scholarly journals Vertical Velocity Statistics in Fair-Weather Cumuli at the ARM TWP Nauru Climate Research Facility

2010 ◽  
Vol 23 (24) ◽  
pp. 6590-6604 ◽  
Author(s):  
Pavlos Kollias ◽  
Bruce Albrecht
Keyword(s):  
Boundary Layer ◽  
Numerical Models ◽  
Radar Data ◽  
Cloud Fraction ◽  
Cloud Base ◽  
Fair Weather ◽  
Research Facility ◽  
Climate Research ◽  
Velocity Statistics ◽  
Air Motion

Abstract Fair-weather cumuli are fundamental in regulating the vertical structure of water vapor and entropy in the lowest 2–3 km of the earth’s atmosphere over vast areas of the oceans. In this study, a long record of profiling cloud radar observations at the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) at Nauru Island is used to investigate cloud vertical air motion statistics over an 8-yr observing period. Appropriate processing of the observed low radar reflectivities provides radar volume samples that contain only small cloud droplets; thus, the Doppler velocities are used as air motion tracers. The technique is applied to shallow boundary layer clouds (less than 1000 m thick) during the 1999–2007 period when radar data are available. Using the boundary layer winds from the soundings obtained at the Nauru ACRF, the fair-weather cumuli fields are classified in easterly and westerly boundary layer wind regimes. This distinction is necessary to separate marine-forced (westerlies) from land-forced (easterlies) shallow clouds because of a well-studied island effect at the Nauru ACRF. The two regimes exhibit large diurnal differences in cloud fraction and cloud dynamics as manifested by the analysis of the hourly averaged vertical air motion statistics. The fair-weather cumuli fields associated with easterlies exhibit a strong diurnal cycle in cloud fraction and updraft strength and fraction, indicating a strong influence of land-forced clouds. In contrast over the fair-weather cumuli with oceanic origin, land-forced clouds are characterized by uniform diurnal cloudiness and persistent updrafts at the cloud-base level. This study provides a unique observational dataset appropriate for testing fair-weather cumulus mass flux and turbulence parameterizations in numerical models.

scholarly journals Temporal Variability of Fair-Weather Cumulus Statistics at the ACRF SGP Site

2008 ◽  
Vol 21 (13) ◽  
pp. 3344-3358 ◽  
Author(s):  
Larry K. Berg ◽  
Evgueni I. Kassianov
Keyword(s):  
Great Plains ◽  
Sensible Heat Flux ◽  
Cloud Fraction ◽  
Surface Fluxes ◽  
Chord Length ◽  
Cloud Base ◽  
Fair Weather ◽  
Cloud Properties ◽  
Cloud Thickness ◽  
Cloud Base Height

Abstract Continental fair-weather cumuli exhibit significant diurnal, day-to-day, and year-to-year variability. This study describes the climatology of cloud macroscale properties, over the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site. The diurnal cycle of cloud fraction, cloud-base height, cloud-top height, and cloud thickness were well defined. The cloud fraction reached its maximum value near 1400 central standard time. The average cloud-base height increased throughout the day, while the average cloud thickness decreased with time. In contrast to the other cloud properties, the average cloud-chord length remained nearly constant throughout the day. The sensitivity of the cloud properties to the year-to-year variability of precipitation and day-to-day changes in the height of the lifting condensation level (zLCL) and surface fluxes were compared. The cloud-base height was found to be sensitive to both the year, zLCL, and the surface fluxes of heat and moisture; the cloud thickness was found to be more sensitive to the year than to zLCL; the cloud fraction was sensitive to both the low-level moisture and the surface sensible heat flux; and cloud-chord length was sensitive to zLCL. Distributions of the cloud-chord length over the ACRF SGP site were computed and were well fit by an exponential distribution. The contribution to the total cloud fraction by clouds of each cloud-chord length was computed, and it was found that the clouds with a chord length of about 1 km contributed most to the observed cloud fraction. This result is similar to observations made with other remote sensing instruments or in modeling studies, but it is different from aircraft observations of the contribution to the total cloud fraction by clouds of different sizes.

