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Peter Siegmund

Abstract

The skill of stratospheric and tropospheric predictors in predicting near-surface quantities at the extended range (∼10 days–2 months) has been investigated, using 40 yr of reanalysis data from the European Centre for Medium-Range Weather Forecasts. The predictors are 1) the geopotential height (Z) at various levels, 2) the difference between Z and the 1000-hPa geopotential [ZZ(1000)], and 3) the temperature at various levels. The predictors are averages over the area north of 65°N. The predictands are Z(1000) averaged over the same area and geographical fields of several near-surface quantities. The predictive skill has been investigated for different lead times between predictor and predictand and different averaging periods of the predictor and the predictand.

The results show that the predictive skill of Z in the troposphere is mainly due to the predictive skill of sea level pressure, whereas the predictive skill of Z in the stratosphere is mainly due to the predictive skill of stratospheric temperature. The predictive skill is largest in the end of December, for the predictor Z at 50 hPa and the temperature between 250 and 50 hPa. The temperature also has significant predictive skill in the upper stratosphere in the summer. In winter, for lead times larger than 5 days the stratospheric Z is a better predictor of the daily Z(1000) than Z(1000) itself. Whereas the predictive skill of the stratospheric Z is largest for zero lead time, the predictive skill of the stratospheric ZZ(1000) and temperature are largest for lead times of about 10 days, evidencing the finite propagation time of geopotential anomalies from the stratosphere to the surface. The skill of the stratospheric height and temperature in predicting the wintertime monthly mean field of Z(1000) is mainly limited to the region north of 60°N. The stratospheric predictive skill for the monthly mean fields of the zonal wind at 850 hPa and the near-surface temperature is particularly large around 60°N. The correlation pattern of the near-surface temperature field and the stratospheric temperature is qualitatively similar to the corresponding pattern for the Arctic Oscillation index, except at middle latitudes over Eurasia and over the subtropical Pacific.

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Peter Siegmund

Abstract

The cloud diabatic forcing (CDF) of the atmosphere, defined as the difference between the diabatic heating in average and in clear-sky conditions is estimated from time series of simultaneous observations of diabatic heating and cloud amount. The beating is evaluated by the residual method using four-time-daily ECMWF(European Centre for Medium-Range Weather Forecasts) initialized analyses for three January and three July months. The cloud data are obtained from the International Satellite Cloud Climatology Project (ISCCP).

The CDF is dominated by release of latent heat in the storm track regions and the intertropical convergence zone (ITCZ). Over the summer continents the CDF is weakly negative, due to the decrease of sensible heating with increasing cloud amount. The error in the CDF due to random errors in the daily heating varies from less than 10 W m−2 over subtropical continents to more than 30 W m−2 over storm track regions and parts of the ITCZ. In the tropical high atmosphere (above 440 hPa) the estimated CDF is most probably too small.

The accuracy of the time-average heating is determined by comparing the distributions of the net source of atmospheric total energy (Q tot ≡ diabatic heating plus net source of latent energy) with simultaneous observations of the net radiation at the top of the atmosphere (R), obtained from the Earth Radiation Budget Experiment (ERBE). Since for time-mean conditions over land the flux of heat into the earth's surface, represented by RQ, is very small, over land the difference between R and the estimated Q tot is a measure for the error in R and Q tot. The value of the three-monthly average of RQ tot over large continental areas varies from 4 W m−2 over Europe in July to −56 W m−2 over Canada in January. Locally over land the value of the three-monthly average RQ tot is generally between 25 and 50 W m−2.

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Peter Siegmund, Henk Eskes, and Peter van Velthoven

Abstract

The ozone budget in the Antarctic region during the stratospheric warming in 2002 is studied, using ozone analyses from the Royal Netherlands Meteorological Institute (KNMI) ozone-transport and assimilation model called TM3DAM. The results show a strong poleward ozone mass flux during this event south of 45°S between about 20 and 40 hPa, which is about 5 times as large as the ozone flux in 2001 and 2000, and is dominated by eddy transport. Above 10 hPa, there exists a partially compensating equatorward ozone flux, which is dominated by the mean meridional circulation. During this event, not only the ozone column but also the ozone depletion rate in the Antarctic region, computed as a residual from the total ozone tendency and the ozone mass flux into this region, is large. The September–October integrated ozone depletion in 2002 is similar to that in 2000 and larger than that in 2001. Simulations for September 2002 with and without ozone assimilation and parameterized ozone chemistry indicate that the parameterized ozone chemistry alone is able to produce the evolution of the ozone layer in the Antarctic region in agreement with observations. A comparison of the ozone loss directly computed from the model’s chemistry parameterization with the residual ozone loss in a simulation with parameterized chemistry but without ozone assimilation shows that the numerical error in the residual ozone loss is small.

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