Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle

Abstract. Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave. Along 20° S the MBL is generally deeper offshore (1600 m at 85° W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74–76° W. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ~74° W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18–25° S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion results in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth.

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Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-4491-2010 ◽

2010 ◽

Vol 10 (10) ◽

pp. 4491-4506 ◽

Cited By ~ 92

Author(s):

D. A. Rahn ◽

R. Garreaud

Keyword(s):

Boundary Layer ◽

Diurnal Cycle ◽

Marine Boundary Layer ◽

Lower Troposphere ◽

Vertical Motion ◽

Atmospheric Model ◽

Diurnal Cycles ◽

The Andes ◽

Southeast Pacific ◽

Westerly Flow

Abstract. Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave. Along 20° S the MBL is generally deeper offshore (1600 m at 85° W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74–76° W. Weather Research and Forecasting (WRF) simulations underestimate MBL height the most near the coast but improve offshore. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ~74° W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18–25° S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion results in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth. Interference is clearly seen in the soundings at Iquique which are driven by a strong upsidence wave contrary to the radiation-driven cycle, leading to a diurnal cycle opposite of the other sites. Because WRF simulations have a lower MBL height, the speed of the simulated gravity wave is slower than observations and accounts for most of the discrepancy between observed and simulated phase speeds.

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Some Insights into the Characteristics and Dynamics of the Chilean Low-Level Coastal Jet

Monthly Weather Review ◽

10.1175/2010mwr3368.1 ◽

2010 ◽

Vol 138 (8) ◽

pp. 3185-3206 ◽

Cited By ~ 21

Author(s):

Qingfang Jiang ◽

Shouping Wang ◽

Larry O’Neill

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Mesoscale Model ◽

Primary Role ◽

Pressure System ◽

Low Level ◽

The Andes ◽

Coastal Jet ◽

Southeast Pacific ◽

Baroclinic Zone

Abstract The characteristics and dynamics of the Chilean low-level coastal jet (CLLCJ) are examined here through diagnosing real-time mesoscale model forecasts in support of the Variability of the American Monsoon System (VAMOS) Ocean–Cloud–Atmosphere Land Study (VOCALS) and additional sensitivity simulations. The forecasted surface winds over the southeast Pacific compare favorably with available observations. According to the forecasts and sensitivity simulations, the Southeast Pacific high pressure system (SEPH) plays a primary role in driving the CLLCJ. The Andes significantly intensify the CLLCJ mainly through interacting with the SEPH and anchoring a baroclinic zone along the Chilean coast. The land–sea differential heating also enhances the CLLCJ by strengthening the coastal baroclinic zone. Based on the location of the SEPH center, the CLLCJ can be separated into two types: a strong-forcing jet, with the SEPH close to the central Chilean coastline; and a weak-forcing jet, with the SEPH centered far away from the coastline. The former is much more intense and associated with stronger interaction between the SEPH and the Andes. The CLLCJ is slightly supergeostrophic within the marine boundary layer top inversion, where weak easterlies develop, and subgeostrophic in the turbulent boundary layer below, where westerlies are present. The inversion easterlies induce strong subsidence along the coast, which contributes to the formation of the coastal low and the coastal baroclinic zone.

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Evaluation of Forecasted Southeast Pacific Stratocumulus in the NCAR, GFDL, and ECMWF Models

Journal of Climate ◽

10.1175/2008jcli2479.1 ◽

2009 ◽

Vol 22 (11) ◽

pp. 2871-2889 ◽

Cited By ~ 86

Author(s):

Cécile Hannay ◽

David L. Williamson ◽

James J. Hack ◽

Jeffrey T. Kiehl ◽

Jerry G. Olson ◽

...