Measuring the variability of the shallow convective mass flux profiles in the tropical trade wind region

2020 ◽  
Author(s):  
Marcus Klingebiel ◽  
Heike Konow ◽  
Bjorn Stevens
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Mass Flux ◽  
Doppler Radar ◽  
Cloud Fraction ◽  
Cloud Base ◽  
Trade Wind ◽  
Geophysical Research ◽  
Wind Region ◽  
Convective Mass Flux

<p>Mass flux is a key parameter to represent shallow convection in global circulation models. To estimate the shallow convective mass flux as accurately as possible, observations of this parameter are necessary. Prior studies from Ghate et al. (2011) and Lamer et al. (2015) used Doppler radar measurements over a few months to identify a typical shallow convective mass flux profile based on cloud fraction and vertical velocity. In this study, we extend their observations by using long term remote sensing measurements at the Barbados Cloud Observatory (13° 09’ N, 59° 25’ W) over a time period of 30 months and check a hypothesis by Grant (2001), who proposed that the cloud base mass flux is just proportional to the sub-cloud convective velocity scale. Therefore, we analyze Doppler radar and Doppler lidar measurements to identify the variation of the vertical velocity in the cloud and sub-cloud layer, respectively. Furthermore, we show that the in-cloud mass flux is mainly influenced by the cloud fraction and provide a linear equation, which can be used to roughly calculate the mass flux in the trade wind region based on the cloud fraction.</p><p> </p><p>References:<br>Ghate,  V.  P.,  M.  A.  Miller,  and  L.  DiPretore,  2011:   Vertical  velocity structure of marine boundary layer trade wind cumulus clouds. Journal  of  Geophysical  Research: Atmospheres, 116  (D16), doi:10.1029/2010JD015344.</p><p>Grant,  A.  L.  M.,  2001:   Cloud-base  fluxes  in  the  cumulus-capped boundary layer. Quarterly Journal of the Royal Meteorological Society, 127 (572), 407–421, doi:10.1002/qj.49712757209.</p><p>Lamer, K., P. Kollias, and L. Nuijens, 2015:  Observations of the variability  of  shallow  trade  wind  cumulus  cloudiness  and  mass  flux. Journal of Geophysical Research: Atmospheres, 120  (12), 6161–6178, doi:10.1002/2014JD022950.</p>

A 19-Month Record of Marine Aerosol–Cloud–Radiation Properties Derived from DOE ARM Mobile Facility Deployment at the Azores. Part I: Cloud Fraction and Single-Layered MBL Cloud Properties

2014 ◽  
Vol 27 (10) ◽  
pp. 3665-3682 ◽  
Author(s):  
Xiquan Dong ◽  
Baike Xi ◽  
Aaron Kennedy ◽  
Patrick Minnis ◽  
Robert Wood
Keyword(s):  
Boundary Layer ◽  
High Pressure ◽  
Liquid Water ◽  
Mixed Boundary ◽  
Cloud Condensation Nuclei ◽  
Low Pressure ◽  
Cloud Fraction ◽  
Cloud Base ◽  
Microphysical Properties ◽  
Cloud Properties

