-band, we refer measures) to the mother domain results. to Fig. 8, which shows a time-latitude cross section of Figure 9 shows the 30-year mean spatial distribution zonally averaged precipitation along the band 110 E– of surface air temperature for two seasons (summer 142 E. The development of the monsoon is charac- and winter). The first set of panels presents the results terized by a northward movement of the rain-band directly obtained from the 57 station dataset. The until mid-July and a subsequent retreat during the fall. second set of panels shows the nested model results, The simulated precipitation by RegCM3 captures this while the third set presents the results from the coarse seasonal propagation, although the rain-band extends mother domain simulation. The observed temperature too far to the north in the summer and retreats too field shows substantial spatial variability with a number rapidly in the late fall. In ECHO-G the precipitation of topographically induced fine-scale regional features, amounts during the summer monsoon season are possibly also related to data at individual stations. The underestimated and a strong winter precipitation band topographic signal related to the mountain chains of can be observed to extend up to about 32 N. Although northeast and south central Korea (see Figs. 1, 2)is the spatial and temporal patterns of the precipitation however evident in each month as well as the season band in the RegCM3 and ECHO-G simulations show average. similarities, the RegCM3 precipitation is in much bet- The results from the nested domain simulation ter quantitative agreement with reanalysis data. reproduce these regional features in the two seasons,
30 9 30 9 Mother Mother ECHO-G ECHO-G NCEP NCEP 20 Mother-NCEP 6 20 Mother-NCEP 6 ECHO-NCEP ECHO-NCEP
10 3 10 3 Diff. (degree) 0 0 Diff. (degree) 0 0 Temperature (degree) Temperature (degree) -10 -3 -10 -3 1357911 1357911 Month Month
Fig. 6 Time series of area-averaged surface air temperature over East Asia (left) and Korean peninsula (right)
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 767
8 8 Mother Mother ECHO-G ECHO-G 6 CMAP 6 CMAP
4 4
2 2 Precipitation rate (mm/day) Precipitation rate Precipitation rate (mm/day) Precipitation rate 0 0 1357911 1357911 Month Month
Fig. 7 Time series of area-averaged precipitation over East Asia (left) and Korean peninsula (right)
Fig. 8 Time-latitude cross (a) CMAP (b) NCEP/NCAR (c) RegCM3 (d) ECHO-G section of zonal averaged precipitation along 110–142E for 30-year period
although the cold bias present in the mother domain mother domain is quite evident, for each month as well simulation is inherited by the nested one. Orographic as the season average. In fact, the spatial pattern cor- detail not found in the mother domain is clearly relation between simulated and observed precipitation reproduced in the nested domain, where the tempera- at the 20 km grid during the winter season is higher in ture field reflects well the location of the Taebaek and the nested domain (0.69) than the mother domain Sobaek mountains. Simulated and observed summer (0.47). The nested domain simulation reproduces quite and winter temperatures show similar topographically well the observed winter maxima over northeast and induced spatial patterns. Daily minimum and maxi- southwest Korea, in magnitude as well as location. mum temperatures show the same spatial patterns as The average precipitation improvement by the nes- average temperature (not shown). ted simulation is less evident in the summer (Fig. 10). Figure 10 shows the same quantities as Fig. 9 except This improvement is seen in the summer average field for precipitation. From the analysis of winter precipi- (JJA), when the model captures high precipitation tation, we first find an agreement between simulated bands extending from northwest to northeast Korea and and observed amounts. The observations show a band from the southern coasts to central Korea. However in of large winter precipitation over the northeastern the individual months the model fails to capture local- coasts. Considering the topography of the Korean ized maxima found in the observations (and perhaps peninsula (Figs. 1, 2), these are associated with the related to individual stations). In general, the mother upslope topographic forcing of northeasterly flow tak- and nested domain results are of similar magnitude, and ing place in the winter over this region (Im et al. the nested domain simulation just improves the repre- 2006a). Another maximum occurs over southwestern sentation of the spatial patterns of summer precipitation Korea, evidently in response to topographic uplift of compared to the mother domain simulation. Note that westerly flow by the Sobaek chain in southern Korea. we expect precipitation to be more stochastic in nature For the winter case the improvement in the simulation during summer than winter, so that the effect of finer of spatial detail by the nested domain compared to the scale topography is less evident in the summer.
