Transcript The Lifecycle of the Convective Boundary Layer: Morning Transition

Boundary layer observations with radar wind profilers
and other ground-based remote sensors
Wayne M. Angevine
CIRES, University of Colorado,
and
NOAA ESRL
Outline
 Wind profiler
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Principles of operation
Quantities measured
Time & height resolution
Uncertainties
 Other ground-based remote sensors
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Lidars
Sodars
 Applications
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Air quality
Weather forecasting / modeling
 Science examples
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Assimilation into mesoscale models
Morning transition
Entrainment
Afternoon transition
Coastal flows
Diurnal / slope flows
Thermal structure (statistics by lidar)
What’s a profiler?
 Properly “radar wind profiler”
 Sensitive Doppler radar
 Vertical beam and 2-4 beams at
15-20° off vertical
 Low power, long dwell time, and
low cost compared to weather
radars
 Return signal is Bragg scattered
from refractivity variations in
clear air
• Any hydrometeors or insects
may contribute or even
dominate
 Range depends on frequency
• BL profilers are at UHF
(typically ~1 GHz)
 Radio acoustic sounding
(RASS) attachment for
temperature profiling
Doppler Beam Swinging vs Spaced Antenna
Doppler shift along 3 or 5 beam
directions to measure winds
Traces backscattered signal
motions over 3 or 4 receivers
10 – 30 minute wind
measurement
1 – 10 minute wind
measurement
Source: Bill
Brown NCAR
Performance of a typical BL profiler
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Wind measurement time 10 – 60 min
Height resolution 60 – 200 m
Minimum range 120 m
Max range: at least to BL top
Wind component precision ~1 m/s
• May be better but no way to prove it
 Careful QA required:
• Low signal
• Birds
• Hydrometeors
What data does a profiler produce?
 Winds
• from radial Doppler velocities
 Reflectivity
• in clear air: product of humidity gradient and turbulence
intensity
• in precipitation: dominated by hydrometeor scattering
• insects: 10 microbugs = typical clear air reflectivity
 Spectral width
• a measure of velocity distribution in the sample volume
• in clear air: turbulence intensity (qualitative)
• in precipitation: information about size distribution and/or
turbulence
Example of a
Precipitating Cloud
System Passing
over a Profiler
during TEFLUN B
Horizontal Axis:
Time – 6 hours
Vertical Axis:
Altitude – 11 km
Data are collected:
Every minute
30 second dwell
100 meter vertical
resolution
(actual-105m)
Courtesy of Ken Gage
Other ground-based remote sensors
 Lidar and Sodar use principles similar to radar
 Many types of lidars exist
 Lidars provide:
• very fine resolution
• fast sampling
• measurements of water vapor, ozone, particulate characteristics
(some types)
 Lidar disadvantages:
• cost (capital and operational)
• limited by cloud
 Sodar advantages:
• low cost
• low minimum range
 Sodar disadvantages:
• noise pollution
• impacted by ambient noise (including wind and rain)
• low maximum range
Applications
 Weather analysis & forecasting
 Air quality (non-weather analysis and forecasting)
 Process studies
Current Profiler Displays on AWIPS
Time-Height Section of Hourly Data
Isobaric Map of Hourly Data
Perspective Wind Profile Display
Source: Steve
Koch, FSL
Assimilating profiler data into a
mesoscale model for process studies
 How often does a sea breeze occur
in the simulation AND
measurement?
 Definition: Northerly component >1
m/s between 0600 and 1200 UTC
and southerly >1 m/s after 1200
UTC
 Assimilating 1 profiler with FDDA
 WRF at 5 km grid for Houston
 FDDA or FDDA+1hSST run closer
to measurement at all 7 sites (at
least a little)
 Results not sensitive to threshold
Red is FDDA run
Blue has FDDA, 1-h SST, and reduced soil moisture
Green has reduced soil moisture only
Coastal winds
 Pease is on the
mainland
 Appledore is on an
island ~10 km
offshore
 Coastline oriented
northwestsoutheast locally
 Low-level jet
stronger offshore
early
 Sea breeze in
afternoon
Atmospheric Boundary Layer
Diurnal Variation
2000
Height (meters)
Inversion
1500
1000
Residual Layer
Convective
Mixed Layer
Residual Layer
500
Stable
Stable (nocturnal)
(nocturnal) Layer
Layer
0
Sunrise
Noon
Sunset
Sunrise
Adapted from Introduction to Boundary Layer Meteorology -R.B. Stull, 1988
How does a
profiler see the
ABL?