Abstract A 19-month record of total and single-layered low (<3 km), middle (3–6 km), and high (>6 km) cloud fractions (CFs) and the single-layered marine boundary layer (MBL) cloud macrophysical and microphysical properties was generated from ground-based measurements at the Atmospheric Radiation Measurement Program (ARM) Azores site between June 2009 and December 2010. This is the most comprehensive dataset of marine cloud fraction and MBL cloud properties. The annual means of total CF and single-layered low, middle, and high CFs derived from ARM radar and lidar observations are 0.702, 0.271, 0.01, and 0.106, respectively. Greater total and single-layered high (>6 km) CFs occurred during the winter, whereas single-layered low (<3 km) CFs were more prominent during summer. Diurnal cycles for both total and low CFs were stronger during summer than during winter. The CFs are bimodally distributed in the vertical with a lower peak at ~1 km and a higher peak between 8 and 11 km during all seasons, except summer when only the low peak occurs. Persistent high pressure and dry conditions produce more single-layered MBL clouds and fewer total clouds during summer, whereas the low pressure and moist air masses during winter generate more total and multilayered clouds, and deep frontal clouds associated with midlatitude cyclones. The seasonal variations of cloud heights and thickness are also associated with the seasonal synoptic patterns. The MBL cloud layer is low, warm, and thin with large liquid water path (LWP) and liquid water content (LWC) during summer, whereas during winter it is higher, colder, and thicker with reduced LWP and LWC. The cloud LWP and LWC values are greater at night than during daytime. The monthly mean daytime cloud droplet effective radius re values are nearly constant, while the daytime droplet number concentration Nd basically follows the LWC variation. There is a strong correlation between cloud condensation nuclei (CCN) concentration NCCN and Nd during January–May, probably due to the frequent low pressure systems because upward motion brings more surface CCN to cloud base (well-mixed boundary layer). During summer and autumn, the correlation between Nd and NCCN is not as strong as that during January–May because downward motion from high pressure systems is predominant. Compared to the compiled aircraft in situ measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX), the cloud microphysical retrievals in this study agree well with historical aircraft data. Different air mass sources over the ARM Azores site have significant impacts on the cloud microphysical properties and surface CCN as demonstrated by great variability in NCCN and cloud microphysical properties during some months.

scholarly journals Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed-layer

2010 ◽  
Vol 10 (4) ◽  
pp. 9219-9252 ◽  
Author(s):  
A. Ansmann ◽  
J. Fruntke ◽  
R. Engelmann
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Unit Length ◽  
Mixing Layer ◽  
Average Frequency ◽  
Cloud Base ◽  
Doppler Lidar ◽  
Fair Weather ◽  
Pronounced Increase ◽  
Horizontal Extent

Abstract. For the first time, a comprehensive, height-resolved Doppler lidar study of updrafts and downdrafts in the mixing layer is presented. The Doppler lidar measurements were performed at Leipzig, Germany, in the summer half year of 2006. The conditional sampling method is applied to the measured vertical velocities to identify, count, and analyze significant updraft and downdraft events. Three cases of boundary layer evolution with and without fair weather cumuli formation are discussed. Updrafts occur with an average frequency of 1–2 per unit length zi (boundary layer depth zi), downdrafts 20%–30% more frequently. In the case with cumuli formation, the draft occurrence frequency is enhanced by about 50% at cloud level or near cloud base. The counted updraft events cover 30%–34%, downdrafts 53%–57% of the velocity time series during the main period of convective activity. By considering all drafts with horizontal extent >36 m in the analysis, the updraft mean horizontal extent ranges from 200–350 m and is about 0.15zi in all three cases. Downdrafts are a factor of 1.3–1.5 larger. The average value of the updraft mean vertical velocities is 0.5–0.7 m/s or 0.4w∗ (convective velocity scale w∗), and the negative downdraft mean vertical velocities are weaker by roughly 10%–20%. The analysis of the relationship between the size (horizontal extent) of the updrafts and downdrafts and their mean vertical velocity reveals a pronounced increase of the average vertical velocity in updrafts from 0.4–0.5 m/s for small thermals (100–200 m) to about 1.5 m/s for large updrafts (>600 m) in the case with fair weather cumuli. At cloudless conditions, the updraft velocities were found to be 20% smaller for the large thermals.

Boundary Layer Turbulence and Orographic Precipitation Growth in Cold Clouds: Evidence from Profiling Airborne Radar Data

2011 ◽  
Vol 68 (10) ◽  
pp. 2344-2365 ◽  
Author(s):  
Bart Geerts ◽  
Qun Miao ◽  
Yang Yang
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Radar Data ◽  
Windward Side ◽  
Inertial Subrange ◽  
Cloud Base ◽  
Orographic Precipitation ◽  
Turbulent Layer ◽  
Near Surface ◽  
Wide Range

