Severe mesoscale weather events over the Nordic Seas: Numerical modelling and sensitivity experiments

The Nordic seas region, due to its unique geographical and climatological conditions,
experiences a number of adverse mesoscale weather events that have proven fatal
for human activities in the area. Most of these weather events occur in the winter season
and include orography-induced wind extremes, shallow and sharp fronts that often form
in the vicinity of the ice edge and intense storms called polar lows. In order to assess
such high impact weather conditions, a field campaign was conducted over the Nordic
seas in February-March 2008 as part of the IPY-THORPEX project. During this field
campaign, measurements of a number of severe weather cases were collected through
the dropsonde and lidar sensors on-board the DLR-FALCON aircraft. This thesis deals
with high resolution modeling experiments of some of the cases observed during the
IPY-THORPEX field campaign, with focus on the sensitivity of these events to underlying
surface conditions and several physical factors. The results of these experiments
are presented in the form of three scientific articles.
Paper-I presents the airborne measurements of a case with strong gap winds observed
at the exit region of the Hinlopen Strait in Svalbard archipelago. The DLRFALCON
aircraft flew along the Hinlopen strait and recorded winds of strength ~ 20
ms−1, which was about four times higher than the upstream wind, near to the surface.
In addition, formation of a wake with return currents in the lee of the terrain was also
recorded by the lidar sensors carried by the aircraft. This case was simulated in a fineresolution
numerical model with a high degree of accuracy. The simulations showed the
existence of small-scale cloud banners extending downstream of the lee of the mountains.
The causal mechanisms for such cloud features were investigated through an
idealised approach. The ideal experiments indicated that the vertical circulations set
up by the terrain features cause the cloud banners and a relatively warm ocean surface
maintains them by supplying moisture and buyoancy.
In paper-II, numerical simulations of another adverse weather case occurred during
the IPY-THORPEX field campaign is described. The simulations indicated the formation
of a low pressure system at the surface over the Barents Sea on 29 February
2008. Later on, the low intensified while propagating in a northwestward direction and
crossed the ice-edge on 1 March followed by its dissipation. The sensitivity of this surface
low to underlying surface conditions is explored in two sensitivity experiments,
wherein the ice-cover was removed in the first and the sea-surface temperature was
raised by 5 K in the second. The Barents Sea surface low persists for a longer duration
in the no-ice experiment but eventually dissolves, perhaps due to high static stability in
the region. In the second experiment with warmer sea-surface temperature, the surface
low is found to persist near the Svalbard archipelago, leading to a significant advection
of cold and stable Arctic air over to the warmer Norwegian Sea. This situation triggers an intense polar low over the Norwegian Sea. Certain dynamical aspects of this polar
low are also presented in this paper.
The sensitivity of polar lows to upper-level features and important physical factors
such as static stability, baroclinicity and surface heating is the subject of paper-III. The
study was carried out in a numerically set idealised baroclinic channel with a horizontal
grid spacing of 10 km. The formation of polar lows was forced in the channel by
imposing a finite amplitude perturbation at the tropopause level at the initial conditions.
A number of sensitivity experiments were conducted, each with a different set of initial
conditions that differ in the degree of baroclinicity, static stability, surface heating and
the scale of upper-level perturbation. This paper shows that the pattern of the vertical
motion associated with a polar low is highly sensitive to the amount of baroclinicity
and surface heating. Static stability appears to have more profound influence on the
growth rate of the polar low than on the associated vertical motion. The structure and
scale of the polar low happens to be directly proportional to the features of upper-level
perturbation