The seasonal cycle of gravity wave drag in the middle atmosphere

PULIDO M; THUBURN J.

Exeter (UK)

Conferencia; Royal Meteorological Society Conference; 2005

Royal Meteorological Society

A technique based on four-dimensional varitional data assimilation is used to estimate gravity wave drag (GWD). Using an adjoint model of a middle atmosphere dynamical model, a cost function, that measures the differences between the model state and observations, is minimised. The only control variables are the horizontal components of GWD, therefore the minimisation determines the optimal drag.In this work a GWD estimation with the variational technique is performed using UKMO middle atmosphere analyses as observations for the year 2002. For each month, UKMO analyses are taken as the initial condition and also as observations, for each successive day. In this way we obtain a  three dimensional GWD field for all the year with a temporal resolution of one day. GWD shows a pronounced seasonal cycle. During solstices, there are  deceleration regions of the polar jet at the winter hemisphere, the summer hemisphere also presents a deceleration region but much weaker. During equinoxes, GWD is weak and represents the transition (due to the small GWD estimates, the technique does not appear to have a robust estimation during equinoxes contrary to solstice estimates).  The predominant features are similar to previous estimations with other techniques. We also find important interhemispheric differences, the deceleration center in winter at the Southern hemisphere (with a peak of $50 m/s/day$) is much stronger than the Northern hemisphere one (with a peak of $20 m/s/day$). The causes of the differences are discussed.