Abstract Airborne vertically pointing Doppler radar data collected in 10 winter storms over the Medicine Bow Range in Wyoming are used to examine the importance of boundary layer (BL) turbulence for orographic precipitation growth. In all 10 cases, the cloud-base temperature was below 0°C and the bulk Froude number was more than 1.0, implying little or no blocking of the flow by the mountain barrier. Seven of the 10 storms sampled were postfrontal, with weak static stability and relatively shallow cloud tops. Doppler vertical velocity transects depict an approximately 1-km-deep turbulent layer draped over the terrain, sometimes clearly distinct from the stratified flow in the free troposphere aloft, where vertical motion is largely controlled by gravity wave dynamics. Spectral analysis of near-surface Doppler vertical velocity data in terrain-following coordinates reveals an inertial subrange with decreasing power with height toward the BL top. The composite of radar data profiles from the 10 flights is analyzed in frequency-by-altitude diagrams, with altitude expressed above ground level. These diagrams indicate a wide range of vertical velocities in the BL, and rapid snow growth within the BL as air rises through the cloud base, especially when BL turbulence is more intense. This snow growth is concentrated on the windward side of mountains, above the terrain–cloud base intersection. The dominant snow growth mechanism in the BL (i.e., by accretion or vapor deposition) cannot be established because of restrictions in aircraft flight level over complex terrain. Snow aggregation may have contributed to the observed rapid increase in reflectivity in the BL along the windward slope.

scholarly journals Validation and deployment of the first Lidar based weather observation network in New York State: The NYS MesoNet Project

2018 ◽  
Vol 176 ◽  
pp. 09010
Author(s):  
L. Thobois ◽  
J. Freedman ◽  
P. Royer ◽  
J. Brotzge ◽  
E. Joseph
Keyword(s):  
Boundary Layer ◽  
New York ◽  
Wind Profile ◽  
Numerical Models ◽  
New York State ◽  
Data Availability ◽  
Extreme Weather Events ◽  
Cloud Base ◽  
York State ◽  
Severe Thunderstorms

The number and quality of atmospheric observations used by meteorologists and operational forecasters are increasing year after year, and yet, consistent improvements in forecast skill remains a challenge. While contributing factors involving these challenges have been identified, including the difficulty in accurately establishing initial conditions, improving the observations at regional and local scales is necessary for accurate depiction of the atmospheric boundary layer (below 2km), particularly the wind profile, in high resolution numerical models. Above the uncertainty of weather forecasts, the goal is also to improve the detection of severe and extreme weather events (severe thunderstorms, tornadoes and other mesoscale phenomena) that can adversely affect life, property and commerce, primarily in densely populated urban centers. This paper will describe the New York State Mesonet that is being deployed in the state of New York, USA. It is composed of 126 stations including 17 profiler sites. These sites will acquire continuous upper air observations through the combination of WINDCUBE Lidars and microwave radiometers. These stations will provide temperature, relative humidity & “3D” wind profile measurements through and above the planetary boundary layer (PBL) and will retrieve derived atmospheric quantities such as the PBL height, cloud base, momentum fluxes, and aerosol & cloud optical properties. The different modes and configurations that will be used for the Lidars are discussed. The performances in terms of data availability and wind accuracy and precision are evaluated. Several profiles with specific wind and aerosol features are presented to illustrate the benefits of the use of Coherent Doppler Lidars to monitor accurately the PBL.

scholarly journals Relationship between a Weakening Cold Front, Misocyclones, and Cloud Development on 10 June 2002 during IHOP

2006 ◽  
Vol 134 (1) ◽  
pp. 311-335 ◽  
Author(s):  
Nettie R. Arnott ◽  
Yvette P. Richardson ◽  
Joshua M. Wurman ◽  
Erik M. Rasmussen
Keyword(s):  
Boundary Layer ◽  
Cold Front ◽  
Radar Data ◽  
Cloud Base ◽  
Small Scale ◽  
Upward Motion ◽  
Cumulus Clouds ◽  
Dry Air ◽  
Cloud Development ◽  
Nearly Continuous

