A Lagrangian analysis of a developing and non-developing disturbance observed during the PREDICT experiment Naval Postgraduate School, Monterey, CA, USA
03 Dec 2012
Received: 01 Sep 2011 – Published in Atmos. Chem. Phys. Discuss.: 19 Dec 2011Abstract. The problem of tropical cyclone formation requires among other
things an improved understanding of recirculating flow regions on
sub-synoptic scales in a time evolving flow with typically sparse real-time
data. This recirculation problem has previously been approached assuming as a
first approximation both a layer-wise two-dimensional and nearly steady flow
in a co-moving frame with the parent tropical wave or disturbance. This paper
provides an introduction of Lagrangian techniques for locating flow
boundaries that encompass regions of recirculation in time-dependent flows
that relax the steady flow approximation.
Revised: 10 Aug 2012 – Accepted: 14 Nov 2012 – Published: 03 Dec 2012
Lagrangian methods detect recirculating regions from time-dependent data and
offer a more complete methodology than the approximate steady framework. The
Lagrangian reference frame follows particle trajectories so that flow
boundaries which constrain particle transport can be viewed in a
frame-independent setting. Finite-time Lagrangian scalar field methods from
dynamical systems theory offer a way to compute boundaries from grids of
particles seeded in and near a disturbance.
The methods are applied to both a developing and non-developing disturbance
observed during the recent pre-depression investigation of cloud systems in
the tropics (PREDICT) experiment. The data for this analysis is derived from
global forecast model output that assimilated the dropsonde observations as
they were being collected by research aircraft. Since Lagrangian methods
require trajectory integrations, we address some practical issues of using
Lagrangian methods in the tropical cyclogenesis problem. Lagrangian
diagnostics are used to evaluate the previously hypothesized import of dry
air into ex-Gaston, which did not re-develop into a tropical cyclone, and the
exclusion of dry air from pre-Karl, which did become a tropical cyclone and
later a major hurricane.
Citation: Rutherford, B. and Montgomery, M. T.: A Lagrangian analysis of a developing and non-developing disturbance observed during the PREDICT experiment, Atmos. Chem. Phys., 12, 11355-11381, doi:10.5194/acp-12-11355-2012, 2012.