TY - JOUR
T1 - The characteristics of low-speed streaks in near-neutral and unstable atmospheric boundary layer
AU - Quan, Li Hong
AU - Longhetto, A.
AU - Ferrero, E.
PY - 2009/3
Y1 - 2009/3
N2 - Low-speed streaks are considered as narrow regions of low-speed velocity fluid stretched in the streamwise direction. Two kinds of low-speed streaks and corresponding high-temperature streaks obtained with LES (Large Eddy Simulation) simulations of the near-neutral and unstable ABLs controlled by both wind shear and buoyancy are investigated to understand the characteristics of streaks in different stratifications. In the near-neutral boundary layer, quasi-streamwise low-speed and high-temperature streaks are in almost 45° to initial streamwise direction and more, thinner and longer near surface. While the low-speed and hightemperature streaks meander and sometimes are not in the streamwise direction in the unstable boundary layer. Furthermore, they are thinner and shorter near surface and many low-speed structures cannot be connected to form streaks. The low-speed structures induced by vortices associated with low-speed streaks are deep and can have a few low-speed centers in vertical direction due to the convergence and divergence of flow. These structures are different in shape in different boundary layers and are deeper in the unstable boundary layer than in the near-neutral boundary layer. In the near-neutral boundary layer, the low-speed structures are high-temperature. Therefore, temperature can be an index of low-speed structures and momentum and sensible heat fluxes are significant at low-speed and high-temperature centers. While in the unstable boundary layer, the corresponding high-temperature streaks lag behind the low-speed streaks along the local flow and low-speed structures are not always high-temperature. Therefore, temperature cannot be an index of vortices or low-speed structures and low momentum and high sensible heat fluxes deviate from the low-speed streak.
AB - Low-speed streaks are considered as narrow regions of low-speed velocity fluid stretched in the streamwise direction. Two kinds of low-speed streaks and corresponding high-temperature streaks obtained with LES (Large Eddy Simulation) simulations of the near-neutral and unstable ABLs controlled by both wind shear and buoyancy are investigated to understand the characteristics of streaks in different stratifications. In the near-neutral boundary layer, quasi-streamwise low-speed and high-temperature streaks are in almost 45° to initial streamwise direction and more, thinner and longer near surface. While the low-speed and hightemperature streaks meander and sometimes are not in the streamwise direction in the unstable boundary layer. Furthermore, they are thinner and shorter near surface and many low-speed structures cannot be connected to form streaks. The low-speed structures induced by vortices associated with low-speed streaks are deep and can have a few low-speed centers in vertical direction due to the convergence and divergence of flow. These structures are different in shape in different boundary layers and are deeper in the unstable boundary layer than in the near-neutral boundary layer. In the near-neutral boundary layer, the low-speed structures are high-temperature. Therefore, temperature can be an index of low-speed structures and momentum and sensible heat fluxes are significant at low-speed and high-temperature centers. While in the unstable boundary layer, the corresponding high-temperature streaks lag behind the low-speed streaks along the local flow and low-speed structures are not always high-temperature. Therefore, temperature cannot be an index of vortices or low-speed structures and low momentum and high sensible heat fluxes deviate from the low-speed streak.
UR - http://www.scopus.com/inward/record.url?scp=72149105031&partnerID=8YFLogxK
U2 - 10.1393/ncb/i2009-10767-9
DO - 10.1393/ncb/i2009-10767-9
M3 - Article
SN - 1594-9982
VL - 124
SP - 325
EP - 340
JO - Nuovo Cimento della Societa Italiana di Fisica B
JF - Nuovo Cimento della Societa Italiana di Fisica B
IS - 3
ER -