Two-year Microwave Radiometric Observations of Low-level Boundary-layer Temperature Inversion Signatures

2006 ◽  
Author(s):  
Y.-A. Liou ◽  
S.-K. Yan
Keyword(s):  
Boundary Layer ◽  
Temperature Inversion ◽  
Low Level

scholarly journals The Low-Level Atmospheric Circulation near Tongoy Bay–Point Lengua de Vaca (Chilean Coast, 30°S)

2011 ◽  
Vol 139 (11) ◽  
pp. 3628-3647 ◽  
Author(s):  
David A. Rahn ◽  
René D. Garreaud ◽  
José A. Rutllant
Keyword(s):  
Boundary Layer ◽  
Ocean Circulation ◽  
Sea Breeze ◽  
Temperature Inversion ◽  
High Elevation ◽  
Marine Terrace ◽  
Offshore Structure ◽  
Local Circulation ◽  
Wind Maximum ◽  
Low Level

Abstract Strong southerly, terrain parallel winds often occur along the coast of north-central Chile (25°–35°S) embedded in the marine atmospheric boundary layer and the lower part of the capping temperature inversion. Their offshore structure and variability have received considerable attention because of the effect on open-ocean processes and connection with the southeast Pacific cloud layer. Mesoscale low-level circulations linked to the coastal topography (e.g., coastal jets and sea breeze) are less studied in Chile, but are particularly relevant as they alter the upper-ocean circulation and the cloud pattern in the nearshore strip. Surface, radiosonde, and airborne meteorological observations near point Lengua de Vaca (LdV)–Tongoy Bay (TB) at 30°S are used alongside numerical modeling to understand the local circulation near a prominent upwelling center. Most observations were gathered during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Chilean Upwelling Experiment (VOCALS-CUpEx) during two weeks in late spring 2009. The regional topography resembles other major capes, but south of TB and east of LdV there is a low (100–300 m), dry marine terrace bounded by high elevation at the coast (~600 m) and farther inland. Coastal soundings 25 km upstream of LdV revealed a southerly wind maximum near the surface and another at 900 m separated by a destabilized layer, deviating from the two-layer model often applied to coastal flow. In the morning a shallow sea breeze penetrates from TB to the marine terrace, but is overridden by southerly flow in the afternoon. Furthermore, between 400 and 900 m, warm continental air is advected from over the marine terrace creating a residual boundary layer over TB. Concurrent with slower changes offshore, the low-level warming over TB leads to a marked cross-shore pressure gradient enhancing the coastal jet just north of LdV.

scholarly journals The Coastal Boundary Layer at the Eastern Margin of the Southeast Pacific (23.4°S, 70.4°W): Cloudiness-Conditioned Climatology

2011 ◽  
Vol 24 (4) ◽  
pp. 1013-1033 ◽  
Author(s):  
Ricardo C. Muñoz ◽  
Rosa A. Zamora ◽  
José A. Rutllant
Keyword(s):  
Boundary Layer ◽  
Lower Troposphere ◽  
Temperature Inversion ◽  
Austral Winter ◽  
Radiative Balance ◽  
Low Level ◽  
Southeast Pacific ◽  
Coastal Margin ◽  
Coastal Boundary Layer ◽  
Coastal Boundary