Keyword(s):

Boundary Layer ◽

Diurnal Cycle ◽

Mixed Boundary ◽

Eddy Diffusivity ◽

Atmospheric Model ◽

Surface Energy Budget ◽

Liquid Water Path ◽

Community Atmosphere Model ◽

Southeast Pacific ◽

Pbl Scheme

Abstract Forecasts of southeast Pacific stratocumulus at 20°S and 85°W during the East Pacific Investigation of Climate (EPIC) cruise of October 2001 are examined with the ECMWF model, the Atmospheric Model (AM) from GFDL, the Community Atmosphere Model (CAM) from NCAR, and the CAM with a revised atmospheric boundary layer formulation from the University of Washington (CAM-UW). The forecasts are initialized from ECMWF analyses and each model is run for 3–5 days to determine the differences with the EPIC field observations. Observations during the EPIC cruise show a well-mixed boundary layer under a sharp inversion. The inversion height and the cloud layer have a strong and regular diurnal cycle. A key problem common to the models is that the planetary boundary layer (PBL) depth is too shallow when compared to EPIC observations. However, it is suggested that improved PBL depths are achieved with more physically realistic PBL schemes: at one end, CAM uses a dry and surface-driven PBL scheme and produces a very shallow PBL, while the ECWMF model uses an eddy-diffusivity/mass-flux approach and produces a deeper and better-mixed PBL. All the models produce a strong diurnal cycle in the liquid water path (LWP), but there are large differences in the amplitude and phase when compared to the EPIC observations. This, in turn, affects the radiative fluxes at the surface and the surface energy budget. This is particularly relevant for coupled simulations as this can lead to a large SST bias.

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Evidence of a Diurnal Cycle in Precipitation over the Southern Ocean as Observed at Macquarie Island

Atmosphere ◽

10.3390/atmos11020181 ◽

2020 ◽

Vol 11 (2) ◽

pp. 181

Author(s):

Francisco Lang ◽

Yi Huang ◽

Steven T. Siems ◽

Michael J. Manton

Keyword(s):

Boundary Layer ◽

Southern Ocean ◽

Diurnal Cycle ◽

Marine Boundary Layer ◽

Lower Troposphere ◽

Early Morning ◽

Substantial Contribution ◽

Stability Parameters ◽

Macquarie Island ◽

Boundary Layer Clouds

Due to a lack of observations, relatively large discrepancies exist between precipitation products over the Southern Ocean. In this manuscript, surface hourly precipitation observations from Macquarie Island (54.62 ° S, 158.85 ° E) are analysed (1998–2016) to reveal a diurnal cycle. The precipitation rate is at a maximum during night/early morning and a minimum in the afternoon at Macquarie Island station. Seasonally, the diurnal cycle is strongest in summer and negligible over winter. Such a cycle is consistent with precipitation arising from marine boundary layer clouds, suggesting that such clouds are making a substantial contribution to total precipitation over Macquarie Island and the Southern Ocean. Using twice daily upper air soundings (1995–2011), lower troposphere stability parameters show a stronger inversion at night, again consistent with precipitation arising from marine boundary layer clouds. The ERA-Interim precipitation is dominated by a 12 hourly cycle, year around, which is likely to be a consequence of the twice-daily initialisation. The implication of a diurnal cycle in boundary layer clouds over the Southern Ocean to derived A-Train satellite precipitation products is also discussed.

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Dynamical Downscaling of the Climate for the Hawaiian Islands. Part II: Projection for the Late Twenty-First Century

Journal of Climate ◽

10.1175/jcli-d-16-0038.1 ◽

2016 ◽

Vol 29 (23) ◽

pp. 8333-8354 ◽

Cited By ~ 29

Author(s):