 Reflectivity is roughly
the product of
humidity gradient and
turbulence intensity
Coastal BL with sea breeze
 Pease day 215 2002
Marine BL
 Appledore day 181 2002
Overcast and rain
 Pease day 196 2002
Spatial variation of BL height
 Urban dome or urban
heat island measured by
profilers in urban core
and in surrounding rural
areas
 Implications for pollutant
concentration and
transport
Lidar time-height
cross-sections of
w with the same
time scale
comparing a
day with light wind
(top: U = 2.2 m/s)
with moderate wind
(bottom:
U = 7.2 m/s).
Courtesy of Don
Lenschow
Lidar time-height crosssections of w with the same
aspect ratio (AR ≈ 7.8)
comparing a day with light
winds (top: U = 2.2 m/s)
with moderate wind (bottom:
U = 7.2 m/s). Courtesy of Don
Lenschow
Time-height cross-section of w for
16 August 2996 with U = 2.2 m/s
and AR ≈ 1.0
Courtesy of Don Lenschow
Morning and evening transitions and BL
top entrainment
 Truly stationary BLs are unusual
 Transitions are critical for air quality and dispersion
applications
 Temporal transitions may cast light on spatial (e.g.
coastal) transitions
 Entrainment is poorly characterized
 Profiler (and lidar) data provide a BL-top perspective
to supplement more traditional in-situ surface or
tower viewpoints
Morning transition
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Establishes initial conditions for ABL growth
Prognostics require initialization
Models must be calibrated and validated
Profiler observations provide estimate of end of
transition (onset of daytime convective ABL)
 Data from two sites
• Tower observations from Cabauw provide detailed insights
• Long profiler and surface flux dataset from Flatland (Illinois)
Timing of transition events
(composite median)
Entrainment
 Definition: Incorporation of air from the free
troposphere into the turbulent (convective) ABL
 A change of condition (laminar to turbulent) but not
necessarily of position
 One of the two largest terms in the ABL heat and
moisture budgets
 Poorly understood and crudely parameterized
 Difficult to measure
Entrainment from heat budget
Entrainment flux =
– heat storage + surface flux + radiative heating – advection
Entrainment ratio = – entrainment flux / surface flux
Measurements during Flatland (Illinois) experiments
 ABL depth from profiler reflectivity (3 profilers)
 Temperature change from RASS (BL average)
 Surface flux from 3 Flux-PAM stations (NCAR)
 Radiative heating from radiation model + aerosol measurements
 Advection from Eta model
Heat budget results
(mean of all good hours)
zi
Fraction of total heating rate
Partitioning
Entrainment flux
-0.050.01 K m s-1
Radiative heating
Advection?
0.030.002 K m s-1
0.0010.005 K m s-1
0.100.004 K m s-1
Surface flux
Variability of partitioning
Afternoon transition
 Transition between fully-developed daytime
convective ABL and nocturnal ABL
 How does turbulence vary with time and height in the
afternoon?
• Sudden collapse or a gradual decline?
• When does transition start?
 Timing and shape of transition are critical to initiation
of inertial oscillation / low-level jet, nighttime
transport, distribution of pollutants, etc.
 Unforced transition – all budget terms are important,
few simplifications are possible
 Measurements from Flatland profiler
• Simple homogeneous terrain
Profiler reflectivity and spectral width
patterns for a “typical” day
Doppler spectral width
When does transition start?