123 768 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
(a) OBS
(b) Nest
(c) Mother
Fig. 9 Spatial distribution of surface air temperature averaged climate stations, the second set of panels is the results for 20 km over 30 years for winter (a–c) and summer (d–f) season. Here, grid (nested domain) and the third set of panels is the results for the first set of panels corresponds to direct plotting of the 57 60 km grid (mother domain). Units are in oC
For a more quantitative evaluation of the nested precipitation, we first notice from the comparison of domain simulation, Fig. 11 compares station data, Figs. 11 and 7 that the station data exhibit substantially mother domain and nested domain temperature and higher summer precipitation amounts than the CMAP precipitation averaged over the Korean territory. This data. As a result, the precipitation underprediction of figure can also be compared with Figs. 6 and 7, based both the mother and nested domain simulations in on NCEP/NCAR reanalysis and CMAP observations. summer appears larger than in Fig. 7. An analysis of The cold temperature bias found in the mother domain Fig. 10 indicates that this bias is not ubiquitous simulation is essentially inherited by the nested domain throughout the Korean peninsula but it is mostly due simulation, leading to very similar bias magnitudes. For to the failure to reproduce some local maxima found
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 769
(d) OBS
(e) Nest
(f) Mother
Fig. 9 continued over the coastal regions. This may be due to the rep- 3.3 Analysis of daily properties resentation of mesoscale convective systems at the relatively coarse model topography. Both the mother Most commonly, local scenario construction has con- and nested domain precipitation amounts show an centrated on monthly, seasonal, or even annual values excellent agreement with station observations from of meteorological variables such as temperature and October to June. In this regard, the precipitation precipitation. However, these scenarios are often of amounts are similar in the mother and nested domain limited value to impact assessment studies, which often experiments. This result differs from a previous appli- require information at daily time scales. For example, cation of the RegCM to the European region (Giorgi shifts in the timing of thresholds such as the dates of and Marinucci 1996), in which precipitation was shown the first and last frosts have important implications for to increase with horizontal resolution. agriculture. Such changes can be evaluated only from
123 770 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
(a) OBS
(b) Nest
(c) Mother
Fig. 10 Same as Fig. 9, except for precipitation. Units are in millimeters/day daily time series of the relevant climate variable there are no significant differences between mother (Palutikof et al. 1997). In this section we thus validate and nested domain simulated daily temperature sta- daily statistics of temperature and precipitation against tistics. The annual PDF shows two peaks, corre- the station data over the Korean territory (see Fig. 2). sponding to the winter and summer conditions. More specifically, we compare station data with model Although the model reproduces this double-peaked data at the grid point closest to the station location, an structure, the distribution is shifted to the left because approach justified by the high model resolution. of the cold bias previously mentioned. In the winter the Figure 12 describes the frequency distribution of shape of the simulated PDF is the same as the observed daily mean temperature at all station locations over one, and in particular the observed variance is repro- Korea, which gives a measure of both mean and vari- duced. In the summer season, however, the model ance of the daily values. First of all, we notice that shows a narrower distribution than observed, i.e. a
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 771
(d) OBS
(e) Nest
(f) Mother
Fig. 10 continued
lower variance and a higher incidence of mean values. winter than in summer, although the winter PDF shows The lower tail of the summertime distribution and the a greater shift compared to observations. upper tail of the wintertime distribution are well sim- Figure 13 compares simulated and observed distri- ulated. However, the lower tail of the wintertime dis- butions of daily temperature anomalies, calculated as tribution is shifted by about 2 C compared to the daily temperature minus the 30-year average daily observations. It is noticeable that the shape of the temperature at all stations. We can see that the an- winter and summer distributions are quite different, nual and winter anomaly distributions are captured the winter one being more symmetrical and wider. The well, while the summer anomaly distribution is nar- model reproduces the shape of the PDF better in rower and more peaked than in the observations
