Can We Predict the Fall Turnaround in Zonal Wind over Vanscoy?

D. Wunch, M. Tingley, G. W. K. Moore, T. G. Shepherd, D. Sankey, K. Strong, J. R. Drummond

University of Toronto

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Outline

• Definition of Turnaround• Motivation for this Work• Methodology• Simple Historical Statistical Prediction Methods

• scoring

• fitting summer winds

• Looking for Memory in the System• autocorrelations

• cross-correlations

• Summary

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What is Zonal Wind Turnaround?

• Turnaround occurs when the zonal winds change direction: that is, when the vertical shear in zonal wind is zero.

• Turnaround is driven by temperature and is largely described by the thermal wind equation.

• Turnaround occurs twice per year: in March/April and August.

• The focus of this work has been to investigate the turnaround event in the Northern Hemisphere Fall, where the stratospheric winds change from easterly to westerly.

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Motivation for this Work

• The Middle Atmosphere Nitrogen TRend Assessment (MANTRA) Campaign consists of high-altitude balloon flights during which a suite of instruments are lifted to 40-km and atmospheric gas measurements are taken.

• The stratospheric winds during a high-altitude balloon campaign need to be very low for three reasons:

1. Launch2. Telemetry3. Recovery

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Motivation Continued

• We would like to know when turnaround will occur over our launch site, Vanscoy, Saskatchewan (52°N, 107°W), within a couple of days, at least 30 days in advance.

• This will allow us to prepare our field campaign• determine arrival dates at the field site

• allow the appropriate amount of time to prepare our instruments and the payload

• give us the opportunity to launch a sufficient number of sondes for flight validation

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What Happens when the Launch Misses Turnaround: The Flight of the MANTRA

1998 Balloon

The day following launch

The two weeks following launch

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105.5106106.5107107.5108108.5

LONGITUDE (degrees East)

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Balloon Launch

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Methodology

• We used NCEP Reanalysis and CMAM data to search for predictors for Fall turnaround over Vanscoy.

• First, simple historical trending was used to determine the most likely date for turnaround to occur.

• Second, we attempted to find measurable predictors that have a high correlation with turnaround at Vanscoy.

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0 50 100 150 200 250 300 350-40

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Julian Day

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Mean Zonal Wind Velocity over Vanscoy for 19 years of CMAM Data

Mean Zonal WindMaximum/Minimum Windspeed

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NCEP Reanalysis Data

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Two Simple Predictions of Turnaround Date

This work uses the NCEP reanalysis data.

1. For each year, the days corresponding to acceptably low stratospheric zonal wind speed were recorded, and a score was generated for each day of the year.

• The results give a predicted turnaround day of Julian day 240 (August 28th) with a standard deviation of almost 6 days.

2. A polynomial was fit to the region of low variability in the NH summertime, and the maximum easterly value of the zonal winds was correlated with the minimum wind value.

• No correlation was found.

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NCEP Reanalysis Data

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50 100 150 200 250 300 350

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Julian Day

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Zonal Wind Velocity over Vanscoy

NCEP Reanalysis Data

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175 180 185 190 195232

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Day of the Year for Minimum Zonal Wind

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Relation between Minimum Zonal Wind and Turnaround

y = -0.065866x + 253.9305

mean = 241.6774, = 3.3205

NCEP Reanalysis Data

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Looking for Memory in the System• We have narrowed down the predicted date of turnaround,

based on the previous historical trends.• However, we have not found an acceptably accurate prediction

of when turnaround will occur in a given year.• We are looking for measurable predictors that will give

information about when Fall turnaround will occur.• We sought correlations to predict the zonal winds over

Vanscoy at 10mb.• Autocorrelations of the zonal winds at Vanscoy.• Autocorrelations of the mean zonal wind at 52°N.• Cross-correlations of the mean zonal wind to the zonal wind over

Vanscoy.• Cross-correlations of the NAM (Northern Hemisphere Annular Mode)

Index to the zonal winds over Vanscoy.• Cross-correlations of the mean zonal winds at 62.5°N to the zonal

winds over Vanscoy.

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Summary - Can We Predict the Fall Turnaround in Zonal Wind over

Vanscoy?

• Not yet.• We can narrow down the turnaround date to day

241 (August 29th), with a standard deviation of just more than 3 days.

• There seems (so far) to be little memory in the system that extends to the fall turnaround time.

• We will continue to look for other possible correlations.