Arctic Surface DJF Biases in CAM3 Viewed As Forcing Fields

1. Arctic Surface DJF Biases in CAM3 Viewed As Forcing Fields Richard Grotjahn and Linlin Pan Dept. of LAWR, Univ. of California, Davis With assistance from Joe Tribbia, Grant Branstator and Andy Mai

2. Snapshot of CAM3 DJF Bias q = ln(Psfc) bias • T42 bias (CAM3 AMIP minus ERA-40) shown for U, V, T. (NCEP RAI used for q) • Bias has prominent values in NE Atlantic. • Over N Europe: SLP & T too low, ζ too high • Over Barents Sea/NW Russia region: opposite • Over Beaufort Sea: SLP too low • Representative level shown for U, V, & T U bias @ σ =0.5 V bias @ σ =0.5 T bias @ σ =0.5

3. Study Contributions to the Temperature bias equation: • T bias equation is CAM T equation minus ERA-40 T eqn. • = TC – TE . Where TC is CAM3, TE is era-40 DJF temperature • Term 1 has CAM bias and ERA-40 combinations found in a linear stationary wave (LSW) model such as Branstator (1990) • Term 2 (nonlinear bias terms) • Term 3 (all transient terms) • diabatic heating bias. QC & QE use independent eqn • T equation in LSW is Term 1 + forcing term

4. T bias equation terms @ σ=0.3, DJF LSW terms Nonlinear terms • For upper troposphere along each midlatitude storm track: • LSW terms mainly balanced by transient eddy fluxes (lower right) & diabatic heating (lower left) • Nonlinear terms NOT as important in T bias equation Diabatic from θ eqn residual Transient terms

5. T bias equation terms @ Sigma=0.5, DJF LSW terms • At mid & lower troposphere levels LSW terms mainly balanced by diabatic heating • Transient eddy heat fluxes bias also oriented along the track. • Consistent with well known storm track error: CAM sends storms into N. Europe instead of further N into Barents Sea. Diabatic from θ eqn residual Transient terms

6. Linear Model to Study CAM3 Bias • We treat CAM3 bias as if it is a forced linear stationary solution. • Use Linear Stationary Wave (LSW) model of Branstator (1990) • Use model to input bias field and find forcing. (Can input subset of bias) • Can input forcing and find solution: as check or to test subsets of the forcing. • Model severely limited to low resolution: R17 with 8 levels (R11 unaliased) • R11 bias shown; differs from T42; warm bias over N Canada and NW Russia much larger. BUT R11 has similar dipole in N Atlantic due to storm track error. • LSW tests find: Arctic bias linked to forcing in the local region of the Arctic AND to the N Atlantic storm track forcing. NOT linked to tropics. T bias @ σ=0.5 Vorticity bias @ σ=0.5 q=ln(Ps) bias

7. Conclusions • A temperature bias equation has been examined. • T bias eqn terms comprise 4 groups: • linear combinations of the T bias, • diabatic heating bias, • transient heat fluxes bias, • nonlinear (bias x bias) terms. • The diabatic heating and and transient eddy groups are prominent at latitudes higher than 30N (mainly in the midlatitude storm tracks) Nonlinear bias terms are small • The diabatic heating and transient heat fluxes are consistent with the well know storm track error in the N Atlantic. • The linear terms are included in a linear stationary wave (LSW) model, so such a model can be used to study the bias as if the bias were a stationary wave pattern. (The LSW model also includes divergence, vorticity, and log of surface pressure equations.) • LSW model severely limited by low resolution but previously we showed: • Bias in Arctic can be reproduced from forcing in Arctic and the N Atlantic. • Vorticity & T forcing have larger contribution than divergence and ln(Ps) forcing. • Future work may include testing the forcing in CAM from this T eqn analysis and from a corresponding vorticity eqn analysis.

8. The End • Thanks for your attention! • (Thanks Joe for giving this presentation!)

9. Storage below • Slides below may be useful if there are questions.

10. T bias equation terms @ Sigma=0.3, DJF LSW terms For upper troposphere along each midlatitude storm track: LSW terms mainly balanced by transient eddy fluxes (lower right) & diabatic heating (lower left) Nonlinear terms not as important in T equation at this level Nonlinear terms Transient terms Diabatic from θ eqn residual

11. T bias equation terms @ Sigma=0.5, DJF LSW terms At mid & lower troposphere levels LSW terms mainly balanced by diabatic heating Upper troposphere transient eddy fluxes at downstream end of storm track important Nonlinear terms not important in T equation Nonlinear terms Transient terms Diabatic from θ eqn residual

12. Bias in the heat flux components • The 3 components of the heat fluxes. • These include transient and stationary contributions. UT bias @ 500 mb VT bias @ 500 mb ωT bias @ 500 mb

13. Compare: term1 & LSW forcing σ=0.5 T42 R11 • Top row T bias at T42(left) and R11(right) • Bottom row: term1 at t42 (left) and LSW forcing (right) • LSW forcing excludes S of 30N. Cannot compare with tropics. • Truncation explains some differences: • R11 too large warmth in N Canada and NW Russia. (bias and consequent forcing) • R11 misses cool bias along US Canada border (hence forcing opposite) • R11 misses central Asia cool bias. • Prominent features shifted: • N. Atlantic (30W) has dipole in both with R11 about 1/2 of the T42 amplitude • Resolution explains some of the difference as judged from preliminary tests. R11 version of term1 being calculated, at this writing.