123 772 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
30 12 10 Mother Mother Nest Nest OBS 8 20 Mother - OBS 8 OBS Nest - OBS 6 10 4 4
0 0 Diff. (degree) 2 Temperature (degree)
- 10 -4 Precipitation rate (mm/day) 0 1 3 5 7 9 11 1 3 5 7 9 11 Month Month
Fig. 11 Time series of area-averaged monthly surface air temperature (left) and precipitation (right) over the Korean Peninsula
8 20 20 Station Station Station Nest 16 Nest 16 Nest 6 Mother Mother Mother 12 12 4 8 8 Probability (%) Probability (%) 2 Probability (%) 4 4
0 0 0 - 30 - 10 10 30 0 10 20 30 40 50 - 25 - 15 - 5 5 15 25 Degree Degree Degree
Fig. 12 Probability density function of the distribution for daily mean temperature in Korea. Left, middle, and right panel is annual, summer, and winter distribution, respectively
20 30 30 Station Station Station Nest Nest Nest 16 25 25 Mother Mother Mother 20 20 12 15 15 8 10 10 Probability (%) Probability (%) Probability (%) 4 5 5
0 0 0 - 20 - 10 0 10 20 - 20 - 10 0 10 20 - 20 - 10 0 10 20 Degree Degree Degree
Fig. 13 Same as Fig. 12, except for daily mean temperature anomaly
Fig. 14 Probability density 6 6 function of annual Station Station distribution for daily Nest Nest Mother Mother maximum (left) and minimum 4 4 (right) temperature in Korea
2 2 Probability (%) Probability (%)
0 0 –200 20 40 –30 –10 10 30 Degree Degree
(although this bias does not contribute greatly to the nearly normal, with a skewness near zero and a kur- annual anomaly PDF). For winter, simulated and tosis near 3. For summer the model results show some observed temperature anomaly distributions are deviations compared to observations, with a slightly
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 773
Fig. 15 Time series of 30- 40 year averaging daily Max_0bs Min_obs maximum and minimum Max_mo Min_mo Max_ne Min_ne temperature 30
20
Degree 10
0
–10 0 50 100 150 200 250 300 350 Day positive skewness (0.13–0.16) and higher kurtosis mid-winter conditions. It is clear from Fig. 15 that the (3.64 vs. 3.23) indicating, as mentioned, more peaked cold model bias is mostly due to the contribution of distributions. Tmax and that a generally better agreement with In order to investigate the cause of the biases in the observations is found in the intermediate seasons daily mean temperature distribution, daily maximum (MAM and SON) than the extreme ones. and minimum temperature distributions were exam- Figure 16 shows the frequency distribution of ob- ined and compared to observations in Fig. 14. We find served and simulated daily precipitation at all Korean that the greatest contribution to the winter cold bias is station locations. During winter, when the model due to an underestimate of winter daily maximum average precipitation is close to observed, the simu- temperature. This is at least partially due to an under- lated frequency distribution matches observations estimate of sunshine duration associated with an over- remarkably well, expect for precipitation episodes estimate of weak precipitation events (Kato et al. 2001), above 100 mm/day. As already discussed, the model as will be discussed in more detail later. Conversely, underestimates precipitation in the summer. Figure 16 summer minimum temperature appears to provide the shows that this low precipitation bias is reflected greatest contribution to the underestimate of the width throughout the entire frequency distribution, that is the of the distribution (and related overestimate of the number of precipitation events is underestimated at all peak). Overall, except for the shift in winter maximum intensities, and especially for intensities in the mid- temperature, the distributions of minimum and maxi- range of 100–200 mm/day. An important result of mum temperature are realistically reproduced. Fig. 16 derives from the comparison of the mother Figure 15 shows the seasonal evolution of the daily domain and nested domain frequency distributions. Tmax and Tmin averaged over 30-years of observa- We can see that for low to mid-intensities the two tions and simulations. The observed and simulated frequency distributions are essentially the same. Tmax and Tmin time series show a good seasonal However, the nested domain frequency distribution phase coherence. Tmax is underestimated throughout has a longer tail at the high intensity range. In other the year, except for the early summer and late summer words, only at the higher resolution of the nested periods, while Tmin is mostly underestimated in domain simulation the model is capable of producing
1000000 1000000 1000000 Station Station Station 100000 Nest 100000 Nest 100000 Nest Mother Mother Mother 10000 10000 10000
1000 1000 1000 Event No. 100 Event No. 100 Event No. 100 10 10 10 1 1 1 0 200 400 600 0 200 400 600 0 50 100 150 200 mm/day mm/day mm/day