Chunxi Zhang ◽

Yuqing Wang ◽

Kevin Hamilton ◽

Axel Lauer

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Dynamical Downscaling ◽

Lower Troposphere ◽

Surface Air Temperature ◽

First Century ◽

Trade Wind ◽

Coupled Models ◽

Extreme Rainfall Events ◽

Twenty First Century

Abstract A 20-yr simulation with a fine-resolution regional atmospheric model for projected late twenty-first-century conditions in Hawaii is presented. The pseudo-global-warming method is employed, and the boundary conditions are based on a multimodel mean of projections made with global coupled models run with a moderate greenhouse gas emissions scenario. Results show that surface air temperature over land increases ~2°–4°C with the greatest warming at the highest topographic heights. A modest tendency for the warming to be larger on the leeward sides of the major islands is also apparent. Climatological rainfall is projected to change up to ~25% at many locations. The currently wet windward sides of the major islands will have more clouds and receive more rainfall, while the currently dry leeward sides will generally have even less clouds and rainfall. The average trade wind inversion–base height and the mean marine boundary layer cloud heights are projected to exhibit only small changes. However, the frequency of days with clearly defined trade wind inversions is predicted to increase substantially (~83% to ~91%). The extreme rainfall events are projected to increase significantly. An analysis of the model’s moisture budget in the lower troposphere shows that the increased mean rainfall on the windward sides of the islands is largely attributable to increased boundary layer moisture in the warmer climate. Rainfall changes attributable to mean low-level circulation changes are modest in comparison.

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Constraining the concentration of the hydroxyl radical in a stratocumulus-topped marine boundary layer from sea-to-air eddy covariance flux measurements of dimethylsulfide

Atmospheric Chemistry and Physics ◽

10.5194/acp-9-9225-2009 ◽

2009 ◽

Vol 9 (23) ◽

pp. 9225-9236 ◽

Cited By ~ 33

Author(s):

M. Yang ◽

B. W. Blomquist ◽

B. J. Huebert

Keyword(s):

Boundary Layer ◽

Hydroxyl Radical ◽

Eddy Covariance ◽

Diurnal Cycle ◽

Marine Boundary Layer ◽

High Reactivity ◽

Simple Method ◽

Bromine Oxide ◽

Dms Oxidation ◽

Strong Inversion

Abstract. The hydroxyl radical (OH) is an important oxidant in the troposphere due to its high reactivity and relative abundance. Measuring the concentration of OH in situ, however, is technically challenging. Here we present a simple method of estimating an OH-equivalent oxidant concentration ("effective OH") in the marine boundary layer (MBL) from the mass balance of dimethylsulfide (DMS). We use shipboard eddy covariance measurements of the sea-to-air DMS flux from the Vamos Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in October and November of 2008. The persistent stratocumulus cloud-cover off the west coast of South America and the associated strong inversion between MBL and the free troposphere (FT) greatly simplify the dynamics in this region and make our budget estimate possible. From the observed diurnal cycle in DMS concentration, the nighttime entrainment velocity at the inversion is estimated to be 4 mm s−1. We calculate 1.4(±0.2)×106 OH molecules cm−3 from the DMS budget, which represents a monthly effective concentration and is well within the range of previous estimates. Furthermore, when linearly proportioned according to the intensity of solar flux, the resultant diel OH profile, together with DMS surface and entrainment fluxes, enables us to accurately replicate the observed diurnal cycle in DMS (correlation coefficient over 0.9). The nitrate radical (NO3) is found to have little contribution to DMS oxidation during VOCALS-REx. An upper limit estimate of 1 pptv of bromine oxide radical (BrO) would account for 30% of DMS oxidation and lower the OH concentration to 1.0)×106 OH molecules cm−3. Our effective OH estimate includes the oxidation of DMS by such radicals.

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The Influence of Uniform Flow on Tropical Cyclone Intensity Change

Journal of the Atmospheric Sciences ◽

10.1175/jas3531.1 ◽

2005 ◽

Vol 62 (9) ◽

pp. 3193-3212 ◽

Cited By ~ 11

Author(s):

Joey H. Y. Kwok ◽

Johnny C. L. Chan

Keyword(s):

