Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 103 -
Modelling the Impact of Inertia-Gravity Waves on Wind and Precipitation
Christoph Zülicke and Dieter Peters
Institute of Atmospheric Physics, Schlossstraße 6, 18225 Kühlungsborn, Germany < zuelicke@iap-kborn.de >
1. Introduction
Inertia-gravity waves (IGW for short) are features of the
atmospheric dynamics, which may influence the regional
momentum and energy balance significantly (Holton, 1992).
They may be generated for example by unbalanced jet
currents, frontal systems, convection or orography. During
their evolution IGW-s transport momentum upwards into the
upper atmosphere. Local weather events are also
occasionally modified by such IGW-s - they are accounted
for extreme weather events such as wind gusts or deep
convection (Bosart et al., 1998).
In a case study study we investigated such an event, which
took place over the North-European / Baltic region. The
general weather situation was characterised by a poleward
breaking Rossby wave connected with an intense polar jet in
the tropopause (Peters et al., 2002). We model the
generation of IGW-s, trace their propagation down to the
surface and finally discuss their impact on the wind and
precipitation fields in the Gotland Basin.
2. Model setup
The Fifth-generation mesoscale model (in short: MM5) has
been developed by the National Center for Atmospheric
Research (Boulder, CO) and the Pennsylvania State
University (NCAR, 2003). It has a nonhydrostatic dynamics
and is implemented on a staggered grid. We have defined
three nested domains covering about an area of 7000 km *
5000 km. The domains have a horizontal resolution of ∆x =
72, 24 and 8 km and a vertical of ∆z = 0.75, 0.25 and 0.1
km. We used for the full-physics run the MRF PBL module,
Grell-parameterisation of cumulus convection and a Dudhia
ice scheme for the microphysics. In order to cover the
LEWIZ-campaign (17.-19.12.1999) the model run was
started at 16.12.1999-00:00. For the construction of initial
and boundary conditions ECMWF analyses were used.
Figure 1: MM5 model domains
3. IGW parameters
A snapshot from the model simulation is discussed below.
Figure 2 shows the jet current at about 300 hPa, approaching
the Baltic Sea from West. During the process of geostrophic
adjustment of the unbalanced jet, IGW-s are generated at its
tip. These waves are essentially ageostrophic – we make
them visible with the horizontal divergence
δ = ∂u/∂x + ∂v/∂y
Figure 2: Map at tropopause level (300 hPa, ca. 8 km
height) for t = 48 h (1 8.12.1999-00:00): shown wind
speed U (values larger than 30 m / s are green) and
horizontal divergence δ (value larger / smaller than 2 10 -5
1 / s are red/blue). Three IGW-s aere indicated with thick
green lines at the positions where the divergence crosses
the Zero line from negative (blue) to positive (red) values.
The position of Gotland is marked with a red dot.
A part of the precipitable water near the surface is
modulated by these IGW-s, in particular some areas with
convective precipitation (see figure 3)
Figure 3: Surface map at 900 hPa (ca. 1 km height) at t =
48 h. In addition to the quantities above, the total
precipitation mixing ratio (values larger 0.1 g / kg are
orange) is shown.
In the simulation we detected IGW-s with a mean
horizontal wavelength of Λ = 240 km, a vertical
wavelength of λz = 2.9 km and an intrinsic period of τ i =
6.9 h (Zülicke & Peters, 2004). The source of the three
indicated IGW-s is the polar jet at about 8 km height – the
wave train appear in both figures but shifted by ca. 100
km.