Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: Victoria A. Sinclair x
  • Refine by Access: All Content x
Clear All Modify Search
Victoria A. Sinclair

Abstract

A 6-yr climatology of the frequency, characteristics, and boundary layer structure of synoptic-scale fronts in Helsinki, Finland, was created using significant weather charts and observations from a 327-m-tall mast and from the Station for Measuring Ecosystem–Atmosphere Relationships III. In total, 855 fronts (332 cold fronts, 236 warm fronts, and 287 occluded fronts) affected Helsinki during the 6-yr period, equating to one front every 2.6 days. Seasonal and diurnal cycles were observed, with frontal frequency peaking during the cold season and during daytime. Composites of warm and cold fronts were developed to provide observationally based conceptual models of the low-level structure of fronts at the end of the North Atlantic Ocean storm track. The composite warm front displays a temperature increase of 4.0°C; a broad, forward-tilting frontal zone; and prolonged, weak-to-moderate precipitation. The composite cold front is characterized by a temperature decrease of 4.4°C, a narrow and slightly rearward-tilting frontal zone, and moderate precipitation collocated with the surface front. Relationships between frontal characteristics and the direction from which fronts approached, the season, time of day, prefrontal boundary layer lapse rate, and the location of the wind shift relative to the thermal gradient were investigated. The prefrontal lapse rate was the single most important variable in determining the temperature change, the height of the maximum temperature change, and the near-surface tilt of both warm and cold fronts. This result demonstrates the interaction between boundary layer and synoptic-scale processes that must be captured by numerical weather prediction models to accurately forecast surface fronts.

Full access
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.

Full access
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.

Full access
Minttu Tuononen
,
Ewan J. O’Connor
,
Victoria A. Sinclair
, and
Ville Vakkari

Abstract

Over two years of meteorological observations from Utö, a small island in the Finnish outer archipelago in the Baltic Sea, were used to investigate the occurrence and characteristics of low-level jets (LLJs) and the diurnal and seasonal variations in these properties. An objective LLJ identification algorithm that is suitable for high-temporal-and-vertical-resolution Doppler lidar data was created and applied to wind profiles obtained from a combination of Doppler lidar data and two-dimensional sonic anemometer observations. This algorithm was designed to identify coherent LLJ structures and requires that they persist for at least 1 h. The long-term mean LLJ frequency of occurrence at Utö was 12%, the mean LLJ wind speed was 11.6 m s−1, and the vast majority of identified LLJs occurred below 150 m above ground level. The LLJ frequency of occurrence was much higher during summer (21%) and spring (18%) than in autumn (8%) and winter (3%). During winter and spring, the LLJ frequency of occurrence is evenly distributed throughout the day. In contrast, the LLJ frequency of occurrence peaks at night (1900–0100 UTC) during summer and during the evening hours (1700–1900 UTC) in autumn. The highest and strongest LLJs come from the southwest, which is also the predominant LLJ direction in all seasons. LLJs below 100 m are common in spring and summer, are weaker, and do not show a strong directional dependence.

Full access
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.

Open access
Tuukka Petäjä
,
Ewan J. O’Connor
,
Dmitri Moisseev
,
Victoria A. Sinclair
,
Antti J. Manninen
,
Riikka Väänänen
,
Annakaisa von Lerber
,
Joel A. Thornton
,
Keri Nicoll
,
Walt Petersen
,
V. Chandrasekar
,
James N. Smith
,
Paul M. Winkler
,
Olaf Krüger
,
Hannele Hakola
,
Hilkka Timonen
,
David Brus
,
Tuomas Laurila
,
Eija Asmi
,
Marja-Liisa Riekkola
,
Lucia Mona
,
Paola Massoli
,
Ronny Engelmann
,
Mika Komppula
,
Jian Wang
,
Chongai Kuang
,
Jaana Bäck
,
Annele Virtanen
,
Janne Levula
,
Michael Ritsche
, and
Nicki Hickmon