Tropical Cyclone ◽

Wind Shear ◽

Mesoscale Model ◽

Vertical Wind Shear ◽

Lower Troposphere ◽

Vertical Motion ◽

Uniform Flow ◽

Vertical Wind ◽

Intensity Change ◽

Westerly Flow

Abstract The influence of a uniform flow on the structural changes of a tropical cyclone (TC) is investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Idealized experiments are performed on either an f plane or a β plane. A strong uniform flow on an f plane results in a weaker vortex due to the development of a vertical wind shear induced by the asymmetric vertical motion and a rotation of upper-level anticyclone. The asymmetric vertical motion also reduces the secondary circulation of the vortex. On a β plane with no flow, a broad anticyclonic flow is found to the southeast of the vortex, which expands with time. Similar to the f-plane case, asymmetric vertical motion and vertical wind shear are also found. This beta-induced shear weakens the no-flow case significantly relative to that on an f plane. When a uniform flow is imposed on a β plane, an easterly flow produces a stronger asymmetry whereas a westerly flow reduces it. In addition, an easterly uniform flow tends to strengthen the beta-induced shear whereas a westerly flow appears to reduce it by altering the magnitude and direction of the shear vector. As a result, a westerly flow enhances TC development while an easterly flow reduces it. The vortex tilt and midlevel warming found in this study agree with the previous investigations of vertical wind shear. A strong uniform flow with a constant f results in a tilted and deformed potential vorticity at the upper levels. For a variable f, such tilting is more pronounced for a vortex in an easterly flow, while a westerly flow reduces the tilt. In addition, the vortex tilt appears to be related to the midlevel warming such that the warm core in the lower troposphere cannot extent upward, which leads to the subsequent weakening of the TC.

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Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10541-2010 ◽

2010 ◽

Vol 10 (21) ◽

pp. 10541-10559 ◽

Cited By ~ 52

Author(s):

S. J. Abel ◽

D. N. Walters ◽

G. Allen

Keyword(s):

Boundary Layer ◽

Diurnal Cycle ◽

Liquid Water ◽

Large Scale ◽

Marine Boundary Layer ◽

Coastal Region ◽

Forecast Model ◽

Good Representation ◽

Liquid Water Path ◽

Water Path

Abstract. Observations in the subtropical southeast Pacific obtained during the VOCALS-REx field experiment are used to evaluate the representation of stratocumulus cloud in the Met Office forecast model and to identify key areas where model biases exist. Marked variations in the large scale structure of the cloud field were observed during the experiment on both day-to-day and on diurnal timescales. In the remote maritime region the model is shown to have a good representation of synoptically induced variability in both cloud cover and marine boundary layer depth. Satellite observations show a strong diurnal cycle in cloud fraction and liquid water path in the stratocumulus with enhanced clearances of the cloud deck along the Chilean and Peruvian coasts on certain days. The model accurately simulates the phase of the diurnal cycle but is unable to capture the coastal clearing of cloud. Observations along the 20° S latitude line show a gradual increase in the depth of the boundary layer away from the coast. This trend is well captured by the model (typical low bias of 200 m) although significant errors exist at the coast where the model marine boundary layer is too shallow and moist. Drizzle in the model responds to changes in liquid water path in a manner that is consistent with previous ship-borne observations in the region although the intensity of this drizzle is likely to be too high, particularly in the more polluted coastal region where higher cloud droplet number concentrations are typical. Another mode of variability in the cloud field that the model is unable to capture are regions of pockets of open cellular convection embedded in the overcast stratocumulus deck and an example of such a feature that was sampled during VOCALS-REx is shown.

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An Idealized Cloud-Resolving Framework for the Study of Midlatitude Diurnal Convection over Land

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3640.1 ◽

2011 ◽

Vol 68 (5) ◽

pp. 1041-1057 ◽

Cited By ~ 28

Author(s):

Linda Schlemmer ◽

Cathy Hohenegger ◽

Jürg Schmidli ◽

Christopher S. Bretherton ◽

Christoph Schär

Keyword(s):

