Eun-Soon Im Æ Won-Tae Kwon Æ Joong-Bae Ahn Æ Filippo Giorgi

Transcription

1 Clim Dyn (7) 28: DOI.7/s z Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system. Part 1: recent climate simulation (1971 ) Eun-Soon Im Æ Won-Tae Kwon Æ Joong-Bae Ahn Æ Filippo Giorgi Received: 21 January 6 / Accepted: 6 October 6 / Published online: 8 December 6 Ó Springer-Verlag 6 Abstract We present an analysis of a high resolution multi-decadal simulation of recent climate (1971 ) over the Korean Peninsula with a regional climate model (RegCM3) using a one-way double-nested system. Mean climate state as well as frequency and intensity of extreme climate events are investigated at various temporal and spatial scales, with focus on surface air temperature and precipitation. The mother intermediate resolution model domain encompasses the eastern regions of Asia at 6 km grid spacing while the high resolution nested domain covers the Korean Peninsula at km grid spacing. The simulation spans the 3-year period of January 1971 through December, and initial and lateral boundary conditions for the mother domain are provided from ECHO-G fields based on the IPCC SRES B2 scenario. The model shows a good performance in reproducing the climatological and regional characteristics of surface variables, although some persistent biases are present. Main results are as follows: (1) The RegCM3 successfully simulates the fine-scale structure of the temperature field due to topographic forcing but it shows a systematic cold bias mostly due to an underestimate of maximum temperature. (2) The frequency distribution of simulated daily mean temperature agrees well with the observed seasonal and spatial patterns. In the summer season, however, daily variability is underestimated. (3) The RegCM3 simulation adequately captures the seasonal evolution of precipitation associated to the East Asia monsoon. In particular, the simulated winter precipitation is remarkably good, clearly showing typical precipitation patterns that occur on the northwestern areas of Japan during the winter monsoon. Although summer precipitation is underestimated, area-averaged time series of precipitation over Korea show that the RegCM3 agrees better with observations than ECHO-G both in terms of seasonal evolution and precipitation amounts. (4) Heavy rainfall phenomena exceeding 3 mm/day are simulated only at the high resolution of the double nested domain. (5) The model shows a tendency to overestimate the number of precipitation days and to underestimate the precipitation intensities. (6) A CSEOF analysis reveals that the model captures the strength of the annual cycle and the surface warming trend throughout the simulated period. E.-S. Im (&) W.-T. Kwon Climate Research Lab, METRI, KMA, Seoul, South Korea esim@metri.re.kr J.-B. Ahn Department of Atmospheric Sciences, Pusan National University, Pusan, South Korea F. Giorgi Abdus Salam ICTP, Trieste, Italy 1 Introduction The climate of Korea has experienced a gradual warming throughout the twentieth century (Oh et al. 4) in agreement with the warming observed at the global scale (Intergovernmental Panel on Climate Change, IPCC 1). Average temperature over South Korea has increased by about 1.5 C during the twentieth century (more than twice the corresponding

