پس‌پردازش آماري برونداد بارش الگوي RegCM4 روي شمال‌ غرب ايران

Statistical Post-Processing of the RegCM4 Model Precipitation Output in Northwest Iran

اين پژوهش به‌منظور تعيين روش برتر پس‌پردازش آماري بر بارش حاصل از اجراي الگوي RegCM4 در شمال ‌غرب انجام گرفت. داده‌هاي مورد ‌نياز اجراي الگوي RegCM4 شامل داده‌هاي بازتحليل وضع جوي (NNRP1)، داده‌هاي دماي سطح دريا (SST) و داده‌هاي سطح زمين (SURFACE) از مركز ICTP دريافت شد. به‌منظور اجراي اين الگو، آزمون تعيين طرحواره همرفت و جداسازي افقي مناسب انجام گرفت كه با استناد به آن، طرحواره Kuo به نسبت دو طرحواره Grell و Emanuel خطاي كمتري را در اجراي الگو‌سازي بارش منطقه داشت. جداسازي افقي نيز سي كيلومتر انتخاب شد. پس از اجراي الگو، برونداد‌ بارش با استفاده از روش‌هاي شبكه پرسپترون چندلايه (MLP) و ميانگين متحرك (MA) پس‌پردازش شد. براساس نتايج، اعمال پس‌پردازش روي داده‌هاي بارش خام سالانه الگو، موجب كاهش ميانگين اريبي خطاي داده‌هاي به‌دست‌آمده و رسيدن به رقم 9/8 ميلي‌متر شد؛ در حالي كه اين آماره براي برونداد خام سالانه الگو 3/124 ميلي‌متر بود. در مقياس‌هاي زماني فصلي و ماهانه نيز ميانگين اريبي خطاي بارش شبيه‌سازي به‌ترتيب برابر 1/31 و 4/10 ميلي‌متر بود كه پس از اعمال پس‌پردازش به روش‌هاي يادشده، اين آماره به ترتيب به 3/0- و صفر ميلي‌متر رسيده است. روش برتر پس‌پردازش نيز در تمام مقاطع زماني، MA بوده است.

Extended Abstract Introduction The main perspective in seasonal prediction of precipitation, is presenting a qualitative prediction for upcoming seasons. Information gained from such predictions can be used for decision making in various deciplines such as agriculture, water management and hydropower production. Besides, it can help for reducing the adverse effects of climatic changes like drought and flood. But General Ciculation Models (GCMs) outputs have coarse resolution ( > 100 km). Dynamical downscaling is a method for obtaining high-resolution climate data from relatively coarse resolution global climate models which do not capture the effects of local and regional forcing in areas of complex surface physiography. GCMs outputs at spatial resolution of 150-300 km are unable to resolve important sub-grid scale features such as clouds and topography. Many impacts models require information at scales of 50 km or less. As suchm, several statistical and dynamical methods are developed to estimate the smaller-scale information. Dynamical downscaling uses a limited area, high resolution model (a regional climate model: RCM) driven by boundary conditions from a GCM to derive smaller scale information. Methodology The aim of this study was application of RegCM4 dynamic model (Reginal climate model) in forecasting rainfall and improving the outputs using post processing techniques in northwest Iran during period 1982-2011. The recorded data of precipitation were collected from Urmia, Tabriz, Ardebil and Khuy Stations. The data required for running the regional climate model RegCM4 were adopted from center ICTP (International Centre for Theoretical Physics), in the format of NetCDF including three sets of weather data, NNRP1 with a 6-hour-scale and a horizontal resolution of 2.5°×2.5° on the reanalysis databases of National Center of environmental prediction of United States, sea surface temperature, (SST) with a horizontal resolution of in 1°×1° from the type of SST belonged to America and National Oceanic and Atmospheric surface SURFACE, which were consisted of three topographic data GTOPO, the vegetation or land use ,GLCC, and the soil type data GLZB, with a horizontal resolution of 30×30 seconds from United States Geological Survey, for the period 1982 to 2011. In order to implement the dynamic model, a test was conducted to determine the Convective Precipitation scheme and the amount of horizontal resolution for the year 2009 (as a normal year), Accordingly, Kuo scheme with minimum mean bias error (MBE), in comparison with observed precipitation in 36 synoptic stations of the region, was implemented as the main scheme, horizontal resolution of 30 × 30 square kilometers, the number of grid points including 152 in longitude (iy) and 168 in latitude (ix) was conducted during the statistical period of 1982 to 2011. Geographical area center implemented in the intended period was located in 30.5 degrees north latitude and 50 degrees east longitude respectively. The output of the model included atmospheric data (ATM), surface cover (SRF) and radiation cover with the format of NetCDF, each containing a large number of meteorological variables among which except precipitation from the Model (tpr), 9 variables that were associated more with precipitation including q2m و t2m ،ps ،v1000 ،v500 ،u1000 ،u500،omega1000, omega500 were extracted. For post-processing the output of the model, the Multi-Layer Perceptron (MLP) and in Moving Average (MA) methods were used. For MLP Entering variables were 10 aforementioned variables and the target variable was observatory precipitation in the stationary point. At any one time, 80% of the data at the beginning of the series were for train and final 20 percent of data was used for test. Results and Discussion The results of the study demonstrated that, in the study area, the mean bias error of raw annual precipitation outputs of the RegCM4 model was 124.3 mm in the validation period, which by conducting Post Processing, reduced to 8.9 mm. In the seasonal and monthly time scales, also, mean bias error of the were 31.1 and 10.4 mm, respectively, which were reduced to -0.3 and zero mm, respectively, after post processing. The MA model was the prefered post processing method, in all time scales. Conclusion In conclusion, it can be stated that the RegCM4 regional climate model with the said implementing conditions and in the study area, contained, mainly, overestimate in precipitation forecasting. However, the application of post-processing will optimally reduce bias. The appropriate method is also the simple moving average (MA) method. Keywords: Post-processing, Dynamical downscaling, MLP, Convective Precipitation scheme, MA