 Three different
definitions based
near daytime
max. ABL height
 All definitions
show transition
starting well
before sunset
sunset
Final thoughts
 Ground-based remote sensors provide continous
data in a column or volume
• a valuable complement to sparse aircraft measurements
 Can be (and usually should be) deployed in groups
 Wind profilers are good for much more than just wind
 Output must be used carefully – beware of “black
boxes”
References (1)
Angevine, W.M., A.B. White, and S.K. Avery, 1994: Boundary layer depth and entrainment zone characterization with a
boundary layer profiler. Boundary Layer Meteor., 68, 375-385.
Angevine, W.M., and J.I. MacPherson, 1995: Comparison of wind profiler and aircraft wind measurements at Chebogue
Point, Nova Scotia. J. Atmos. Oceanic Technol., 12, 421-426.
Carter, D.A., K.S. Gage, W.L. Ecklund, W.M. Angevine, P.E. Johnston, A.C. Riddle, J. Wilson, and C.R. Williams, 1995:
Developments in UHF lower tropospheric wind profiling at NOAA's Aeronomy Laboratory. Radio Sci., 30, 977-1001.
Riddle, A.C., W.M. Angevine, W.L. Ecklund, E.R. Miller, D.B. Parsons, D.A. Carter, and K.S. Gage, 1996: In situ and
remotely sensed horizontal winds and temperature intercomparisons obtained using Integrated Sounding Systems
during TOGA COARE. Contributions to Atmospheric Physics, 69, 49-62.
Angevine, W.M., 1997: Errors in mean vertical velocities measured by boundary layer wind profilers. J. Atmos. Oceanic.
Technol., 14, 565-569.
Angevine, W.M., P.S. Bakwin, and K.J. Davis, 1998: Wind profiler and RASS measurements compared with measurements
from a 450 m tall tower. J. Atmos. Oceanic. Technol., 15, 818-825.
Grimsdell, A.W., and W.M. Angevine, 1998: Convective boundary layer height measured with wind profilers and compared
to cloud base. J. Atmos. Oceanic Technol., 15, 1332-1339.
Angevine, W.M., 1999: Entrainment results including advection and case studies from the Flatland boundary layer
experiments. J. Geophys. Res., 104, 30947-30963.
References (2)
Cohn, S.A., and W.M. Angevine, 2000: Boundary layer height and entrainment zone thickness measured by lidars
and wind profiling radars. J. Appl. Meteorol., 39, 1233-1247.
Angevine, W.M., and K. Mitchell, 2001: Evaluation of the NCEP mesoscale Eta model convective boundary layer
for air quality applications. Mon. Wea. Rev., 129, 2761-2775.
Angevine, W.M., H. Klein Baltink, and F.C. Bosveld, 2001: Observations of the morning transition of the convective
boundary layer. Boundary-Layer Meteorol., 101, 209-227.
Grimsdell, A.W., and W.M. Angevine, 2002: Observations of the afternoon transition of the convective boundary
layer. J. Appl. Meteorol., 41, 3-11.
Angevine, W.M., C.J. Senff, and E.R. Westwater, 2002: Boundary Layers/Observational techniques -- Remote.
Encyclopedia of Atmospheric Sciences, J.R. Holton, J. Pyle, and J.A. Curry, Eds., Academic Press, 271-279.
Angevine, W.M., A.B. White, C.J. Senff, M. Trainer, and R.M. Banta, 2003: Urban-rural contrasts in mixing height
and cloudiness over Nashville in 1999. J. Geophys. Res., 108(D3), doi:10.1029/2001JD001061.
Nielsen-Gammon, J.W., R.T. McNider, W.M. Angevine, A.B. White, and K. Knupp, 2007: Mesoscale model
performance with assimilation of wind profiler data: Sensitivity to assimilation parameters and network
configuration. J. Geophys. Res., 112, D09121, doi:10.1029/2006JD007633.
Angevine, W.M., 2008: Transitional, entraining, cloudy, and coastal boundary layers. Acta Geophysica, 56, 2-20.
Acknowledgements
 Ken Gage, Don Lenschow for slides