Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls

Boundary layer rolls are a rather frequent phenomena, where regions of alternating up- and downdraft motion causes clouds to form in elongated, parallel rows oriented with the mean wind direction. The clouds can be seen during certain atmospheric conditions and are often called ”cloud streets” becau...

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Main Author: Bergstedt, Josefine
Format: Others
Language:English
Published: Uppsala universitet, Luft-, vatten- och landskapslära 2020
Subjects:
Online Access:http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-424449
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spelling ndltd-UPSALLA1-oai-DiVA.org-uu-4244492020-12-17T05:28:30ZCloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer RollsengMolngator - En studie över hur molnrullar uppkommer i gränsskiktetBergstedt, JosefineUppsala universitet, Luft-, vatten- och landskapsläraUppsala University2020Cloud streetsboundary layer rollsdynamic instabilitythermal instabilityhorizontal con- vective rollsinflection pointMeteorology and Atmospheric SciencesMeteorologi och atmosfärforskningBoundary layer rolls are a rather frequent phenomena, where regions of alternating up- and downdraft motion causes clouds to form in elongated, parallel rows oriented with the mean wind direction. The clouds can be seen during certain atmospheric conditions and are often called ”cloud streets” because of their characteristic appearance. By performing a linear instability analysis, the underlying mechanisms causing the onset of boundary layer rolls has been analysed in this study. There are two governing mech- anisms that cause the boundary layer rolls to form, the thermal instability and the dynamic instability. The thermal instability is caused by convection in an unstable airmass, while the dynamic instability usually is associated with neutral or stable conditions. The dynamic instability arise due to an inflection point in the wind profile, around which eddies develop. In a previous study by Svensson et al. (2017), rolls were observed over the Swedish east-coast, stretching out over sea during four days; 2 of May 1997, 3 of May 1997, 17 of May 2011 and 25 of May 2011. The aim of this study is to simulate the rolls on these four dates, analyse the underlying mechanisms and establish what type of instability that primarily causes the rolls to form. The linear stability analysis performed in this study indicate that the dynamic instability is the main mechanism giving rise to the rolls on all four studied dates. The rolls are found to arise over the Swedish mainland and are advected out over the sea. Both the orientation of the rolls and the modeled wind direction are in accordance with the observations. A qualitative agreement is found for the wavelength, the amplitude and the altitude of the rolls, when comparing the results of this study with the observations. Student thesisinfo:eu-repo/semantics/bachelorThesistexthttp://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-424449Examensarbete vid Institutionen för geovetenskaper, 1650-6553 ; 504application/pdfinfo:eu-repo/semantics/openAccess
collection NDLTD
language English
format Others
sources NDLTD
topic Cloud streets
boundary layer rolls
dynamic instability
thermal instability
horizontal con- vective rolls
inflection point
Meteorology and Atmospheric Sciences
Meteorologi och atmosfärforskning
spellingShingle Cloud streets
boundary layer rolls
dynamic instability
thermal instability
horizontal con- vective rolls
inflection point
Meteorology and Atmospheric Sciences
Meteorologi och atmosfärforskning
Bergstedt, Josefine
Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls
description Boundary layer rolls are a rather frequent phenomena, where regions of alternating up- and downdraft motion causes clouds to form in elongated, parallel rows oriented with the mean wind direction. The clouds can be seen during certain atmospheric conditions and are often called ”cloud streets” because of their characteristic appearance. By performing a linear instability analysis, the underlying mechanisms causing the onset of boundary layer rolls has been analysed in this study. There are two governing mech- anisms that cause the boundary layer rolls to form, the thermal instability and the dynamic instability. The thermal instability is caused by convection in an unstable airmass, while the dynamic instability usually is associated with neutral or stable conditions. The dynamic instability arise due to an inflection point in the wind profile, around which eddies develop. In a previous study by Svensson et al. (2017), rolls were observed over the Swedish east-coast, stretching out over sea during four days; 2 of May 1997, 3 of May 1997, 17 of May 2011 and 25 of May 2011. The aim of this study is to simulate the rolls on these four dates, analyse the underlying mechanisms and establish what type of instability that primarily causes the rolls to form. The linear stability analysis performed in this study indicate that the dynamic instability is the main mechanism giving rise to the rolls on all four studied dates. The rolls are found to arise over the Swedish mainland and are advected out over the sea. Both the orientation of the rolls and the modeled wind direction are in accordance with the observations. A qualitative agreement is found for the wavelength, the amplitude and the altitude of the rolls, when comparing the results of this study with the observations.
author Bergstedt, Josefine
author_facet Bergstedt, Josefine
author_sort Bergstedt, Josefine
title Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls
title_short Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls
title_full Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls
title_fullStr Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls
title_full_unstemmed Cloud Streets. A Study of the Instability Mechanisms Giving Rise to Boundary Layer Rolls
title_sort cloud streets. a study of the instability mechanisms giving rise to boundary layer rolls
publisher Uppsala universitet, Luft-, vatten- och landskapslära
publishDate 2020
url http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-424449
work_keys_str_mv AT bergstedtjosefine cloudstreetsastudyoftheinstabilitymechanismsgivingrisetoboundarylayerrolls
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