Downscaling of current and future rainfall climatologies for southern Morocco . Part I : Downscaling method and current climatology

The use of a statistical-dynamical downscaling approach to obtain a high-resolution rainfall climatology for a subtropical region in southern Morocco is analysed. The statistical part of the downscaling uses Circulation Weather Types (CWTs) as a measure for near-surface wind fields calculated from sea-level pressure (SLP) data. The daily CWTs are correlated with daily rainfall data at three different climate stations in the region for the period 1978–1997 and systematic differences are discussed. Results are viewed for one extreme dry year (1984) and one extreme wet year (1989) to show the limits of purely statistical downscaling. Dynamical downscaling is realized for representative days to enable statistical-dynamical downscaling. Comparison of statistically-dynamically downscaled rainfall with measured rainfall for the year 2002 and climatological results show that the method is capable of capturing the relevant mechanisms triggering rainfall in the area. For extreme dry (wet) years, rainfall is overestimated (underestimated) by the method and errors might occur even for normal years. However, application to a climatology calculated for the period 1958–1997 via analysing daily SLP fields from NCEP Reanalyses gives satisfactory results. For sufficiently long periods, the method is well capable of producing a reliable high-resolution rainfall climatology. We will therefore apply the method to climate change simulations in part II of this paper. Copyright  2007 Royal Meteorological Society

[1]  P. Knippertz.  Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa: Statistical Analysis and Seasonal Variations , 2003 .

[2]  J. Murphy,et al.  Simulation of climate change over europe using a nested regional‐climate model. II: Comparison of driving and regional model responses to a doubling of carbon dioxide , 1997 .

[3]  D. Heimann,et al.  An Improved Statistical-Dynamical Downscaling Scheme and its Application to the Alpine Precipitation Climatology , 2000 .

[4]  T. A. Buishand,et al.  COMPARISON OF CIRCULATION CLASSIFICATION SCHEMES FOR PREDICTING TEMPERATURE AND PRECIPITATION IN THE NETHERLANDS , 1997 .

[5]  C. Schär,et al.  Validation of present-day regional climate simulations over Europe: LAM simulations with observed boundary conditions , 1997 .

[6]  D. Funnell,et al.  Climate change in mountain regions: some possible consequences in the Moroccan High Atlas , 1999 .

[7]  P. Camberlin,et al.  Validation of a coupled GCM and projection of summer rainfall change over South Africa, using a statistical downscaling method , 2005 .

[8]  Jean Palutikof,et al.  Generating rainfall and temperature scenarios at multiple sites: Examples from the Mediterranean , 2002 .

[9]  S. Nicholson,et al.  THE RELATIONSHIP OF THE EL NIÑO–SOUTHERN OSCILLATION TO AFRICAN RAINFALL , 1997 .

[10]  P. Alpert,et al.  The Sharav Cyclone: Observations and some theoretical considerations , 1989 .

[11]  Jean Palutikof,et al.  Precipitation Scenarios over Iberia: A Comparison between Direct GCM Output and Different Downscaling Techniques , 2001 .

[12]  Peter Knippertz,et al.  Long-term precipitation variability in Morocco and the link to the large-scale circulation in recent and future climates , 2003 .

[13]  F. Molteni,et al.  Wintertime variability of Mediterranean precipitation and its links with large-scale circulation anomalies , 2001 .

[14]  R. Trigo,et al.  Circulation weather types and their influence on the precipitation regime in Portugal. , 2000 .

[15]  Xiaolan L. Wang,et al.  Downscaling GCM information to regional scales: a non-parametric multivariate regression approach , 1995 .

[16]  C. Goodess,et al.  DEVELOPMENT OF DAILY RAINFALL SCENARIOS FOR SOUTHEAST SPAIN USING A CIRCULATION-TYPE APPROACH TO DOWNSCALING , 1998 .

[17]  Mike Hulme,et al.  A COMPARISON OF LAMB CIRCULATION TYPES WITH AN OBJECTIVE CLASSIFICATION SCHEME , 1993 .

[18]  J. Corte-Real,et al.  Decadal changes in the link between El Niño and springtime North Atlantic oscillation and European–North African rainfall , 2003 .

[19]  Brent Yarnal,et al.  Developments and prospects in synoptic climatology , 2001 .

[20]  P. Alpert,et al.  Numerical experiments on the genesis of Sharav cyclones: idealized simulations , 1995 .

[21]  Mike Hulme,et al.  A comparison of Lamb circulation types with an objective classification derived from grid-point mean-sea-level pressure data , 1993 .

[22]  Dietrich Heimann,et al.  A model-based wind climatology of the eastern Adriatic coast , 2001 .

[23]  P. Knippertz,et al.  Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa , 2003 .

[24]  Yaping Shao,et al.  Effects of land-surface heterogeneity upon surface fluxes and turbulent conditions , 2001 .

[25]  C. Hoste,et al.  Rangeland production and annual rainfall relations in the Mediterranean Basin and in the African Sahelo Sudanian zone. , 1977 .

[26]  W. May,et al.  A time-slice experiment with the ECHAM4 AGCM at high resolution: the impact of horizontal resolution on annual mean climate change , 2001 .

[27]  W. Enke,et al.  A novel scheme to derive optimized circulation pattern classifications for downscaling and forecast purposes , 2005 .

[28]  B. Timbal.  Southwest Australia past and future rainfall trends , 2004 .

[29]  Michael Schmidt,et al.  Simulating evapotranspiration in a semi-arid environment , 2005 .