**Office:**

** Geological Museum 452
24 Oxford Street
Cambridge, MA 02138**

**Tele** : (617) 495-2998

**Fax **: (802) 329-2152

**E-mail** : farrell@seas.harvard.edu

Professor Farrell's primary interest is in the field of dynamic meteorology.
Dynamic meteorologists apply the principles of physics and mathematics to gain
fundamental understanding of the operation of the atmosphere. A central problem
in dynamic meteorology that Professor Farrell has been particularly involved in
is explaining the origin of the midlatitude cyclone
which is responsible for producing the high and low pressure regions that give
rise to much of the variation in mid-latitude weather. The methods used in this
study exploit the high degree of non-normality of the dynamical system
underlying these processes. Theoretical insights gained from this work have
also been applied to understanding tropical cyclones as well as the
fascinating, if less known, polar lows of the arctic night.

At the present time, Professor Farrell and co-workers are applying the methods
of stochastic analysis of non-normal dynamical systems to study the statistical
properties of the climate system including those processes that control the
organization of storm tracks and the transfer of heat and momentum from
subtropical regions to higher latitudes. Understanding these fundamental
processes regulating the statistical properties of earth's climate give insight
into what changes in the global climate will result from man's intervention in
the climate system.

A specific area of current study in dynamic meteorology at Harvard is the
observed extreme sensitivity of regional climate to small perturbations of the
atmosphere as was seen in the El Nino related floods in the southwest in 1982,
1992 and 1998. By studying the equations governing the statistics of the
weather we are learning how to make specific predictions of climate sensitivity
building on what we have learned from studying the predictability of individual
cyclones.

Improving the accuracy of weather forecasts is the goal of another project being pursued presently by Farrell and co-workers. This work proceeds from a long term effort to formulate a generalized stability theory that takes account of nonmodal growth perturbation growth processes as well as of asymptotic modal growth mechanisms. A current focus is on the stability of nonautonomous (time dependent) linear dynamical systems. These are of particular interest because the growth of small errors in a forecast model is governed by a time dependent linear operator, the tangent linear operator about the nonlinear forecast trajectory in its state space.

Generalized stability theory has been used by Farrell and co-workers to produce a theory for turbulence both at synoptic scale in the atmosphere and in boundary layer flows. This turbulence theory is based on the dominance of nonmodal interactions between the perturbations and the background flow in the turbulence dynamics of highly sheared flows. This theory has been used to find active control methods for preventing transition to turbulence in boundary layer flows.