Basic characteristics of stratospheric predictability: Results from 1-month ensemble hindcast experiments for Masakazu Taguchi Aichi University of Education Kariya, JAPAN

Despite potential importance, stratospheric PR for days to months is relatively unexplored. Ref.: Baldwin and Dunkerton (2001) ■Suggestion Importance of stratosphere in extended weather forecasts ■Stratospheric PR for intraseasonal timescales Relatively unexplored ⇒ JMA 1-mo. ensemble HC data became available (thru MSJ). Intro

We use data from JMA 1-mo. hindcast (HC) experiments for JMA HC data Ref.: JMA （ 2010, 2011 ） ■System JMA 1-mo. EP system of March 2011 version Global model w/ TL159(Δx≈110km), L60 to 0.1hPa ■Initial conditions 10 th, 20 th, last day of each month for Perturbations in troposphere of NH and tropics 1-month ensemble predictions with N=5 ■Boundary conditions Assume persistent SST anomalies

We use some basic quantities and measures to examine stratospheric PR. Quantities and measures Ref.: None ■JRA-25/JCDAS reanalysis and HC data A JRA, A HC,n, A HC,EM where A= [U], Z10, Z500 EM: ensemble mean, [ ]: zonal mean ■Measures to evaluate HC data for Z10 or Z500 (Φ≥20N) Root of spatial mean of error 2 of EM from JRA Spatial mean of spread at each gridpoint i: index for space i = 1, 2, …, I n: ensemble n =1, 2, …, N N = 5

Winter stratosphere has larger average and variability in RMSE, with longer timescale. RMSE Normalized RMSE of Z, poleward of 20N. 10hPa 500hPa Ref.: Black lines denote SD of interannual variability of JRA monthly Z for Φ≥20N. PL Results for each year Mean wrt year SD= 59.1 SD=362.5SD= 63.6 SD= 33.7 Normalized RMSE

PL is longer on average and has larger variability at 10hPa (in winter) than at 500hPa. PL Ref.: PL is defined when RMSE exceeds 1xSD poleward of 20N. Bin widths differ between 10 and 500 hPa. PDFs of PL (Predictable Limit) 10hPa 500hPa Will be meaningless because RMSE is very small. 15.8± ±4.1 MN±SD=5.5±1.14.1±0.7

Both average increase and variability in RMSE Z10 become large around 15 days. RMSE Ref.: None. Time variations of RMSE of Z10 (Φ ≧ 20N) INITIAL=0110 MN±1xSD of RMSE RMSE for each year

SPREAD&RMSE of Z10 are positively correlated, with large skewness of RMSE (clear outliers). SPREAD-RMSE Ref.: This figure uses time means of SPREAD/RMSE for t=15±3 d, after 11/10-02/10. Scatter plot between SPREAD and RMSE of Z10 Additional lines Best-fit line At 60N, 10hPa, Δ[U] ≥ +15 m/s Δ[U] ≤ -5 m/s Δ = EM of HC – JRA. SPREAD (m) RMSE (m) R=+0.46 FREQ. (%) FREQ. (%)

SPREAD&RMSE of Z500 are positively correlated, with less marked outliers. SPREAD-RMSE Ref.: This figure uses time means of SPREAD/RMSE for t=8±3 d, after 11/10-02/10. Scatter plot between SPREAD and RMSE of Z500 At 60N, 10hPa, Δ[U] ≥ +5 m/s Δ[U] ≤ -5 m/s Δ = EM of HC – JRA. SPREAD (m) RMSE (m) R=+0.41 Best-fit line FREQ. (%) FREQ. (%)

PDFs of RMSE and DISTANCE (to best-fit lines) are more highly skewed at 10hPa. RMSE and DISTANCE Ref.: Distance is measured from each data point perpendicularly to the best-fit line. PDFs of RMSE and DISTANCE at 10/500hPa (Normalized) 10hPa 500hPa 10hPa 500hPa LevelSkew. 10hPa hPa0.62 LevelSkew. 10hPa hPa0.64

In the following, we look at meteorological conditions for a few outliers. SPREAD-RMSE Ref.: This figure uses time means of SPREAD/RMSE for t=15±3 d, after 11/10-02/10. Scatter plot between SPREAD and RMSE of Z10 SPREAD (m) RMSE (m) (4) (5)

HC data (#1) completely miss occurrence of vortex split MSSW in Jan., #1 (INIT= ) Ref.: Color shades denote Z10. Black contours denote anomalies of JRA from climatology, or error of HC EM from JRA. SPREAD Z10 (m) RMSE Z10 (m) INITIAL+1 wk+2 wks HC JRA Z10 JRA HC EM 1 (4) 2006 (5) /01/10 ↓

HC data (#2) fail to reproduce horizontal vortex structure in recovery after mSSW in ND, #2 (INIT= ) Ref.: Color shades denote Z10. Black contours denote anomalies of JRA from climatology, or error of HC EM from JRA. SPREAD Z10 (m) RMSE Z10 (m) INITIAL+1 wk+2 wks HC JRA Z10 JRA HC EM /11/30 ↓

HC data (#3) fail to reproduce horizontal vortex structure during MSSW in January, #3 (INIT= ) Ref.: Color shades denote Z10. Black contours denote anomalies of JRA from climatology, or error of HC EM from JRA. SPREAD Z10 (m) RMSE Z10 (m) INITIAL+1 wk+2 wks HC JRA Z10 JRA HC EM /01/10 ↓

Using JMA HC data, this study reveals following characteristics of stratospheric PR for NH winter. Summary Ref.: None ■SPREAD and RMSE Larger average and variability ⇒ Longer average (≈2 wks) and larger variability of PL ■SPREAD-RMSE (skill) relationship ◇ Positive correlation ◇ Large skewness in RMSE, or outliers from the relationship Contributed by SSWs in onset and also recovery phases ⇒ Suggest importance and difficulty of good SSW predictions in extended weather forecasts

Back-ups

The climatological bias of HC data from JRA reanalysis is generally small. Bias Ref.: Initial dates is 1210 or 0610, and t=34 d (last day of predictions). Dots denote 95% significance in two-tail test, i.e., HC clim differs from JRA clim. Clim. bias of [U] (m/s) in mid-January/July

Winter stratosphere has large spread on average. Yr-to-yr (case-to-case) variability is also large. SPREAD SPREAD of Z (m) poleward of 20N. Ref.: Cyan lines denote results for each year. Blue lines denote the mean for all years. 10hPa 500hPa

HC data (#4) fail to reproduce the onset of the MSSW in January, #4 ( ) Ref.: Color shades denote Z10. Black contours denote anomalies of JRA from climatology, or error of HC EM from JRA. SPREAD Z10 (m) RMSE Z10 (m) INITIAL+1 wk+2 wks HC JRA Z10 JRA HC EM

HC data (#5) mostly fail to reproduce the onset of the MSSW in Dec./Jan., 1984/85. #5 ( ) Ref.: Color shades denote Z10. Black contours denote anomalies of JRA from climatology, or error of HC EM from JRA. SPREAD Z10 (m) RMSE Z10 (m) INITIAL+1 wk+2 wks HC JRA Z10 JRA HC EM