Boundary Layer ◽

Relaxation Time ◽

Planetary Boundary Layer ◽

Diurnal Cycle ◽

Lower Troposphere ◽

Relaxation Method ◽

Lower Atmosphere ◽

Soil Conditions ◽

Humidity Profile ◽

Atmospheric Profiles

Abstract This paper introduces an idealized cloud-resolving modeling (CRM) framework for the study of midlatitude diurnal convection over land. The framework is used to study the feedbacks among soil, boundary layer, and diurnal convection. It includes a setup with explicit convection and a full set of parameterizations. Predicted variables are constantly relaxed toward prescribed atmospheric profiles and soil conditions. The relaxation is weak in the lower troposphere and upper soil to allow the development of a realistic diurnal planetary boundary layer. The model is run to its own equilibrium (30 days). The framework is able to produce a realistic timing of the diurnal cycle of convection. It also confirms the development of deeper convection in a more unstably stratified atmosphere. With this relaxation method, the simulated “diurnal equilibrium convection” determines the humidity profile of the lower atmosphere, and the simulation becomes insensitive to the reference humidity profile. However, if a faster relaxation time is used in the lower troposphere, the convection and rainfall become much more sensitive to the reference humidity, consistent with previous studies.

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First model evaluations of height-resolved diurnal water vapour cycles using lidar observations in an Arctic environment

10.5194/egusphere-egu21-10175 ◽

2021 ◽

Author(s):

Shannon Hicks-Jalali ◽

Zen Mariani ◽

Barbara Casati ◽

Sylvie Leroyer ◽

Francois Lemay ◽

...

Keyword(s):

Solar Cycle ◽

Water Vapour ◽

Diurnal Cycle ◽

Weather Prediction ◽

Vertical Motion ◽

The Arctic ◽

Diurnal Cycles ◽

Surface Station ◽

Nwp Model ◽

Total Column

<p>Atmospheric water vapour is a critical component of both meteorological and climatological processes. It is the dominant gas in the greenhouse effect and its diurnal cycle is an essential component of the hydrological cycle. Diurnal water vapour cycles are complex and are a product of several mechanisms, including (but not necessarily limited to): evapotranspiration, advection, large-scale vertical motion, and precipitation. They are dependent on local geography, as well as latitude. Numerical Weather Prediction (NWP) models rely on high-quality water vapour input to provide accurate forecasts, which is particularly difficult in the Arctic due to its extreme weather and harsh environment. Diurnal water vapour cycle observations are also excellent tools for evaluating NWPs due to their complex nature and dependence on multiple processes. Integrated water vapour (IWV), or total column, diurnal water vapour cycles, usually calculated with Global Navigation Satellite Systems (GNSS) instruments, have been the focus of most previous diurnal WV studies; however, height-resolved diurnal cycles provide a more complete picture of the diurnal mechanisms and include vertical motion, which cannot be discerned via IWV measurements. Differential Absorption Lidars (DIALs) are well suited to providing height-resolved diurnal cycles in the boundary layer due to their high vertical and temporal resolution.</p><p>We use the novel Vaisala pre-production DIAL, installed in Iqaluit, Nunavut (63.75 N, 68.55 W), to calculate seasonal height-resolved diurnal WV cycles from 100 m to 1500 m altitude. We also calculate the surface and total column WV diurnal cycles using co-located surface station and GNSS measurements. We find that the first 250 m of the DIAL diurnal cycle magnitudes agree well with the surface station measurements. The phases of the cycle do shift with altitude, and the amplitudes generally increase with altitude. In the summer, all instruments observe a strong 24 hr cycle. As the amount of solar radiation decreases over the year, the 24 hr cycle weakens and the 12 hr cycle begins to dominate in all instruments. While we find a strong correlation between the 24 hr cycle and the solar cycle, we do not observe any correlation between the 12 hr cycle and the solar cycle. Finally, we also compare the DIAL observations to the Environment and Climate Change Canada (ECCC) NWP model. We evaluate both the assimilation of the humidity input and initial water vapour fields, as well as the diurnal cycle over the 24 hour forecast. Future work will include case study comparisons with the Canadian NWP model to assess the model&#8217;s ability to resolve rapid changes in diurnal water vapour.</p>

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