Arctic observing system for regional NWP Harald Schyberg (met.no), Frank Thomas Tveter (met.no) Roger Randriamampianina (met.no), Trygve Aspelien (met.no) (h.schyberg@met.no)

Outline Introduction Observing system and impact studies Ongoing work on improving satellite data usage in high-latitude Results are from projects EUCOS space-terrestrial network studies Norwegian IPY-THORPEX project DAMOCLES (EU FP7 project)

Operational NWP LAM modeling – some characteristics Should add information to products from global modeling centres (ECMWF, …) Higher resolution More frequent data assimilation, shorter cutoff times (some satellite data types arrive late)  Forecasts from a given analysis time available before those from global model

Numerical Weather Prediciton in the Arctic NWP quality in general determined by: Quality of model formulation Physics of processes in the Arctic Quality of boundary forcing In particular lower boundary: Ice/snow/ocean: heat and moisture flux, momentum flux, radiative fluxes For LAMs: Lateral boundaries Quality of initial state estimate As determined in data assimilation – issues of observation coverage/usage in the Arctic

WMO workshop on impact of obs systems, 2008: Synopsis from many studies using “observing system experiments” Figure shows impact of observing systems in terms of gain in forecast range for “medium range” forecasts

Regional experiments – do we get consistent results with this picture? Two approaches in anaysing data denial impact studies: Statistical, long term Case studies No weighting to favor high-impact weather Statistical significance?

EUCOS impact studies Two questions in these “Space-terrestrial studies”: In the presence of the full conventional observing system, what is the impact of adding the various satellites In the presence of the full satellite observing system, what it the impact of the various components of the conventional observing system Met.no (and several other global and regional centres) participated in the second part funded by EUCOS

EUCOS impact trials at met.no Use of HIRLAM 3D-Var Winter period 2004-05 and summer period 2005

Results- All scenarios (more details later) Control Scenario (all available in-situ observations) Baseline scenario Add E-ASAPs Add AIREPs

Results from met.no HIRLAM OSEs in EUCOS terrestrial network study Conventional observations have large positive impact in our system in the prescence of satellite data Probably higher impact than in ECMWF system (we use less satellite obervation types) TEMPs still dominating factor for analysis quality, wind more than temperature Aircraft obs complement radiosondes (positive impact of adding aircraft in the presence of sondes) Significant positive impact from E-ASAP network

The Arctic observing system for regional NWP An observation gap in the conventional observing network: Lack of radiosondes Lack of surface observations, but some buoys, more during IPY The routinely available Arctic Ocean observing system consists basically of buoys and satellite data AMSU, AIRS/IASI radiances and MODIS winds Most of the satellite data are free atmosphere, surface/lower troposphere info lacking The total observing system in the Arctic Ocean: Lacks surface/near surface observations Relies on efficient use of satellite data in assimilation schemes Use of satellite sounding data is less straightforward over sea ice and land than over ocean  Met.no focus on including IASI and on use of surface affected AMSU measurements

Challenges for Arctic data assimilation: Radiosonde observation coverage ff

Challenges for Arctic data assimilation: aricraft obs coverage

Challenges for Arctic data assimilation: SYNOP surface obs coverage

Challenges for Arctic data assimilation: buoy surface obs coverage

Coverage from some satellite observation types

The Norwegian IPY-THORPEX project: Assimilation of extra campaign data and IASI observations (R.Randriamampianina, T. Aspelien) Observing system experiments during the Norwegian THORPEX campaign (several polar lows): After implementing and optimizing IASI assimilation: What is the impact of assimilating IASI data (Norwegian version of HARMONIE NWP system) What is the impact of assimilating campaign data: Extra radiosonde launches from Norwegian and Russian Arctic stations, Norwegian coast guard vessels, dropsondes, … (HARMONIE and LAMEPS)

HARMONIE and its assimilation system (Hirlam Aladin Regional/Meso-scale Operational NWP In Europe) Model domain: rotated Lambert pr. Dx=dy= 11 km, 60 vertical levels up to 0.2 hPa 3D-Var assimilation system Use of conventional and satellite data Obs operator for radiance data: RTTOV- 8.7