2 76 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 and rapid urbanization (Kwon 5). The Korean peninsula appears thus to be particularly responsive to possible anthropogenically induced climatic changes. As the global mean surface temperature is projected to further increase in the twenty-first century, it is reasonable to expect that Korea will be strongly vulnerable to climate change. In fact, also associated with global warming are changes in the frequency and intensity of extreme climatic events (Bell et al. 4), and discernable evidence of increased rainfall intensity, shifts of climatic seasons and lengthening of the growing season has already been observed over Korea (Kwon et al. 5). It is thus important that credible scenarios of climate change over Korea are developed in order to evaluate related impacts and adaptation/ mitigation measures. The primary tools used to generate climate change scenarios are coupled atmosphere ocean general circulation models (AOGCMs), and several generations of AOGCMs have been used to produce such scenarios (Kittle et al. 1998; Cubasch et al. 1; Giorgi et al. 1; Giorgi and Bi 5). AOGCMs represent many broad features of current climate reasonably well (McAvaney et al. 1) and can reproduce the observed large-scale changes in climate over the recent past (Mitchell et al. 1). They can therefore be used with some confidence to produce projections of the global climate response to anthropogenic activities. However, in areas where complex coastal and mountainous features have a significant effect on weather and climate, scenarios based AOGCMs, whose resolution is still of the order of km, generally fail to capture the local detail needed for impact assessments at the national and regional level (Mearns et al. 1). Also, at such coarse resolutions, extreme events such as drought or heavy rainfall are either not captured or substantially underestimated. Korea is a region where these limitations of AOGCMs are particularly important, since the Korean peninsula is small (the area of South Korea is 99,585 km 2 ) and mountainous, and since the climate of Korea is characterized by the occurrence of extreme precipitation episodes (Park et al. 3; Yun et al. 1). One method for overcoming the resolution limitations of AOGCMs and adding regional detail to global projections is to use one-way nested regional climate models (RCMs; Giorgi and Mearns 1999). Because of the reasons presented previously, this technique can be especially useful for the Korean peninsula, and therefore we developed a one-way double-nested RCM system for the Korean region based on the regional model RegCM3 (Giorgi et al. 1993a, b; Pal et al. 6; Im et al. 6a). In the double nested approach (Christensen et al. 1998) an intermediate resolution mother domain simulation is first completed using lateral boundary forcing fields from an AOGCM. Fields from this mother domain simulation are then provided as lateral boundary conditions to high resolution nested RCM simulations over a sub-domain of interest. In our double-nested model, the mother domain encompasses the eastern regions of Asia and adjacent oceans at 6 km grid spacing and the nested domain covers the Korean Peninsula at km grid spacing. In a previous paper (Im et al. 6a) we examined the basic model performance in a perfect boundary condition (Giorgi and Mearns 1999) experiment using NCEP/NCAR reanalysis boundary conditions for the period 1 3. The model was validated against a dense observational network over the Korean territory and showed a realistic representation of Korean climate, however with some persistent biases, such as a cold bias during winter (Im et al. 6a). In this and the companion paper by Im et al. (6b) we use the double-nested model system to investigate possible changes in regional surface climate due to global warming and to produce fine-scale regional climate information for impact assessment studies over the Korean peninsula. Toward this goal, we carried out two 3-year long experiments, one for present day conditions (covering the period 1971 ) and one for near future climate conditions (covering the period 21 5) under forcing from the B2 IPCC emission scenario, which lies towards the low end of the IPCC scenario range. Global fields used to drive the mother domain simulation are obtained from a corresponding scenario experiment with the ECHO-G AOGCM. In this paper, we focus on the analysis of the present day climate simulation (1971 ), which is key for assessing and understanding the future climate change scenario. Im et al. (6b) then focus on the future scenario simulation. Our analysis is primarily centered on large scale circulations affecting the region and different statistics of surface air temperature and precipitation, the two variables most used in impact assessment studies (Mearns et al. 1). Among the statistics analyzed are monthly, seasonal, and annual means and the distribution of daily temperature and precipitation events, including extremes. In addition, we investigate whether the modeling system is capable to reproduce observed trends during the 1971 period using a cyclostationary empirical orthogonal function (CSEOF) technique. The model results are evaluated by comparison with large scale reanalysis data as well as station observations covering the South Korea territory.

3 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 model system, experiment design and observation data. The results for the AOGCM, mother and nested domain simulations are then validated and intercompared in Sect. 3 and conclusion are presented in Sect Models, experiment design and validation strategy 2.1 Atmosphere ocean global climate model The global climate model which provides the initial and lateral boundary fields for the mother domain simulation is the ECMWF Hamburg Atmosphere Model Version 4 (ECHAM4) coupled with the Hamburg Ocean Primitive Equation-Global model (HOPE- G) of the Max Planck Institute for Meteorology (MPI) (hereafter referred to as ECHO-G; Min et al. 5, 6). The horizontal and vertical resolution of EC- HAM4 is spectral T3 grid (approximately ~3.75 ) and 19 hybrid sigma-pressure levels (highest level at hpa), respectively. The horizontal resolution of HOPE-G corresponds to a Gaussian T42 grid (~2.8 ) and the model employs vertical levels. HOPE-G also includes a dynamic-thermodynamic sea-ice model with snow cover. The simulation of ECHO-G used in this study is based on the Special Report Emission Scenario (SRES) B2 GHG emission scenario developed by IPCC (). A total of 19 well-mixed gases including CO 2,N 2 O, CH 4 and industrial halocarbons are considered. GHG concentrations are reconstructed from observations for the period , and are taken from the B2 scenario for the period In this scenario the CO 2 concentration increases to 66 ppmv by 2 compared to 199. The ECHO-G simulation covers the period and is described in detail by Min et al. (5, 6) and Oh et al. (4). 2.2 Regional climate model The regional climate model used in this study is the Abdus Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model (RegCM) in its latest version RegCM3. This is an upgraded version of the model originally developed by Giorgi et al. (1993a, b) and then improved as discussed by Giorgi and Mearns (1999) and Pal et al. (6). The dynamic core of the RegCM3 is equivalent to the hydrostatic version of the NCAR/Pennsylvania State University mesoscale model MM5 (Grell et al. 1994). The physics parameterizations employed in this simulation include the comprehensive radiative transfer package of the NCAR Community Climate Model, version CCM3 (Kiehl et al. 1996), the nonlocal boundary layer scheme of Holtslag et al. (199), the BATS land surface scheme (Dickinson et al. 1993), the mass flux cumulus cloud scheme of Grell (1993) and the resolvable scale precipitation scheme of Pal et al. (). The same model physics schemes are used in the mother and nested domain simulations and we adopt a linear orography blending method similar to Hong and Juang (1998) at the lateral boundaries in order to minimize the discernible systematic error due to scale mismatch between driving and model fields. 2.3 Experiment design Figure 1 shows the mother and nested model domains used in this study, which are the same as in Im et al. (6a). The mother domain covers the eastern regions of Asia (including the Korean Peninsula) at 6 km grid spacing, while the nested domain encompasses the South Korean Peninsula at km grid spacing. As shown in the nested domain, the Korean Peninsula is a mountainous region with the most prominent ranges reaching elevations of over 1, m. Approximately 7% of the South Korean territory consists of hills and mountains (Choi et al. 3). These geographical characteristics cause significant fine-scale variations in weather and climate. Comparison of the two domains shows that the mountain ranges in the nested domain are much more realistic than in the mother domain, which emphasizes the necessity of the double-nesting system. We carefully selected the domain area through various sensitivity experiments (Im et al. 6a). In both the mother and nested domain simulations the model employs 18 vertical sigma levels. The regional model can be run with initial and lateral boundary conditions from either global analysis data or the output of a GCM. In our experiments the initial and time-dependent meteorological lateral boundary conditions are interpolated at 6-hourly intervals from a transient ECHO-G B2 scenario simulation which covered the period For the present day climate conditions discussed here the RegCM simulation covers the 3-year period of Sea surface temperature (SST) over the ocean areas, initial soil moisture and GHG concentrations are obtained from the corresponding ECHO-G fields. We analyze the simulation results focusing on the summer (June, July, August; or JJA) and winter (December, January, February; or DJF) seasons.

