Retrospective analysis of NE Atlantic weather (especially storms) EXTROP Miami Workshop: Investigation of Tropical and Extra-tropical cyclones using passive and active microwave radar, Miami, Frauke Feser and Hans von Storch Institute for Coastal Research, GKSS, Germany

Outline … Regional climate modelling as a downscaling problem Determination of added value provided by downscaling with RCMs Applications of “reconstruction”

Climate = statistics of weather The genesis of climate C s = f(C l, Φ s ) with C l = larger scale climate C s = smaller scale climate Φ s = physiographic detail at smaller scale von Storch, H., 1999: The global and regional climate system. In: H. von Storch and G. Flöser: Anthropogenic Climate Change, Springer Verlag, ISBN , 3-36 Anthropogenic Climate Change  “downscaling”

The concept of downscaling does NOT imply that smaller scales are irrelevant for simulating the larger scales in atmosphere and ocean models Small scale processes, such as convection, play a key role in forming the global climate. However only the overall effect of these processes matters, not the space-time details. Therefore “parameterizations” of small scale processes are sufficient for such models. It is this fortuitous arrangement, which allows us simulate the global and continental climate well – even without simulating any regional climate adequately.

2d grid corresponding to a T42 spectral description

global model Well resolved Insufficiently resolved variance Problem: Global models and analyses do not resolve regional detail. Global climate models operate with grid sizes of often 200 km, and analyses with 100 km. Thus, they resolve spatial scales of, possibly, km km ? km Large … small spatial scales

Regional climate modelling as a downscaling problem von Storch, H., H. Langenberg and F. Feser, 2000: A spectral nudging technique for dynamical downscaling purposes. Mon. Wea. Rev. 128:

Dynamical downscaling mit LAM (Limited Area Model)

Concept of Dynamical Downscaling RCM Physiographic detail 3-d vector of state Known large scale state projection of full state on large-scale scale Large-scale (spectral) nudging

Spectral nudging vs. standard formulation

The genesis of regional climate is better framed as a downscaling problem and not as a boundary value problem. Spectral nudging constrains the dynamics on the “large” scales while “small” and “medium” scales remain unconstrained. “Solution” in the interior of a RCM domain is not determined by the boundary values. The problem is mathematically ill posed.

Weisse, pers. comm. January 1980-January 1997 Extreme value analysis of wind speed at platform K13 (southern North Sea) simulated observed

Model output vs radar and buoy measurements Lidia Gaslikova, pers. comm. Significant wave height Wave mean direction (coming from)

Determination of added value provided by downscaling with RCMs Feser, F., and H. von Storch, 2005: Spatial two-dimensional discrete filters for limited area model evaluation purposes. Mon. Wea Rev 133, Feser, F., 2006: Enhanced detectability of added value in limited area model results separated into different spatial scales, Monthly Weather Review (in press)

Spatial scale separation in 1 dimension Design of digital filters for discriminating different spatial scales. Expected added value in medium scales, if large scale sufficiently well described.

N=8 Low pass + medium pass filter Response functions of constructed 2d isotopic digital filters, constructed to approximate regions of unaffected “waves” and suppressed “waves”.

Example of a decomposition of a field into large- and medium scale contributions. Note that the two fields do not add up to the full field. Zonal wind component at a height of 10 m

Determination of added value Comparison with operational regional weather analysis prepared by DWD ( ) – representing supposedly the “truth”. NCEP is geostatistically interpolated to LAM grid (only 2m temp and SLP). Comparison on different spatial scales: large and medium scales. Ratio of temporal standard deviations Pattern correlation coefficients.

Ratio of 2m temperature st’ddev’s DJF (medium scales only) DWD/NCEP [%]DWD/REMO [%]

Pattern correlation coefficients [PCC, %] Positive values indicate an added value provided by the RCM. Significant improvements (rejection of null hypothesis of no added value; 5% risk) are marked by an asterisk *. RCM/sn = REMO with spectral nudging RCM/nn = REMO with lateal forcing only PCC of DWD (“truth”) and NCEP PCC change when RCM/sn is used PCC change if RCM/nn is used

Added value … … in medium scales. Medium scales are determined by both the large scale dynamics and the regional physiographic details (C s = f(C l, Φ s )) More added value with large-scale constraint (spectral nudging) Little improvement for SLP, which is a large- scale variable. Significant improvement for 2m temp, which is strongly affected by regional detail.

Applications of the reconstruction of regional weather Weisse, R., H. von Storch and F. Feser, 2005: Northeast Atlantic and North Sea storminess as simulated by a regional climate model and comparison with observations. J. Climate 18, Feser, F., R. Weisse and H. von Storch, 2001: Multidecadal atmospheric modelling for Europe yields multi-purpose data. EOS 82, Weisse, R. and A. Plüß, 2005: Storm related sea level variations along the North Sea Coast as simulated by a high-resolution model , Ocean Dynamics, DOI: /s y

General Strategy Dynamical downscaling to obtain high-resolution (50 km grid; 1 hourly) description of weather stream. Use of NCEP re-analysis allows reconstruction of regional weather in past decades ( ) Meteorological data are fed into dynamical models of weather- sensitive systems. Integration area used in GKSS reconstruction and regional scenarios

Applications Assessment of changing storminess Storm surges Ocean wave conditions Long-range pollution – examples: gasoline lead and benz-a-pyren “Commercial” applications (e.g., assessment of oil drifts in case of coastal accidents, assessment of fatigue in ships and off-shore constructions (with FGS Flensburg), Planning of harbor, costal defense and off-shore wind constructions)

Stormcount Weisse, pers. comm.  C/year t <  t > 

model estimate Estimated lead depositions into the Baltic Sea, compared to analyses based on observational evidence

Conclusions Regional models can be used to “reconstruct” detailed weather in the past decades by downscaling global re-analyses. In particular information about marine wind can be used for assessing past developments and contemporary risk assessments.

On Wednesday, three more (partly critical) presentations on related issues Matthias Zahn, Uni HH/GKSS: Case studies of polar lows Jörg Winterfeld, GKSS: Added value of wind fields from regional models Stig Wilkenskjeld, GKSS: Extreme wind waves from WAM