Our experiences with EnKF assimilation of high-res radar observations collected

in the 5 June 2009 Vortex 2 tornadic supercell.

Jim Marquis,Yvette Richardson, Paul Markowski, David Dowell,

Josh Wurman, Karen Kosiba, Paul RobinsonPhoto by Sean Waugh

Goals of EnKF analysis• Increase availability of 3-D kinematic and

thermodynamic data (dual-Doppler and in situ obs are spatially/temporally limited).

Courtesy of Paul Markowski

Goals of EnKF analysis• We want a set of smoothly evolving EnKF analyses

(i.e., We are not using EnKF to initialize a forecast).

• Analyze roles that mesocyclone-scale processes play in tornadogenesis, maintenance, and decay:

- trajectory analysis,- circulation/vorticity budgets,

- mid-upper level features.

Model Specifics• WRF-ARW 3.2.1:

-Δx,y = 500 m, 80m < Δz < 2km, [120 x 80 x 20] km3 -LFO microphysics,-open lateral BCs,-no surface fluxes, no radiation, flat terrain.

Homogeneous environment:

DA Specifics• DART:

– Ensemble adjustment filter,

– 50 members,– Localization:• Gaspari-Cohn, W = 0 @ r = 6 km

– Ensemble initiation:• 10 randomly placed warm bubbles at model t0 for each member

– Ensemble spread maintained with:• Additive noise added to T, Td, U, V every 5 min where radar

reflectivity is > 25 dBZ, (Dowell and Wicker 2009) • Perturbations smoothed to 4 km (horiz), 2 km (vert) scales

Courtesy www.vortex2.org

σ2obs = (2 m/s)2

• Radar velocities assimilated every 2 minutes

•

• OBAN: Cressman weighting

500 m horizontal grid spacing (for 500m-model grid experiment)

data along conical slices

Experiment timeline

“synthetic Data”

Thinning the amount of observations assimilated:

Unthinned 50% assim’ed 25% assim’ed

Model gridRadar vel. ob.

Y

X

No thinning done in vertical direction shown today; though, experiments show poor storm structure when low-level elevation angles are neglected.

θρdBZ ζW > 0

Z = 0.5 km AGL

(Note: these experiments conducted with 2-km model grid)

(K)

X

Y

50%

ass

imila

ted

25%

ass

imila

ted

u

nthi

nned

Y

ζradarsDu

al-D

oppl

erEn

KF

Wm/s

Z = 400 m AGL

Dual-Doppler – EnKF (ensemble mean) kinematics comparison(DOW6 & DOW7)

X (km)

Y (k

m)

Model errors/idealized conditions require DA for a good storm.

Storm structure with/without radar assimilation:

Top row: Series of EnKF (Ens. Mean) analyses.

Bottom row: Single member forecasted forward from 2157 (no DA).

ζ

Z = 150 m

X (km)

Y (k

m)

ζ

Ensemble mean analyses – temperature fields

Z = 50 m

(Note: these experiments conducted with 1-km model grid)

ζZ = 50 m

Mob. Mesonets

ζZ = 50 m

Mob. Mesonets

MM obs – EnKF outflow comparison

θ’(K)

ζ

Low-level wmax trace:dBZ MM obs:

Not great,particularly in farleft FF

Better overall, but still disagreement in FF

Decent, but stilldisagreement in FF

Warmth (lasts 1-2analysis times) - Seems to disagree With few available MM obs

Pretty good decent ok

MM’s only just getting into storm, but so far - not good

My < 1 sentence impression of the agreement of MM obs and eachEnKF analysis

Z = 50 m

• Sequence of EnKF analyses of theta’ and MM obs (valid +/- 1 min from EnKF analysis) overlayed:

ζZ = 50 m

Mesocyclone buoyancy (function of height & time)

Circulation and radial motion(function of radius & time)

Periods of tornado lifecycle

Pre-tornadic

T-genesisIntensification

MaturityWeakening

Time (UTC)

Inbound0 m/sOutbound

Positively buoyantNegatively buoyant

Trajectories (storm-rel.) calculated from ens. mean analyses

W (z = 200m)

Ring (radius = 1 km) of 20 parcels centered on peak ζ at z = 200 m; integrated forward in time from 4 times

θρ‘

(K)Most parcels rising into updraft

Some parcels rising into updraft

Few/no parcels rising into updraft

No parcels rising into updraft

θρ‘

(K)

Pre-tornadic T-genesis Intensification Maturity Weakening

Comparison across model grid resolutions

RMSI

Tota

l Sp

read

Cons

isten

cy

Rati

o

θ’(K)

Low-level wmax trace: MM obs:

ζ

Summary• EnKF Kinematic Analyses:

-Compare well with dual-Doppler fields of similar scale.-Storm-scale structure robust with different model scale.

More details come with finer model grid res.

• EnKF Thermo Analyses: - Mixed success with comparisons to MM in situ obs (where

available).- 500-m model grid seems verify with MM obs best in mesocyclone

area (partially due to more details captured?)- Seemingly random, possibly unphysical(?) anomalies appear more

prominently with finer model resolution.

Questions

1) Different way to maintain ensemble spread (adaptive methods)?

2) How else can we determine plausibility of analyses (kinematic and thermodynamic)?- Forcing terms along parcel trajectories?

3) More rigorous methods for optimizing scales/amounts of observations (compared to model resolution)?

Acknowledgements• The EnKF experiments were performed using NCAR CISL

supercomputing facilities with the Data Assimilation Research Testbed (DART) and WRF-ARW software.

• Thank you to Glen Romine, Lou Wicker, Dan Dawson, Chris Snyder, and Nancy Collins for advice/help.

• Thanks to all VORTEX2 crew for their dedication while collecting data on 5 June 2009.

• This research is funded by NSF grants: NSF-AGS-0801035, NSF-AGS-0801041. The DOW radars are NSF Lower Atmospheric Observing Facilities supported by NSF-AGS-0734001.