Exploring the use of IASI during the Norwegian IPY-THORPEX campaign period IASI has 8461 channels, of which we extract 366 for potential use. Four experiments have been performed using 41 active channels Period: 2008022000 – 2008031712 (Warming period 5 days)

Impact as function of forecast range over the period Left effect of IASI with campaign data, right without campaign data Verification against ECMWF analyses

A forecast sensitivity measure to various observations during the campaign period:

Polar low case 16-17 March 2008 +24 hrs forecasts of pressure and precip IASI, Caobs No IASI, Caobs No IASI, no Caobs IASI, no Caobs

3 March 2008: Probabilistic variables (Wind, + 51 hrs) wind > 15 m/s Green= With campaign Red=Without Campaign data

Probabilistic variables (Precipitation + 51 hrs) Green: Regular + Campaign Obs rr > 1 mm/3hr Red: Regular Obs rr > 1 mm/3hr

Developments towards using surface affected AMSU-A data over sea ice

AMSU-A channels over sea ice We simulate observations from NWP fields using radiative transfer model RTTOV-8 (“B”) and compares against the real observations (“O”) When using fixed emissivity and NWP surface temperatures, typical O-B rms magnitudes over sea ice are: Ch 3 ~5K Ch 4 ~3K Ch 5 ~2K Ch 6-9 ~ 0.5K Previously ch 6-10 has been used over sea ice. Can we improve the use of ch 6-7 and add lower peaking channels? How to define emissivity and surface temperature?

3 issues in using surface-affected microwave sounding (AMSU-A) over sea ice The sea ice emissivity Accounting for penetration depth of the radiation and vertical temperature gradient within the sea ice Getting a realistic (first guess of) surface temperature to RTTOV

Emissivity: Help using data from Ocean and Sea ice SAF High-latitude centre: Hosted by Met.no and DMI Daily sea ice products: Concentration, edge, type etc on 10 km grid Will also include info on sea ice microwave emissivity in future See www.osi-saf.org and saf.met.no

Emissivity (1) Use daily OSISAF concentration chart to find near-100% ice covered area In this area multi- year sea ice from OSISAF was used as predictor for sounding ch emissivity

Emissivity (2) An alternative approach is to estimate the emissivity from a window AMSU-A channel using simplified radiative transfer theory leading to:  = (Tb obs – Tb sim(=0)) / (Tb sim (=1)-Tb sim(=0)) One could then use the assumption that emissivity varies slowly with frequency and also apply it for other channels Statistical analysis showed that using the MY ice map from SSM/I gave a better fit between modeled and observed data

Handling of surface emitting temperature (figures from R.Tonboe) Variations in penetration depth, increasing temperature with ice depth: The colder, the more misrepresentative is the surface temperature for the emitting layer

Example: O-B statistics ch 4 using RTTOV-8 over sea ice Right panel: Departures as a function of ground temperature for first-year sea ice Left panel: The same for multi-year sea ice The colder, the more misrepresentative is the surface temperature for the emitting layer a) 0-10% multiyear b) 70-80% multiyear

Implementation in variational data assimilation J(x) = ½ (x-xb)T B-1 (x-xb) + ½ (y-H(x))T O-1 (y-H(x)) Multiyear ice fraction based surface emissivity dependence included in the observation operator H(x) Variational bias correction: A linear correction extension added to H(X) for handling dependence of emitting temperature on surface temperature Control variable x extended with slope coefficient for this dependence An optimal dependence on surface temperature is determined intrisically, constrained by other information available in the analysis

Setup of NWP assimilation experiment over sea ice, using HIRLAM 4D-Var Reference experiment: no variational bias correction, constant sea ice emissivity, emitting temperature equal to surface temp, channels 6 and 7 Experiment 1: variational bias correction, channels 6 and 7 Experiment 2: variational bias correction, channels 5,6 and 7

Impact study of changing the surface handling using channels 6 and 7 in HIRLAM 3D-Var Small effects on”standard” surface verification averaged over the whole European network Some positive effect seen on profile verification against radiosondes Problem of lack of verifying obs near sea ice areas Red: Fixed sea ice emissivity and surface temperature from HIRLAM Blue: Emissivity estimation based on OSISAF products and surface temperature in the VarBC control variable