Fig. 16 Frequency distribution of daily precipitation in Korea. Left, middle, and right panel is annual, summer, and winter distribution, respectively
123 774 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
Fig. 17 Spatial distribution of the annual total number of precipitation days. Units are in number of days
extreme precipitation episodes (about 80 mm/day in fields are transmitted to, and processed by, the fine-scale winter and 250–400 mm/day in summer). In this re- regional model. In this section, we address this issue by gard, the nested domain simulation is much closer to use of cyclostationary empirical orthogonal function observations than the mother domain simulation and (CSEOFs) analysis as a statistical method to extract the this result supports the use of finer scale models to time evolution of the annual cycle and to detect simulate extreme events. warming trends in the simulation period (trends possi- To further investigate the origin of the precipitation bly due to increased GHG concentration) (Kim and underestimation, we analyzed precipitation intensities North 1997). It is in fact more appropriate to adopt a and frequencies. Figure 17 shows the spatial distribu- cyclostationary approach rather than a stationary one in tion of the annual total number of precipitation days, any statistical analysis of climatic variables (such as defined as days with precipitation exceeding 1 mm/day. temperature) that show strong dependence on the phase We find that the model simulates precipitation fre- of the annual (or diurnal) cycle (Kim and Wu 2000). quencies in line with observations or even higher. The CSEOF considers the annual cycle in a data series precipitation underestimation is thus not given by an fluctuating on longer time scales, and assigns a deter- underestimate of the total number of events, but by an ministic cycle representing the fluctuations of an underestimate (mostly during the summer season) of oscillation embedded in the time series. In this study, the intensities of events in the mid to high range (as we assign 12 months as the deterministic cycle. We also seen in Fig. 16) therefore first examine how accurately the evolution of We have seen earlier that the summer underestimate the annual cycle of present day climate is depicted by of precipitation is related to an excessively northward the RegCM3 mother simulation. Towards this goal, we shift of the monsoon front over the Korea-Northeastern apply the CSEOF technique to both the simulation China region. However, a further contribution to this (1971–2000) and the NCEP/NCAR reanalysis (1979– model bias probably derives from the well-known ten- 2003) over a broad region encompassing the mother dency of models to overpredict precipitation frequen- domain. cies and underpredict intensities (Mearns et al. 1995). The variability explained by the first mode is given Part of this problem has been attributed to the spatial by: averaging inherent to the model resolution and part to deficiencies in the model physics parameterizations, Tðr,tÞ¼B1ðr,tÞT1ðtÞ; particularly moist convection.
where, B1(r,t) is the leading vector of the first CSEOF 3.4 Analysis of trend properties mode and T1(t) is the principal component (PC) time series. B1(r,t) is periodic with a 12 month period, i.e., When applying a modeling system to climate change B1(r,t)=B1(r,t + 12), and we can compute T1(r,t) for studies, it is important to test the model performance the entire data series. not only in reproducing the observed climate statistics, The first mode of the cyclostationary leading vector but also observed trends. In our case this of course will is nearly identical with the composite annual cycle depend on the GCM-derived large scale forcing (Figs. 18, 19). This mode explains about 97% of the boundary fields and on how trends in these large scale total variance, and therefore it is the most dominant
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 775
(a) N CEP/NCAR reanalysis - 1st mode
(b) N CEP/NCAR reanalysis - 2nd mode
Fig. 18 a The first and b second mode cyclostationary leading vector of observed (NCEP/NCAR reanalysis) surface air temperature for winter and summer season. Units are in oC mode of variability. The evolution on a monthly time- are very similar. The seasonal cycle of the temperature scale is consistent with the annual mode. It shows field in the RegCM3 thus describes well the time negative variance during the winter season and positive evolution of the spatial pattern of surface air temper- during the summer season. It is found that the leading ature over East Asia. vectors of the seasonal cycle extracted from the simu- Figure 20 shows the time series of the first two PCs lated and observed (NCEP/NCAR reanalysis) datasets for observed and simulated surface air temperature.