Abstract

During Biogenic Aerosols—Effects on Clouds and Climate (BAECC), the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Program deployed the Second ARM Mobile Facility (AMF2) to Hyytiälä, Finland, for an 8-month intensive measurement campaign from February to September 2014. The primary research goal is to understand the role of biogenic aerosols in cloud formation. Hyytiälä is host to the Station for Measuring Ecosystem–Atmosphere Relations II (SMEAR II), one of the world’s most comprehensive surface in situ observation sites in a boreal forest environment. The station has been measuring atmospheric aerosols, biogenic emissions, and an extensive suite of parameters relevant to atmosphere–biosphere interactions continuously since 1996. Combining vertical profiles from AMF2 with surface-based in situ SMEAR II observations allows the processes at the surface to be directly related to processes occurring throughout the entire tropospheric column. Together with the inclusion of extensive surface precipitation measurements and intensive observation periods involving aircraft flights and novel radiosonde launches, the complementary observations provide a unique opportunity for investigating aerosol–cloud interactions and cloud-to-precipitation processes in a boreal environment. The BAECC dataset provides opportunities for evaluating and improving models of aerosol sources and transport, cloud microphysical processes, and boundary layer structures. In addition, numerical models are being used to bridge the gap between surface-based and tropospheric observations.

Full access
Federico Bianchi
,
Victoria A. Sinclair
,
Diego Aliaga
,
Qiaozhi Zha
,
Wiebke Scholz
,
Cheng Wu
,
Liine Heikkinen
,
Rob Modini
,
Eva Partoll
,
Fernando Velarde
,
Isabel Moreno
,
Yvette Gramlich
,
Wei Huang
,
Alkuin Maximilian Koenig
,
Markus Leiminger
,
Joonas Enroth
,
Otso Peräkylä
,
Angela Marinoni
,
Chen Xuemeng
,
Luis Blacutt
,
Ricardo Forno
,
Rene Gutierrez
,
Patrick Ginot
,
Gaëlle Uzu
,
Maria Cristina Facchini
,
Stefania Gilardoni
,
Martin Gysel-Beer
,
Runlong Cai
,
Tuukka Petäjä
,
Matteo Rinaldi
,
Harald Saathoff
,
Karine Sellegri
,
Douglas Worsnop
,
Paulo Artaxo
,
Armin Hansel
,
Markku Kulmala
,
Alfred Wiedensohler
,
Paolo Laj
,
Radovan Krejci
,
Samara Carbone
,
Marcos Andrade
, and
Claudia Mohr

Abstract

This paper presents an introduction to the Southern Hemisphere High Altitude Experiment on Particle Nucleation and Growth (SALTENA). This field campaign took place between December 2017 and June 2018 (wet to dry season) at Chacaltaya (CHC), a GAW (Global Atmosphere Watch) station located at 5,240 m MSL in the Bolivian Andes. Concurrent measurements were conducted at two additional sites in El Alto (4,000 m MSL) and La Paz (3,600 m MSL). The overall goal of the campaign was to identify the sources, understand the formation mechanisms and transport, and characterize the properties of aerosol at these stations. State-of-the-art instruments were brought to the station complementing the ongoing permanent GAW measurements, to allow a comprehensive description of the chemical species of anthropogenic and biogenic origin impacting the station and contributing to new particle formation. In this overview we first provide an assessment of the complex meteorology, airmass origin, and boundary layer–free troposphere interactions during the campaign using a 6-month high-resolution Weather Research and Forecasting (WRF) simulation coupled with Flexible Particle dispersion model (FLEXPART). We then show some of the research highlights from the campaign, including (i) chemical transformation processes of anthropogenic pollution while the air masses are transported to the CHC station from the metropolitan area of La Paz–El Alto, (ii) volcanic emissions as an important source of atmospheric sulfur compounds in the region, (iii) the characterization of the compounds involved in new particle formation, and (iv) the identification of long-range-transported compounds from the Pacific or the Amazon basin. We conclude the article with a presentation of future research foci. The SALTENA dataset highlights the importance of comprehensive observations in strategic high-altitude locations, especially the undersampled Southern Hemisphere.

Full access