Problem of modeling surface temperatures over sea ice: Several present NWP system utilizes digital ice concentration maps from passive microwave (SSM/I, SSMIS, AMSR) for instance OSISAF Difficult for low-resolution satellite data like SSM/I to detect small areas of open water within the sea ice But the effect of such areas on surface fluxes very important Present passive microwave concentration algorithms show small, but unrealistic, concentration fluctuations over closed sea ice This also affects present assimilation of AMSU data

Typical example: Fraction of ice used in operational HIRLAM NWP model (April 2010) Ice concentrations above 95% set to 100%, but still some unrealistic values remaining between 90 and 95%

Example: Corresponding 2m temperature Temperatures typically in the range from -10 to - 30

Example: Corresponding difference T (2m) – T (0m) Erroneous small water areas creates large differences (and erroneously large heat fluxes) Also a problem for use of satellite sounding data No straightforward solution to this

Summary Gaps in the Arctic observation network: Particularly in lower troposphere A potential for improvement with conventional observations even in precense of satellite. Profile info most valuable. A good coverage of satellite data of the Arctic will be available for the foreseeable future: It is probably cost-effective to exploit the still unrealized potential there Particular issues on surface description for satellite observation usage over sea ice

Thank you …

Backup slides follow

Challenges for NWP in the Arctic application Turbulence modeling Stable boundary layer Unstable boundary layer Surface fluxes over sea ice Heat Radiative Momentum Routine observing system is basically satellite Utilization of satellite data over sea ice: surface properties, cloud detection, moisture/cloud assimilation Arctic cloud microphysics parametrization Leads, meltponds, albedo... Roughness

Some main paths from observations to products for numerical atmospheric modeling Focus of this talk Arctic atmospheric observations Process studies Data assimilation Improved NWP model parametrizations Reanalysis Analysis Modeling of future climate Monitoring of past and present climate Short/medium range operational NWP

Scatter in O-B can be further reduced by introducing surface temperature dependence Physical basis described by Mathew et al, 2008 Leads to empirical expression: Temitting = aT2m + b

Can this be dealt with through linear correction (“bias correction”)? Linear dependence of observed brightness temperature on Ts But: slope is of Tb vs Ts slightly dependent on ice characteristics such as type need to simultaneously include multi- year ice fraction dependence Have chosen to implement this using variational bias correction

Some targets for atmospheric process studies Turbulence modeling Stable boundary layer Unstable boundary layer Surface fluxes over sea ice Leads, meltponds, albedo... Heat Radiative Momentum (Roughness) Arctic cloud microphysics parametrization Should improve numerical modeling capability of the Arctic atmosphere  NWP skills, reanalysis quality, climate modeling capability

EUCOS Impact Scenarios at met.no with HIRLAM Winter period: 14 December 2004 – 20 January 2005 Summer period: 1 August – 31 August 2005 6 hours cycling, forecasts up to +48hrs from 00Z cycle stored Baseline scenario: All available satellite observations: AMSU-A over ocean (EUMETSAT retransmission) QuikScat winds (100 km product) MSG cloud drift winds (AMV/SATOBs) SYNOPS from GSN station list Radiosondes from GUAN climate station network. Bouys (no ship data)

Role of the in situ observation network Hard to see an increase in the Arctic conventional network for day-to-day NWP Observation campaigns expected to be important Process studies which feed back to model physics Validation studies – identify NWP model problem areas Validation/calibration of satellite observing system

Some more conclusions Many recent studies shows clear positive impact of adding sounding data and improving use of satellite data: A potential for improving forecasting and reanalysis capability A main contributor to the Arctic observing system for data assimilation will be satellite data Surface observations missing in the Arctic Improvements in accounting for the sea ice contribution to satellite measurements Improvements in describing boundary layer processes over sea ice Future progress in Arctic atmospheric modeling will take place on the interface between expertise on sea ice, data assimilation and atmospheric physical processes