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  • Author or Editor: Victoria A. Sinclair x
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Mirja L. Kemppi
and
Victoria A. Sinclair

Abstract

The purpose of this study is to document the structure of a warm front in northeast Europe, identify the effects that the Finnish coastline has on the evolution of the front, and investigate factors that influence the speed that the warm front moves at within, and above, the boundary layer. The warm front formed over Estonia, traveled northward across the Gulf of Finland, and then crossed the southern coastline of Finland. Surface-based measurements from the Helsinki Testbed are analyzed together with output from a high-resolution numerical weather prediction model, Application of Research to Operations at Mesoscale (AROME). During the early stages of development, the warm front interacted with a stationary baroclinic zone and, consequently, evolved into an S shape. As the front approached the southern coast of Finland, the temperature gradient at 1000 hPa increased, as it merged with a diabatically generated temperature gradient. At 1000 hPa, the front stalled at the coastline due to friction-enhanced convergence, while the front’s speed at 860 hPa was almost uniform and unaffected by the coastline. At both 860 and 1000 hPa, the front moved slower than the wind speed. Hence, the front’s movement had a propagating component that was directed in the opposite direction to that of the front’s movement. The distribution of the ageostrophic winds showed that the front’s propagation component was produced by the front’s secondary circulation and surface friction. These results highlight the importance of surface sensible heat fluxes and friction on the evolution and movement of warm fronts.

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Victoria A. Sinclair
,
Sami Niemelä
, and
Matti Leskinen

Abstract

A narrow and shallow cold front that passed over Finland during the night 30–31 October 2007 is analyzed using model output and observations primarily from the Helsinki Testbed. The aim is to describe the structure of the front, especially within the planetary boundary layer, identify how this structure evolved, and determine the ability of a numerical model to correctly predict this structure. The front was shallow with a small (2.5–3 K) temperature decrease associated with it, which is attributed to the synoptic evolution of the cold front from a frontal wave on a mature, trailing cold front in a region of weak upper-level forcing and where the midtroposphere was strongly stratified. Within the boundary layer, the frontal surface was vertical and the frontal zone was narrow (<8 km). The small cross-front scale was probably a consequence of the weak frontolytical turbulent mixing occurring at night, at high latitudes, combined with strong, localized frontogenetic forcing driven by convergence. The model simulated the mesoscale evolution of the front well, but overestimated the width of the frontal zone. Within the boundary layer, the model adequately predicted the stratification and near-surface temperatures ahead of, and within, the frontal zone, but failed to correctly predict the thermal inversion that developed in the stably stratified postfrontal air mass. This case study highlights the complex structure of fronts both within the nocturnal boundary layer, and in a location far from regions of cyclogenesis, and hence the challenges that both forecasters and operational models face.

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Terhi K. Laurila
,
Victoria A. Sinclair
, and
Hilppa Gregow

Abstract

On 22 September 1982, an intense windstorm caused considerable damage in northern Finland. Local forecasters noted that this windstorm potentially was related to Hurricane Debby, a category 4 hurricane that occurred just 5 days earlier. Due to the unique nature of the event and lack of prior research, our aim is to document the synoptic sequence of events related to this storm using ERA-Interim reanalysis data, best track data, and output from OpenIFS simulations. During extratropical transition, the outflow from Debby resulted in a ridge building and an acceleration of the jet. Debby did not reintensify immediately in the midlatitudes despite the presence of an upper-level trough. Instead, ex-Debby propagated rapidly across the Atlantic as a diabatic Rossby wave–like feature. Simultaneously, an upper-level trough approached from the northeast and once ex-Debby moved ahead of this feature near the United Kingdom, rapid reintensification began. All OpenIFS forecasts diverged from reanalysis after only 2 days indicating intrinsic low predictability and strong sensitivities. Phasing between Hurricane Debby and the weak trough, and phasing of the upper- and lower-level potential vorticity anomalies near the United Kingdom was important in the evolution of ex-Debby. In the only OpenIFS simulation to correctly capture the phasing over the United Kingdom, stronger wind gusts were simulated over northern Finland than in any other simulation. Turbulent mixing behind the cold front, and convectively driven downdrafts in the warm sector, enhanced the wind gusts over Finland. To further improve understanding of this case, we suggest conducting research using an ensemble approach.

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