Abstract A basic climatological description of 29 years of surface and upper-air observations at a coastal site (23.4°S, 70.4°W) in northern Chile is presented. The site is considered to be generally representative of the eastern coastal margin of the southeast Pacific stratocumulus region, which plays an important role in the global radiative balance. The analysis focuses on two of the main elements affecting coastal weather in this region: low-level cloudiness and the state of the subsidence temperature inversion. The objectives of the paper are 1) to present the basic climatological features of these elements and 2) to document the differences in the structure of this coastal boundary layer (BL) associated with the presence or absence of low-level clouds. Low-level clouds (defined here as ceilings less than 1500 m AGL) occur at the site mostly in the night, especially during austral winter and spring. Elevated subsidence inversions show a very large prevalence in the 1200 UTC [0800 local time (LT)] radiosonde profiles analyzed here, with base heights typically between 800 and 1100 m. The seasonal cycle of the subsidence inversion shows an ∼300-m amplitude at inversion base and top and a substantial BL cooling in austral winter. Generally weak and shallow surface-based inversions at 1200 UTC (0800 LT) are present in about 15% of the soundings, with more frequent occurrence in austral fall. The second objective was accomplished by compositing surface meteorology and upper-air profiles conditioned by nighttime low-level cloudiness. More frequent surface inversions in temperature and dewpoint are found for mostly clear nights, as compared to mostly cloudy nighttime conditions. The clear-night BL shows a more stable temperature profile and larger vertical gradients in mixing ratio when compared to the approximately well-mixed cloud-topped BL. Above the BL, the clear composites show a weaker subsidence inversion and more intense northerly winds in the 1000–3000-m layer compared to the cloudy cases. Insights into the physical mechanisms underlying the findings above were sought by comparing the cloudy composites to results of a stationary mixed-layer model of a stratus-capped marine BL, by computing derived parameters pertaining to the temperature budget and the turbulent state of the lower troposphere and by using reanalysis fields to compute regional circulation anomalies associated to coastal low-level cloudiness. The results show physically significant differences in subsidence, horizontal temperature advection, and winds in the lower troposphere associated with the mean clear and cloudy coastal BL. Coastal clear nights appear associated with a cold anomaly in the lower troposphere over the southeast Pacific basin offshore of Peru and Chile, which by thermal wind arguments induce anomalies of southerly winds along the Chilean coast near the surface and northerly winds above the BL, while at the same time reducing the coastal subsidence in the lower troposphere. These results point to the importance of properly representing the sea–land temperature contrast and the topographic impact on the lower-tropospheric flow in order to adequately model the coastal BL mean state over this region.

Role of low-level jets and boundary-layer properties on the NBL budget technique

2005 ◽  
Vol 135 (1-4) ◽  
pp. 35-43 ◽  
Author(s):  
N. Mathieu ◽  
I.B. Strachan ◽  
M.Y. Leclerc ◽  
A. Karipot ◽  
E. Pattey
Keyword(s):  
Boundary Layer ◽  
Low Level ◽  
Low Level Jets

Idealized large-eddy simulations of stratocumulus advecting over cold water. Part 1: Boundary layer decoupling

2021 ◽  
Author(s):  
Youtong Zheng ◽  
Haipeng Zhang ◽  
Daniel Rosenfeld ◽  
Seoung-Soo Lee ◽  
Tianning Su ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Surface Temperature ◽  
Cold Water ◽  
Temperature Inversion ◽  
Atmospheric Modeling ◽  
Sea Surface ◽  
Entrainment Rate ◽  
Surface Cooling ◽  
Near Surface ◽  
Large Eddy

AbstractWe explore the decoupling physics of a stratocumulus-topped boundary layer (STBL) moving over cooler water, a situation mimicking the warm air advection (WADV). We simulate an initially well-mixed STBL over a doubly periodic domain with the sea surface temperature decreasing linearly over time using the System for Atmospheric Modeling large-eddy model. Due to the surface cooling, the STBL becomes increasingly stably stratified, manifested as a near-surface temperature inversion topped by a well-mixed cloud-containing layer. Unlike the stably stratified STBL in cold air advection (CADV) that is characterized by cumulus coupling, the stratocumulus deck in the WADV is unambiguously decoupled from the sea surface, manifested as weakly negative buoyancy flux throughout the sub-cloud layer. Without the influxes of buoyancy from the surface, the convective circulation in the well-mixed cloud-containing layer is driven by cloud-top radiative cooling. In such a regime, the downdrafts propel the circulation, in contrast to that in CADV regime for which the cumulus updrafts play a more determinant role. Such a contrast in convection regime explains the difference in many aspects of the STBLs including the entrainment rate, cloud homogeneity, vertical exchanges of heat and moisture, and lifetime of the stratocumulus deck, with the last being subject to a more thorough investigation in part 2. Finally, we investigate under what conditions a secondary stratus near the surface (or fog) can form in the WADV. We found that weaker subsidence favors the formation of fog whereas a more rapid surface cooling rate doesn’t.

