Brian Farrell
Robert P. Burden Professor of Meteorology and Member of Department of Earth & Planetary Sciences
Harvard University

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.

Farrell, B.F., ``Equable Climate Dynamics,'' J. Atmos. Sci., 47, 2986-2995 (1990).

Montgomery, M. B., and B. F. Farrell, ``Polar low dynamics in a three-dimensional moist geostrophic momentum model,'' J. Atmos. Sci., 49, 2484-2505 (1993).

M.B. Montgomery and B.F. Farrell, ``Tropical Cyclone Formation,'' J. Atmos. Sci., 50, 285-310 (1993).

Farrell, B.F. and Ioannou, P.J., ``A theory for the statistical equilibrium energy spectrum and heat flux produced by transient baroclinic waves,'' J. Atmos. Sci., 51, 2685-2698 (1994).

Farrell, B.F. and Ioannou P.J., ``Stochastic dynamics of the mid-latitude atmospheric jet,'' J. Atmos. Sci., , 1642-1656 (1995).

DelSole, T.M. and Farrell, B.F., ``A stochastically excited linear system as a model for quasigeostrophic turbulence: analytic results for one and two-layer fluids,'' J. Atmos. Sci.52, 2531-2542 (1995).

Farrell, B.F. and Ioannou, P.J., ``Turbulence suppression by active control,'' Phys. Fluids., 8, 1257-1268 (1996).

Farrell, B.F. and Ioannou, P.J., ``Generalized stability theory part I: autonomous operators,'' J. Atmos. Sci., , 2025-2040 (1996).

Farrell, B.F. and Ioannou, P.J., ``Generalized stability theory part II: non-autonomous operators,'' J. Atmos. Sci., , 2041-2053 (1966).

B.S., 1973, Geophysics, Massachusetts Institute of Technology Ph.D., 1981, Applied Mathematics/Meteorology, Harvard University