123 776 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
(a) RegCM - 1st mode
(b) RegCM - 2nd mode
Fig. 19 Same as Fig. 18, except for RegCM3 mother domain simulation. Units are in oC
The first mode describes the temporal fluctuations of a somewhat larger interannual fluctuation, indicating the annual cycle and thus measures interannual vari- that the model is slightly underestimating interannual ability. The first PC values of the RegCM3, which are a variability. Figure 21 shows the power spectrum of the measure of the amplitude of the seasonal cycle, are time series of the first PC. Differences in the position somewhat higher than those of the NCEP/NCAR of the spectral peaks in Fig. 21 indicate that the source reanalysis data. On the other hand the reanalysis shows of the interannual variability of the seasonal cycle is
123 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 777
Fig. 20 The first (left) and (a) NCEP / NCAR (1st) (b) NCEP / NCAR (2nd) second (right) principal 11 2 component time series of observed and simulated 10 1 surface air temperature 9 0
8 -1
7 -2 1979 198219851988 199219951998 2001 1979 198219851988 199219951998 2001
(c) RegCM3 (1st) (d) RegCM3 (2nd) 11 2
10 1
9 0
8 -1
7 -2 1971 1974 1977 19801984198719901993 1997 2000 1971 1974 1977 19801984198719901993 1997 2000
Fig. 21 The power spectral density function of the first and second mode CSEOF PC time series different between the two datasets (K.-Y. Kim, per- climate change studies showing that the warming signal sonal communication). As shown by the spectral is more pronounced over higher latitude land areas and analysis of Fig. 21, the seasonal cycle in the NCEP/ in the cold season (Giorgi et al. 2001). NCAR reanalysis data exhibits stronger variations at a longer time scale than the simulated one. It appears that a 60-month peak is more prominent in the NCEP/ 4 Summary and discussion NCAR reanalysis while a 40-month peak is more prominent in the RegCM3. The first mode of the PC In this paper, the climate of Korean was simulated for time series also exhibits oscillations with a 6-month the period 1971–2000 using a RegCM3-based one-way period that are due to a semiannual cycle caused by the double-nested system. In this system, a 60 km grid phase lag between the annual cycle (warm peak in spacing mother domain simulation was first completed August and cold peak in January) and the solar radi- for the East Asia region with the RegCM3 driven at ation (Kim and Chung 2001). the lateral boundaries by fields from the ECHO-G In addition to resolving the strength of the annual coupled AOGCM. This mother domain simulation cycle from the first PC, we can detect a possible then provided lateral boundary forcing fields for a warming trend from the time series of the second PC nested simulation at 20 km grid spacing over the (Fig. 20). Both observed and simulated datasets de- Korean peninsula. Both the simulated average clima- scribe a gradually increasing trend, indicating a warm- tology and some aspects of extremes and trends were ing effect. The second mode of the cyclostationary investigated and validated against large scale analyses leading vector shows a significant temporal dependency and station observations. with a positive signal in the winter season (Figs. 18, 19). The mother domain simulation reproduced the This feature is consistent with results from many seasonal cycle of temperature and precipitation over
123 778 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system
East Asia, thereby providing good quality boundary improved compared to previous GCM-RCM simula- conditions for the nested simulation. The temperature tions over the region (e.g. Hirakuchi and Giorgi 1995; fields in nested domain reflected well the orographic Kato et al. 1999; Kato et al. 2001). In the companion detail at the model resolution, improving in this regard paper by Im et al. (2006b) we describe a climate sce- compared to the mother simulation. However, the nario experiment with the same double-nested system, model showed a systematic cold bias of several tenths with focus on the possible application of the fine-scale of a degree in summer to a few degrees in winter, simulation results to impact studies. mostly due to an underestimate of maximum temper- ature. The model reproduced the observed distribution Acknowledgments The authors wish to thank two anonymous of daily precipitation events in winter, but produced a reviewers whose comments and suggestions were helpful for narrower and more peaked distribution than observed improving the quality of this paper. We are also grateful to Dr. in summer. Steven Cocke for carefully proofreading our manuscript. We extend our thanks to Dr. Kwang-Yul Kim and Dr. Baek-Min The RegCM3 system reproduced reasonably well Kim for kindly providing the source code of the CS EOF and the seasonal evolution of precipitation as determined valuable comments. This research was supported by a grant by the progression and retreat of the East Asia mon- (code#1–9-2) from Sustainable Water Resources Research st soon. In this regard the mother and nested domain Center of 21 Century Frontier Research Program. simulations showed a substantial improvement com- pared to the ECHO-G model, which severely under- References predicted the seasonal cycle of precipitation. The simulated precipitation in winter and in the interme- Bell JL, Sloan LC, Snyder MA (2004) Regional changes in diate seasons was remarkably good; however, simu- extreme climate events: a future climate scenario. J Climate lated summer precipitation was underestimated 17:81–87 compared to station observations, mostly as a result of Choi Y, Jung H-S, Nam K-Y, Kwon W-T (2003) Adjusting urban bias in the regional mean surface temperature series of an excessively northward shift of the monsoon rain- South Korea, 1968–99. Int J Climatol 23:577–591 band. 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