4 762 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 (6 km grid spacing) and nested ( km grid spacing) simulations 2.4 Verification strategy Various reanalysis and observation datasets are used to validate the performance of the RegCM3 double-nested system. First of all, 57 climate stations with data for the full 3-year period of 1971 throughout South Korea are used, including daily mean, maximum and minimum temperature and precipitation. This dataset allows a validation of the fine-scale structure of the nested model simulation and the relatively high model resolution justifies the comparison between station and model grid point data. The topography and location of the 57 stations are shown in Fig. 2. Note that the dataset includes both low elevation and high elevation stations. Here, two mountain systems are most relevant, the Taebaek Mountains, extending from north to south along the eastern coastal regions of Korea, and the Sobaek Mountains located in the south-central regions of the peninsula. For the validation of results from the ECHO-G and mother domain simulations we use global reanalysis datasets. Precipitation results are compared with the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) (Xie and Arkin 1997) and the NCEP/NCAR Reanalysis dataset (Kalnay et al. 1996), which include monthly data at a 2.5 resolution. The CMAP dataset was produced by merging gauge observations, estimates inferred from a variety of satellite observations, and precipitation forecasts from the NCEP/NCAR reanalysis. On the other hand, the NCEP/NCAR reanalysis precipitation is a model product obtained by assimilating quality-controlled observations from different sources. Therefore, precipitation in the two datasets can be different. Temperature results are compared with the NCEP/NCAR Reanalysis data. Both reanalysis datasets cover the 25- year period of Results 3.1 ECHO-G and mother domain simulation In this section we present an analysis of the 3-year climatology of the mother domain simulation as driven by the ECHO-G fields. This is important to assess whether the driving fields in the mother domain are of adequate quality for use in the double nesting method. Figure 3 first compares NCEP/NCAR reanalysis and simulated (mother domain) average large scale circulation at 85 mb in the winter and summer seasons. Results for ECHO-G are not shown, as they are generally similar to those from the mother domain simulation. The climate of East Asia is greatly influenced by the seasonal evolution of the monsoon circulation. During the winter season, the Siberian high determines the development of the winter monsoon. The wind field at 85 mb shows a predominant northwesterly lower tropospheric flow over East Asia associated with anticyclonic circulation induced by the Siberian high (Jhun and Lee 4). This circulation carries cold air from the polar regions into East Asia

5 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 763 Fig. 2 Topography (m) and location of climate stations used for the assessment of the model performance during the winter season. The model captures the observed wave pattern associated with the Siberian high. During the summer (Fig. 3), the western branch of the Pacific sub-tropical high penetrates inland, producing 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. 1; 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 difficulties in simulating the East Asia monsoon (Gao et al. 6). Figure 4 compares the average surface air temperature in the NCEP/NCAR reanalysis (25 years) and Fig. 3 Observed (NCEP/ NCAR reanalysis) (upper panels a and c), and simulated (lower panels b and d) 3-year averaging 85 hpa winds for the winter and summer season. Units are in meter/s (a) NCEP/NCAR Dec Jan Feb DJF (b) RegCM3 RegCM3 NCEP/NCAR (c) Jun Jul Aug JJA (d)