Assessing the influence of complex terrain on severe convective environments in northeastern Alabama

2021 ◽  
Author(s):  
Branden Katona ◽  
Paul Markowski
Keyword(s):  
Boundary Layer ◽  
Complex Terrain ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Spatial Distributions ◽  
Convective Storm ◽  
Self Organizing Maps ◽  
Low Level ◽  
Wind Regimes

AbstractStorms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective stormenvironments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

scholarly journals Summertime observations of ultrafine particles and cloud condensation nuclei from the boundary layer to the free troposphere in the Arctic

2016 ◽  
Author(s):  
Julia Burkart ◽  
Megan D. Willis ◽  
Heiko Bozem ◽  
Jennie L. Thomas ◽  
Kathy Law ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Ultrafine Particles ◽  
Cloud Condensation Nuclei ◽  
Particle Diameter ◽  
Open Ocean ◽  
The Arctic ◽  
Condensation Nuclei ◽  
Aerosol Composition ◽  
Low Level ◽  
20 Nm

Abstract. The Arctic is extremely sensitive to climate change. Shrinking sea ice extent increases the area covered by open ocean during Arctic summer, which impacts the surface albedo and aerosol and cloud properties among many things. In this context extensive aerosol measurements (aerosol composition, particle number and size, cloud condensation nuclei, and trace gases) were made during 11 flights of the NETCARE July, 2014 airborne campaign conducted from Resolute Bay, Nunavut (74N, 94W). Flights routinely included vertical profiles from about 60 to 3000 m a.g.l. as well as several low-level horizontal transects over open ocean, fast ice, melt ponds, and polynyas. Here we discuss the vertical distribution of ultrafine particles (UFP, particle diameter, dp: 5–20 nm), size distributions of larger particles (dp: 20 nm to 1 μm), and cloud condensation nuclei (CCN, supersaturation = 0.6 %) in relation to meteorological conditions and underlying surfaces. UFPs were observed predominantly within the boundary layer, where concentrations were often several hundreds to a few thousand particles per cubic centimeter. Occasionally, particle concentrations below 10 cm−3 were found. The highest UFP concentrations were observed above open ocean and at the top of low-level clouds, whereas numbers over ice-covered regions were substantially lower. Overall, UFP formation events were frequent in a clean boundary layer with a low condensation sink. In a few cases this ultrafine mode extended to sizes larger than 40 nm, suggesting that these UFP can grow into a size range where they can impact clouds and therefore climate.

scholarly journals Impacts of Modifications to a Local Planetary Boundary Layer Scheme on Forecasts of the Great Plains Low-Level Jet Environment

2018 ◽  
Vol 33 (5) ◽  
pp. 1109-1120 ◽  
Author(s):  
David E. Jahn ◽  
William A. Gallus
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Planetary Boundary Layer ◽  
Great Plains ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Low Level ◽  
Stable Conditions ◽  
Pbl Scheme ◽  
Low Level Jet

Abstract The Great Plains low-level jet (LLJ) is influential in the initiation and evolution of nocturnal convection through the northward advection of heat and moisture, as well as convergence in the region of the LLJ nose. However, accurate numerical model forecasts of LLJs remain a challenge, related to the performance of the planetary boundary layer (PBL) scheme in the stable boundary layer. Evaluated here using a series of LLJ cases from the Plains Elevated Convection at Night (PECAN) program are modifications to a commonly used local PBL scheme, Mellor–Yamada–Nakanishi–Niino (MYNN), available in the Weather Research and Forecasting (WRF) Model. WRF forecast mean absolute error (MAE) and bias are calculated relative to PECAN rawinsonde observations. The first MYNN modification invokes a new set of constants for the scheme closure equations that, in the vicinity of the LLJ, decreases forecast MAEs of wind speed, potential temperature, and specific humidity more than 19%. For comparison, the Yonsei University (YSU) scheme results in wind speed MAEs 22% lower but specific humidity MAEs 17% greater than in the original MYNN scheme. The second MYNN modification, which incorporates the effects of potential kinetic energy and uses a nonzero mixing length in stable conditions as dependent on bulk shear, reduces wind speed MAEs 66% for levels below the LLJ, but increases MAEs at higher levels. Finally, Rapid Refresh analyses, which are often used for forecast verification, are evaluated here and found to exhibit a relatively large average wind speed bias of 3 m s−1 in the region below the LLJ, but with relatively small potential temperature and specific humidity biases.