Abstract The finescale structure and evolution of a cold front in the presence of small-scale circulations are examined using overdetermined dual-Doppler syntheses of mobile radar data along with thermodynamic data and cloud imagery collected on 10 June 2002 during the International H2O Project (IHOP). Linear clear-air reflectivity maxima and open cellular convection intersect the cold front, causing spatial variations in convergence along the front. Small-scale vertical vorticity maxima (misocyclones) often are coincident with these intersections and are associated with vertical velocity maxima. Throughout the deployment, trajectory analyses indicate that parcels entering the frontal circulation travel predominately in the along-front direction. During the first hour of the deployment, upward motion is nearly continuous along the front. Consequently, parcels remain in regions of upward motion as they move along the front, and many eventually ascend to cloud base. In response, a line of cumulus clouds develops along the front. Later in the deployment, however, enhanced warming behind the cold front causes the frontal circulation to weaken. Small-scale features such as misocyclones play a larger role in organizing upward motion along the front than when the front was stronger. Misocyclones contort the weakened cold front and are associated with kinks in radar reflectivity and fractures in upward motion. Parcels moving along the front now experience regions of both upward and downward forcing due to this fracturing. Hence, many of these parcels do not retain upward motion long enough to reach cloud base, and clouds along the cold front dissipate in the dry air above the boundary layer. Parcels that remain coincident with upward motion maxima, such as those maxima associated with misocyclones, however, often still reach the top of the radar domain and presumably cloud base.

scholarly journals Long-Term Observations of the Convective Boundary Layer Using Insect Radar Returns at the SGP ARM Climate Research Facility

2010 ◽  
Vol 23 (21) ◽  
pp. 5699-5714 ◽  
Author(s):  
Arunchandra S. Chandra ◽  
Pavlos Kollias ◽  
Scott E. Giangrande ◽  
Stephen A. Klein
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Convective Boundary Layer ◽  
Turbulent Transport ◽  
Doppler Velocity ◽  
Research Facility ◽  
Convective Boundary ◽  
Climate Research ◽  
Turbulent Statistics

Abstract A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes–thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

scholarly journals Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

2010 ◽  
Vol 10 (16) ◽  
pp. 7845-7858 ◽  
Author(s):  
A. Ansmann ◽  
J. Fruntke ◽  
R. Engelmann
Keyword(s):  
Boundary Layer ◽  
Vertical Velocity ◽  
Unit Length ◽  
Mixing Layer ◽  
Average Frequency ◽  
Cloud Base ◽  
Doppler Lidar ◽  
Fair Weather ◽  
Pronounced Increase ◽  
Horizontal Extent

Abstract. For the first time, a comprehensive, height-resolved Doppler lidar study of updrafts and downdrafts in the mixing layer is presented. The Doppler lidar measurements were performed at Leipzig, Germany, in the summer half year of 2006. The conditional sampling method is applied to the measured vertical velocities to identify, count, and analyze significant updraft and downdraft events. Three cases of atmospheric boundary-layer (ABL) evolution with and without fair-weather cumuli formation are discussed. Updrafts occur with an average frequency of 1–2 per unit length zi (boundary-layer depth zi), downdrafts 20–30% more frequently. In the case with cumuli formation, the draft occurrence frequency is enhanced by about 50% at cloud level or near cloud base. The counted updraft events cover 30–34%, downdrafts 53–57% of the velocity time series in the central part of the ABL (subcloud layer) during the main period of convective activity. By considering all drafts with horizontal extent >36 m in the analysis, the updraft mean horizontal extent ranges here from 200–420 m and is about 0.16 zi–0.18 zi in all three cases disregarding the occurrence of cumulus clouds. Downdraft extents are a factor of 1.3–1.5 larger. The average value of the updraft mean vertical velocities is 0.5–0.7 m/s or 0.40 w*–0.45 w* (convective velocity scale w*), and the negative downdraft mean vertical velocities are weaker by roughly 10–20%. The analysis of the relationship between the size (horizontal extent) of the updrafts and downdrafts and their mean vertical velocity reveals a pronounced increase of the average vertical velocity in updrafts from 0.4–0.5 m/s for small thermals (100–200 m) to about 1.5 m/s for large updrafts (>600 m) in the subcloud layer in the case with fair-weather cumuli. At cloudless conditions, the updraft velocities were found to be 20% smaller for the large thermals.

Sign in / Sign up

Export Citation Format

Share Document