6 764 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 3-year averaging surface air temperature for the winter and summer seasonal mean. Units are in degrees the ECHO-G and RegCM3 mother domain simulations (3 years) for the winter and summer seasons. Overall, the model results agree well with the broad spatial patterns of observed seasonal surface air temperature. In addition, the RegCM3 simulation shows some fine-scale structure missed by the reanalysis and ECHO-G data due to their coarse resolution. Note that the ECHO-G shows an almost zonal temperature pattern that does not properly reflect the detailed geography of the Korean peninsula in the winter season. Overall, the RegCM shows a cold bias of a few degrees, particularly in the cold season. This feature was also found in the perfect boundary condition experiment of Im et al. (6a) and hence it appears to be a persistent bias of the RegCM over this region. More discussion of this bias is presented later. The simulated precipitation field successfully reproduces observed large scale features and captures the seasonal evolution of the precipitation pattern associated with the East Asia monsoon system (Fig. 5). However, the precipitation simulated by ECHO-G shows a smaller than observed range of seasonal variation over East Asia. In the winter season, the ECHO- G overestimates precipitation over eastern China and Korea and misses typical precipitation patterns that occur on the northwestern side of Japan during the winter monsoon (Kato et al. 1). Conversely, the summer precipitation simulated by ECHO-G is underestimated in response to a relatively weak monsoon rain-belt. In the RegCM3 mother domain simulation, winter precipitation agrees with observations, especially over the Korean peninsula. Both the general spatial patterns and the winter monsoon precipitation patterns over northwestern Japan are reproduced clearly in the RegCM3 mother domain run. During the summer, the mother domain simulation produces higher precipitation amounts and a more intense rainbelt than ECHO-G, a result in the direction of a better agreement with the reanalysis data. However, some substantial differences between CMAP and RegCM3 simulated summer precipitation are found. In particular, the CMAP data show a pronounced precipitation maximum over the Korean peninsula while the precipitation band in the RegCM3 simulation extends farther to the north. Surprisingly, we find considerable discrepancies between the CMAP and NCEP precipitation fields in the summer season. In fact, the model simulated precipitation is in agreement with the NCEP precipitation than the CMAP one. To quantitatively assess whether the regional climate model indeed shows an improvement compared to the global climate model ECHO-G, we provide additional objective measures of performance, such as the spatial pattern correlation and bias, between model results and reanalysis data (Table 1). The pattern correlation measures the agreement between the spatial patterns of simulation and observations while the bias is a direct measure of how the model average climatology deviates from observed. Here, we calculate the scores at the two different model resolutions to estimate both the broad patterns and the fine-scale features.

7 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 3-year averaging precipitation for the winter and summer seasonal mean. Units are in millimeters/day Table 1 Spatial pattern correlation and bias between model results and observation Corerlation Bias RegCM3 ECHO-G RegCM3 ECHO-G 2.5 Tem. (NCEP) DJF JJA Pre. (NCEP) DJF JJA Pre. (CMAP) DJF JJA km Tem. (station) DJF JJA Pre. (station) DJF JJA First, the seasonal mean temperature and precipitation fields from the RegCM3 mother domain and ECHO-G are compared with NCEP and CMAP (precipitation only) reanalysis data at a 2.5 resolution over the mother domain interior after both model results are bilinearly interpolated onto the reanalysis grid. For temperature we find generally high correlation coefficients for both model simulations. The spatial pattern correlations of ECHO-G are slightly higher than those of RegCM3 except for summer precipitation. In fact, the topographic signal is almost absent from the reanalysis data at a 2.5 resolution due to the coarse resolution of the data. Conversely, the bias scores illustrate a pronounced deficiency of the ECHO-G simulation, with a severe underestimate of summer precipitation and overestimate of winter precipitation. Both these scores are improved in the RegCM simulation. The improvement deriving from the regional climate model is further measured by the comparison with the Korean station observations (land only: 125 E 13 E, 34 N 38.5 N). The ECHO-G results are interpolated, and the observations are aggregated, onto the mother domain 6 km grid. The RegCM3 pattern correlations are higher than ECHO-G correlations except for summer precipitation. Because of better resolved topographic forcing, the increased resolution of the mother domain (compared to ECHO-G) evidently leads to increased spatial agreement with the fine-scale observation fields. For summer precipitation, we do not find a significant pattern agreement between either of the model results and observations. However, the bias of RegCM3 is much lower than that of ECHO-G. To provide a quantitative measure of model performance related to the seasonal cycle, we calculated the monthly variation of area-averaged temperature