The Impact of Mixed-Phase Microphysical Processes on the Turbulence in Low-level Clouds in the Arctic

2021 ◽  
Author(s):  
Jan Chylik ◽  
Roel Neggers
Keyword(s):  
Boundary Layer ◽  
Climate Models ◽  
Weather Forecast ◽  
Mixed Phase ◽  
The Arctic ◽  
Contributing Factors ◽  
Microphysics Scheme ◽  
Low Level ◽  
Microphysical Processes ◽  
The Impact

<p>The proper representation of Arctic mixed-phased clouds remains a challenge in both weather forecast and climate models. Amongst the contributing factors is the complexity of turbulent properties of clouds. While the effect of evaporating hydrometeors on turbulent properties of the boundary layer has been identified in other latitudes, the extent of similar studies in the Arctic has been so far limited.</p><p>Our study focus on the impact of heat release from mixed-phase microphysical processes on the turbulent properties of the convective low-level clouds in the Arctic. We  employ high-resolution simulations, properly constrained by relevant measurements.<br>Semi-idealised model cases are based on convective clouds observed during the recent campaign in the Arctic: ACLOUD, which took place May--June 2017 over Fram Strait. The simulations are performed in Dutch Atmospheric Large Eddy Simulation (DALES) with double-moment mixed-phase microphysics scheme of Seifert & Beheng.</p><p>The results indicate an enhancement of boundary layer turbulence is some convective regimes.<br>Furthermore, results are sensitive to aerosols concentrations. Additional implications for the role of mixed-phase clouds in the Arctic Amplification will be discussed.</p>

scholarly journals An Idealized Numerical Study of Shear-Relative Low-Level Mean Flow on Tropical Cyclone Intensity and Size

2019 ◽  
Vol 76 (8) ◽  
pp. 2309-2334 ◽  
Author(s):  
Buo-Fu Chen ◽  
Christopher A. Davis ◽  
Ying-Hwa Kuo
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Numerical Study ◽  
Vertical Wind Shear ◽  
Development Stage ◽  
Surface Fluxes ◽  
Mean Flow ◽  
Low Level ◽  
Core Convection

Abstract Given comparable background vertical wind shear (VWS) magnitudes, the initially imposed shear-relative low-level mean flow (LMF) is hypothesized to modify the structure and convective features of a tropical cyclone (TC). This study uses idealized Weather Research and Forecasting Model simulations to examine TC structure and convection affected by various LMFs directed toward eight shear-relative orientations. The simulated TC affected by an initially imposed LMF directed toward downshear left yields an anomalously high intensification rate, while an upshear-right LMF yields a relatively high expansion rate. These two shear-relative LMF orientations affect the asymmetry of both surface fluxes and frictional inflow in the boundary layer and thus modify the TC convection. During the early development stage, the initially imposed downshear-left LMF promotes inner-core convection because of high boundary layer moisture fluxes into the inner core and is thus favorable for TC intensification because of large radial fluxes of azimuthal mean vorticity near the radius of maximum wind in the boundary layer. However, TCs affected by various LMFs may modify the near-TC VWS differently, making the intensity evolution afterward more complicated. The TC with a fast-established eyewall in response to the downshear-left LMF further reduces the near-TC VWS, maintaining a relatively high intensification rate. For the upshear-right LMF that leads to active and sustained rainbands in the downshear quadrants, TC size expansion is promoted by a positive radial flux of eddy vorticity near the radius of 34-kt wind (1 kt ≈ 0.51 m s−1) because the vorticity associated with the rainbands is in phase with the storm-motion-relative inflow.

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