8 766 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 (1 E 142 E, 27 N 45 N) and the Korean peninsula ( E 131 E, 32 N 43 N), including data over ocean areas (Figs. 6, 7). The RegCM3 systematic cold bias varying from 2.3 C in late winter to.3 C in the fall is evident over both the East Asia and Korea regions. By comparison, the ECHO-G simulatin shows a warm bias of up to 2.7 C in the winter and a cold bias of up to 1.3 C in the summer (Fig. 6). Figure 7 compares area-averaged observed and simulated precipitation over East Asia and the Korean peninsula. As already mentioned, ECHO-G severely underestimates precipitation in summer and overestimates it in winter, thereby markedly underestimating the seasonal precipitation cycle. The mother domain simulation substantially improves upon the ECHO-G results, with a much better simulated seasonal precipitation cycle, especially during the summer months. The main model deficiencies are an overestimate of early to mid summer precipitation over East Asia and an underestimate of later summer precipitation over Korea. In order to examine the model performance related to the propagation of the monsoon rain-band, we refer to Fig. 8, which shows a time-latitude cross section of zonally averaged precipitation along the band 1 E 142 E. The development of the monsoon is characterized by a northward movement of the rain-band until mid-july and a subsequent retreat during the fall. The simulated precipitation by RegCM3 captures this seasonal propagation, although the rain-band extends too far to the north in the summer and retreats too rapidly in the late fall. In ECHO-G the precipitation amounts during the summer monsoon season are underestimated and a strong winter precipitation band can be observed to extend up to about 32 N. Although the spatial and temporal patterns of the precipitation band in the RegCM3 and ECHO-G simulations show similarities, the RegCM3 precipitation is in much better quantitative agreement with reanalysis data. In summary, Figs. 3, 4, 5, 6, 7 and 8 indicate that the mother domain simulation adequately simulates the basic features of East Asia climate, including the Korean peninsula, and is thus adequate to provide realistic forcing fields to the nested domain simulation. The Korean peninsula is directly affected by the monsoon activity and is characterized by a marked spatial variability of precipitation as influenced by the topography of the region. In the next section we turn our attention to the analysis of the fine scale nested domain experiment. 3.2 ed domain simulation In the analysis of our nested domain simulation we compare the model results with observations derived from 57 climate stations in South Korea with no missing data during the 3-year period of In assessing the quality of the simulated results for the nested domain, we use as reference the mother domain results over the nested domain area. Use of the double nesting approach is indeed justified when the nested model results are superior (according to selected measures) to the mother domain results. Figure 9 shows the 3-year mean spatial distribution of surface air temperature for two seasons (summer and winter). The first set of panels presents the results directly obtained from the 57 station dataset. The second set of panels shows the nested model results, while the third set presents the results from the coarse mother domain simulation. The observed temperature field shows substantial spatial variability with a number of topographically induced fine-scale regional features, possibly also related to data at individual stations. The topographic signal related to the mountain chains of northeast and south central Korea (see Figs. 1, 2) is however evident in each month as well as the season average. The results from the nested domain simulation reproduce these regional features in the two seasons, Temperature (degree) 3 ECHO-G NCEP -NCEP ECHO-NCEP Diff. (degree) Temperature (degree) 3 ECHO-G NCEP -NCEP ECHO-NCEP Diff. (degree) Month Month Fig. 6 Time series of area-averaged surface air temperature over East Asia (left) and Korean peninsula (right)

9 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 767 Precipitation rate (mm/day) 8 ECHO-G 6 CMAP 4 2 Precipitation rate (mm/day) 8 ECHO-G 6 CMAP Month Month Fig. 7 Time series of area-averaged precipitation over East Asia (left) and Korean peninsula (right) Fig. 8 Time-latitude cross section of zonal averaged precipitation along 1 142E for 3-year period (a) CMAP (b) NCEP/NCAR (c) RegCM3 (d) ECHO-G although the cold bias present in the mother domain simulation is inherited by the nested one. Orographic detail not found in the mother domain is clearly reproduced in the nested domain, where the temperature field reflects well the location of the Taebaek and Sobaek mountains. Simulated and observed summer and winter temperatures show similar topographically induced spatial patterns. Daily minimum and maximum temperatures show the same spatial patterns as average temperature (not shown). Figure shows the same quantities as Fig. 9 except for precipitation. From the analysis of winter precipitation, we first find an agreement between simulated and observed amounts. The observations show a band of large winter precipitation over the northeastern coasts. Considering the topography of the Korean peninsula (Figs. 1, 2), these are associated with the upslope topographic forcing of northeasterly flow taking place in the winter over this region (Im et al. 6a). Another maximum occurs over southwestern Korea, evidently in response to topographic uplift of westerly flow by the Sobaek chain in southern Korea. For the winter case the improvement in the simulation of spatial detail by the nested domain compared to the mother domain is quite evident, for each month as well as the season average. In fact, the spatial pattern correlation between simulated and observed precipitation at the km grid during the winter season is higher in the nested domain (.69) than the mother domain (.47). The nested domain simulation reproduces quite well the observed winter maxima over northeast and southwest Korea, in magnitude as well as location. The average precipitation improvement by the nested simulation is less evident in the summer (Fig. ). This improvement is seen in the summer average field (JJA), when the model captures high precipitation bands extending from northwest to northeast Korea and from the southern coasts to central Korea. However in the individual months the model fails to capture localized maxima found in the observations (and perhaps related to individual stations). In general, the mother and nested domain results are of similar magnitude, and the nested domain simulation just improves the representation of the spatial patterns of summer precipitation compared to the mother domain simulation. Note that we expect precipitation to be more stochastic in nature during summer than winter, so that the effect of finer scale topography is less evident in the summer.

10 768 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system (a) (b) (c) OBS Fig. 9 Spatial distribution of surface air temperature averaged over 3 years for winter (a c) and summer (d f) season. Here, the first set of panels corresponds to direct plotting of the 57 climate stations, the second set of panels is the results for km grid (nested domain) and the third set of panels is the results for 6 km grid (mother domain). Units are in o C For a more quantitative evaluation of the nested domain simulation, Fig. 11 compares station data, mother domain and nested domain temperature and precipitation averaged over the Korean territory. This figure can also be compared with Figs. 6 and 7, based on NCEP/NCAR reanalysis and CMAP observations. The cold temperature bias found in the mother domain simulation is essentially inherited by the nested domain simulation, leading to very similar bias magnitudes. For precipitation, we first notice from the comparison of Figs. 11 and 7 that the station data exhibit substantially higher summer precipitation amounts than the CMAP data. As a result, the precipitation underprediction of both the mother and nested domain simulations in summer appears larger than in Fig. 7. An analysis of Fig. indicates that this bias is not ubiquitous throughout the Korean peninsula but it is mostly due to the failure to reproduce some local maxima found

11 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) (f) Fig. 9 continued over the coastal regions. This may be due to the representation of mesoscale convective systems at the relatively coarse model topography. Both the mother and nested domain precipitation amounts show an excellent agreement with station observations from October to June. In this regard, the precipitation amounts are similar in the mother and nested domain experiments. This result differs from a previous application of the RegCM to the European region (Giorgi and Marinucci 1996), in which precipitation was shown to increase with horizontal resolution. 3.3 Analysis of daily properties Most commonly, local scenario construction has concentrated on monthly, seasonal, or even annual values of meteorological variables such as temperature and precipitation. However, these scenarios are often of limited value to impact assessment studies, which often require information at daily time scales. For example, shifts in the timing of thresholds such as the dates of the first and last frosts have important implications for agriculture. Such changes can be evaluated only from

12 77 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system (a) (b) (c) OBS Fig. Same as Fig. 9, except for precipitation. Units are in millimeters/day daily time series of the relevant climate variable (Palutikof et al. 1997). In this section we thus validate daily statistics of temperature and precipitation against the station data over the Korean territory (see Fig. 2). More specifically, we compare station data with model data at the grid point closest to the station location, an approach justified by the high model resolution. Figure 12 describes the frequency distribution of daily mean temperature at all station locations over Korea, which gives a measure of both mean and variance of the daily values. First of all, we notice that there are no significant differences between mother and nested domain simulated daily temperature statistics. The annual PDF shows two peaks, corresponding to the winter and summer conditions. Although the model reproduces this double-peaked structure, the distribution is shifted to the left because of the cold bias previously mentioned. In the winter the shape of the simulated PDF is the same as the observed one, and in particular the observed variance is reproduced. In the summer season, however, the model shows a narrower distribution than observed, i.e. a

13 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 771 (d) (e) (f) OBS Fig. continued lower variance and a higher incidence of mean values. The lower tail of the summertime distribution and the upper tail of the wintertime distribution are well simulated. However, the lower tail of the wintertime distribution is shifted by about 2 C compared to observations. It is noticeable that the shape of the winter and summer distributions are quite different, the winter one being more symmetrical and wider. The model reproduces the shape of the PDF better in winter than in summer, although the winter PDF shows a greater shift compared to observations. Figure 13 compares simulated and observed distributions of daily temperature anomalies, calculated as the daily temperature minus the 3-year average daily temperature at all stations. We can see that the annual and winter anomaly distributions are captured well, while the summer anomaly distribution is narrower and more peaked than in the observations

14 772 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system Temperature (degree) 3-1 OBS - OBS - OBS Month Diff. (degree) Precipitation rate (mm/day) Month 7 9 OBS 11 Fig. 11 Time series of area-averaged monthly surface air temperature (left) and precipitation (right) over the Korean Peninsula Probability (%) Station Probability (%) Station Probability (%) Station Degree 3 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 Probability (%) Station Probability (%) Station Probability (%) Station - - Degree - - Degree - - Degree Fig. 13 Same as Fig. 12, except for daily mean temperature anomaly Fig. 14 Probability density function of annual distribution for daily maximum (left) and minimum (right) temperature in Korea Probability (%) Station Probability (%) Station Degree Degree (although this bias does not contribute greatly to the annual anomaly PDF). For winter, simulated and observed temperature anomaly distributions are nearly normal, with a skewness near zero and a kurtosis near 3. For summer the model results show some deviations compared to observations, with a slightly

15 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 3- year averaging daily maximum and minimum temperature 4 3 Max_bs Max_mo Max_ne Min_obs Min_mo Min_ne Degree positive skewness (.13.16) and higher kurtosis (3.64 vs. 3.23) indicating, as mentioned, more peaked distributions. In order to investigate the cause of the biases in the daily mean temperature distribution, daily maximum and minimum temperature distributions were examined and compared to observations in Fig. 14. We find that the greatest contribution to the winter cold bias is due to an underestimate of winter daily maximum temperature. This is at least partially due to an underestimate of sunshine duration associated with an overestimate of weak precipitation events (Kato et al. 1), as will be discussed in more detail later. Conversely, summer minimum temperature appears to provide the greatest contribution to the underestimate of the width of the distribution (and related overestimate of the peak). Overall, except for the shift in winter maximum temperature, the distributions of minimum and maximum temperature are realistically reproduced. Figure 15 shows the seasonal evolution of the daily Tmax and Tmin averaged over 3-years of observations and simulations. The observed and simulated Tmax and Tmin time series show a good seasonal phase coherence. Tmax is underestimated throughout the year, except for the early summer and late summer periods, while Tmin is mostly underestimated in Day mid-winter conditions. It is clear from Fig. 15 that the cold model bias is mostly due to the contribution of Tmax and that a generally better agreement with observations is found in the intermediate seasons (MAM and SON) than the extreme ones. Figure 16 shows the frequency distribution of observed and simulated daily precipitation at all Korean station locations. During winter, when the model average precipitation is close to observed, the simulated frequency distribution matches observations remarkably well, expect for precipitation episodes above mm/day. As already discussed, the model underestimates precipitation in the summer. Figure 16 shows that this low precipitation bias is reflected throughout the entire frequency distribution, that is the number of precipitation events is underestimated at all intensities, and especially for intensities in the midrange of mm/day. An important result of Fig. 16 derives from the comparison of the mother domain and nested domain frequency distributions. We can see that for low to mid-intensities the two frequency distributions are essentially the same. However, the nested domain frequency distribution has a longer tail at the high intensity range. In other words, only at the higher resolution of the nested domain simulation the model is capable of producing Event No. Station mm/day Event No. Station mm/day Event No. Station mm/day Fig. 16 Frequency distribution of daily precipitation in Korea. Left, middle, and right panel is annual, summer, and winter distribution, respectively

16 774 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 8 mm/day in winter and 25 4 mm/day in summer). In this regard, the nested domain simulation is much closer to observations than the mother domain simulation and this result supports the use of finer scale models to simulate extreme events. To further investigate the origin of the precipitation underestimation, we analyzed precipitation intensities and frequencies. Figure 17 shows the spatial distribution of the annual total number of precipitation days, defined as days with precipitation exceeding 1 mm/day. We find that the model simulates precipitation frequencies in line with observations or even higher. The precipitation underestimation is thus not given by an underestimate of the total number of events, but by an underestimate (mostly during the summer season) of the intensities of events in the mid to high range (as also seen in Fig. 16) We have seen earlier that the summer underestimate of precipitation is related to an excessively northward shift of the monsoon front over the Korea-Northeastern China region. However, a further contribution to this model bias probably derives from the well-known tendency of models to overpredict precipitation frequencies and underpredict intensities (Mearns et al. 1995). Part of this problem has been attributed to the spatial averaging inherent to the model resolution and part to deficiencies in the model physics parameterizations, particularly moist convection. 3.4 Analysis of trend properties When applying a modeling system to climate change studies, it is important to test the model performance not only in reproducing the observed climate statistics, but also observed trends. In our case this of course will depend on the GCM-derived large scale forcing boundary fields and on how trends in these large scale fields are transmitted to, and processed by, the fine-scale regional model. In this section, we address this issue by use of cyclostationary empirical orthogonal function (CSEOFs) analysis as a statistical method to extract the time evolution of the annual cycle and to detect warming trends in the simulation period (trends possibly due to increased GHG concentration) (Kim and North 1997). It is in fact more appropriate to adopt a cyclostationary approach rather than a stationary one in any statistical analysis of climatic variables (such as temperature) that show strong dependence on the phase of the annual (or diurnal) cycle (Kim and Wu ). CSEOF considers the annual cycle in a data series fluctuating on longer time scales, and assigns a deterministic cycle representing the fluctuations of an oscillation embedded in the time series. In this study, we assign 12 months as the deterministic cycle. We therefore first examine how accurately the evolution of the annual cycle of present day climate is depicted by the RegCM3 mother simulation. Towards this goal, we apply the CSEOF technique to both the simulation (1971 ) and the NCEP/NCAR reanalysis (1979 3) over a broad region encompassing the mother domain. The variability explained by the first mode is given by: Tðr,tÞ ¼B 1 ðr,tþt 1 ðtþ; where, B 1 (r,t) is the leading vector of the first CSEOF mode and T 1 (t) is the principal component (PC) time series. B 1 (r,t) is periodic with a 12 month period, i.e., B 1 (r,t) =B 1 (r,t + 12), and we can compute T 1 (r,t) for the entire data series. The first mode of the cyclostationary leading vector is nearly identical with the composite annual cycle (Figs. 18, 19). This mode explains about 97% of the total variance, and therefore it is the most dominant

17 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 775 (a) NCEP/NCAR reanalysis - 1st mode (b) NCEP/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 o C mode of variability. The evolution on a monthly timescale is consistent with the annual mode. It shows negative variance during the winter season and positive during the summer season. It is found that the leading vectors of the seasonal cycle extracted from the simulated and observed (NCEP/NCAR reanalysis) datasets are very similar. The seasonal cycle of the temperature field in the RegCM3 thus describes well the time evolution of the spatial pattern of surface air temperature over East Asia. Figure shows the time series of the first two PCs for observed and simulated surface air temperature.

18 776 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 o C The first mode describes the temporal fluctuations of the annual cycle and thus measures interannual variability. The first PC values of the RegCM3, which are a measure of the amplitude of the seasonal cycle, are somewhat higher than those of the NCEP/NCAR reanalysis data. On the other hand the reanalysis shows a somewhat larger interannual fluctuation, indicating that the model is slightly underestimating interannual variability. Figure 21 shows the power spectrum of the time series of the first PC. Differences in the position of the spectral peaks in Fig. 21 indicate that the source of the interannual variability of the seasonal cycle is

19 E.-S. Im et al.: Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system 777 Fig. The first (left) and second (right) principal component time series of observed and simulated surface air temperature 11 (a) NCEP / NCAR (1st) 2 1 (b) NCEP / NCAR (2nd) (c) RegCM3 (1st) 2 (d) RegCM3 (2nd) 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, personal communication). As shown by the spectral analysis of Fig. 21, the seasonal cycle in the NCEP/ NCAR reanalysis data exhibits stronger variations at a longer time scale than the simulated one. It appears that a 6-month peak is more prominent in the NCEP/ NCAR reanalysis while a 4-month peak is more prominent in the RegCM3. The first mode of the PC time series also exhibits oscillations with a 6-month period that are due to a semiannual cycle caused by the phase lag between the annual cycle (warm peak in August and cold peak in January) and the solar radiation (Kim and Chung 1). In addition to resolving the strength of the annual cycle from the first PC, we can detect a possible warming trend from the time series of the second PC (Fig. ). Both observed and simulated datasets describe a gradually increasing trend, indicating a warming effect. The second mode of the cyclostationary leading vector shows a significant temporal dependency with a positive signal in the winter season (Figs. 18, 19). This feature is consistent with results from many climate change studies showing that the warming signal is more pronounced over higher latitude land areas and in the cold season (Giorgi et al. 1). 4 Summary and discussion In this paper, the climate of Korean was simulated for the period 1971 using a RegCM3-based one-way double-nested system. In this system, a 6 km grid spacing mother domain simulation was first completed for the East Asia region with the RegCM3 driven at the lateral boundaries by fields from the ECHO-G coupled AOGCM. This mother domain simulation then provided lateral boundary forcing fields for a nested simulation at km grid spacing over the Korean peninsula. Both the simulated average climatology and some aspects of extremes and trends were investigated and validated against large scale analyses and station observations. The mother domain simulation reproduced the seasonal cycle of temperature and precipitation over

20 778 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 conditions for the nested simulation. The temperature fields in nested domain reflected well the orographic detail at the model resolution, improving in this regard compared to the mother simulation. However, the model showed a systematic cold bias of several tenths of a degree in summer to a few degrees in winter, mostly due to an underestimate of maximum temperature. The model reproduced the observed distribution of daily precipitation events in winter, but produced a narrower and more peaked distribution than observed in summer. The RegCM3 system reproduced reasonably well the seasonal evolution of precipitation as determined by the progression and retreat of the East Asia monsoon. In this regard the mother and nested domain simulations showed a substantial improvement compared to the ECHO-G model, which severely underpredicted the seasonal cycle of precipitation. The simulated precipitation in winter and in the intermediate seasons was remarkably good; however, simulated summer precipitation was underestimated compared to station observations, mostly as a result of an excessively northward shift of the monsoon rainband. This shift in the nested domain simulation was mostly inherited from the ECHO-G and mother domain simulations. Compared to the mother domain experiment, the nested one improved the spatial pattern of precipitation, especially in winter, as a result of a more detailed representation of topography. Observed daily precipitation PDFs were reproduced very accurately by the nested domain simulation in winter, but the number of events in the mid to high intensity range was underpredicted in summer. The nested domain simulation produced higher intensity extreme events compared to the mother domain simulation, with the intensity of these events being comparable to observations. This indicates that fine scale is critical for simulating extreme events over the region. The RegCM3 also reproduced the strength and temporal modulation of the annual cycle of the temperature field compared to the NCEP/NCAR reanalysis in terms of the CSEOF first mode. Besides resolving the annual cycle of the first mode, we also found a warming trend from the second mode, which is consistent with a global warming signal amplified over the region during winter conditions. Overall, despite the biases described above, the ECHO-G RegCM3 double-nested system showed an encouraging performance over this region, whose monsoon-dominated climate has been traditionally very difficult to simulate with global climate models (e.g. Gao et al. 6). In particular, the quality of the present modeling system is improved compared to previous GCM-RCM simulations over the region (e.g. Hirakuchi and Giorgi 1995; Kato et al. 1999; Kato et al. 1). In the companion paper by Im et al. (6b) we describe a climate scenario experiment with the same double-nested system, with focus on the possible application of the fine-scale simulation results to impact studies. Acknowledgments The authors wish to thank two anonymous reviewers whose comments and suggestions were helpful for improving the quality of this paper. We are also grateful to Dr. Steven Cocke for carefully proofreading our manuscript. We extend our thanks to Dr. Kwang-Yul Kim and Dr. Baek-Min Kim for kindly providing the source code of the CS EOF and valuable comments. This research was supported by a grant (code#1 9-2) from Sustainable Water Resources Research Center of 21 st Century Frontier Research Program. 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