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Flux Measurements for the Ronald H. Brown
Objectives
- Characterize surface forcing, u*, surface-layer stability
- PBL surface forcing
- Gas/particle Deposition
- Cloud macrophysical, microphysical, and optical properties (connect to aerosol-indirect)
- Chemical fluxes (CO2, Ozone)
- Mixing properties - PBL profiles, dynamics, depth of mixed layer, decay of TKE
- Define horizontal spatial structure PBL
- Model intercomparisons
Background
The near-surface concentrations of pollutant gases and aerosols results
from a complicated balance of sources, sinks, chemical reactions, aerosol
dynamics, advection, and turbulent transport. Boundary layer (BL)
turbulence is the dominant form of vertical mixing near the surface.
The sources of BL turbulence are friction and convection (surface warmer
than air) at the surface plus velocity shear and cloud top cooling
(upside down convection) in the upper BL. Over the ocean, there tend
to be few sources of pollutants of current interest, although the ocean
may be a source of gases that react to produce or remove specific
pollutants. Thus, when pollutants happen to be brought ashore by a
local sea breeze, they typically have their source region somewhere over
land (elsewhere) and have been subject to a variety of physical and
chemical processes both in and above the BL. The balance of BL versus
above the BL processes is highly dependent on the depth of the BL and
the strength of turbulent mixing in the BL - BL characteristics that
are highly sensitive the original meteorological properties of the BL
air overland and the history of its thermal interactions with the
ocean surface.
For the BL itself and for gaseous/aerosol properties, key BL variables
to consider are surface fluxes, depth of the BL, entrainment
velocity/fluxes (BL exchange with the free troposphere), and the BL
turbulence intensity (strength of mixing). The BL wind profile may
also be important, particularly when there is directional shear. For
example, an afternoon convective BL overland (with depth 2 km) might
advect over the ocean and quickly form a stable BL with depth 0.1 km.
Turbulence in the original BL will decay in about an hour and contact with
the surface will be lost for pollutants between 0.1 and 2.0 km. In this
layer changes in concentration that depend on surface and entrainment
processes will essentially cease until the BL is re-energized by passing
over warm land or water. Opposite effects can occur when overland, night
time stable BL's are advected over the ocean; highly stratified
concentrations from, say elevated smokestacks, could be quickly mixed to
the surface over the ocean.
Purpose
The ETL effort for NEAQS will focus primarily on defining the BL properties
and the physical processes that led to the observed properties. This
latter aspect is important for improving numerical model realizations of
BL properties on the coastal region and for interpreting the observed
temporal/spatial evolution of gaseous/aerosol concentrations. The
properties of interest will be separated into meteorological and chemical.
Measurements
The meteorological measurements will feature a combination of episodic
(rawinsondes) and continuous (various remote sensors) definition of the
mean meteorological profiles on roughly 1-hr time scales. The remote
sensors will primarily feature Doppler wind and turbulence profiling
(radar, lidar, and sodar). Direct measurements of turbulent fluxes of
momentum, heat, and moisture (and associated bulk meteorological sea-air
near surface properties) from the ship's foremast will yield key scaling
parameters for BL evolution and characterization of surface removal of
pollutants. We also propose to do direct turbulent flux measurements for
CO2 and O3 to check the NOAA/COARE gas transfer model. A high-resolution
ozone lidar will be used to provide continuous monitoring of ozone
vertical structure. The combination of O3 surface fluxes and O3 profiles
will allow a good 1-D look at the local processes affecting O3
concentration. Solar and IR radiative flux sensors are part of the ETL
flux package; they will allow us to close the surface energy budget to
provide an important check on model BL results. A ceilometer is
included as a simple way to monitor the low cloud statistics; this
system also yields some crude information on aerosol backscatter profiles.
High vertical resolution and the ability to resolve the near surface
part of the profiles will be important because of the expected
occurrences of stable BL cases. Several steps will be taken to improve
on the results from NEAQS-2002.
- The ozone lidar's highest resolution capability will be operating.
- The wind profiler will be converted to RIMS operation
(a multifrequency method that improves vertical resolution from 100-m
to better than 10-m). This will yield much more accurate determinations
of the inversion height, but may or may not improve the low-level
performance of the radar.
- The ETL Doppler sodar will be deployed. The sodar has 1-m resolution
and is ideal for resolving stable BL turbulence regions. This system
has not been used at sea before, so it will be tested at a reasonable
opportunity. The sodar will provide the BL depth in even the most
stable situations. The wind profiler will continue to provide the depth
of the inversion associated with the residual BL above the new stable BL.
The residual inversion height becomes critical when the BL is re-energized
and starts mixing through convective processes.
- The HRDL lidar can perform conical scans at various angles to
determine wind profiles. If low grazing angles are used, then very
high-resolution near-surface profiles are obtained. Turbulence
information can be obtained from the scan-to-scan variability of
the wind profiles.
We also suggest a feasibility study of the scanning C-band Doppler
radar's ability to resolve spatial information on the BL. This radar
is expected to have sufficient sensitivity to acquire usable signal from
the marine BL out to ranges on the order of 30 km. Three-dimensional
volume scans can be processed to map the inversion within 30-km of the
ship. We also anticipate a sharp change in properties at the land-sea
breeze front.
Table 1. Instruments and measurements suggested for deployment by ETL for
the NOAA Ship Ronald H. Brown NEAQS-04 project.
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Item
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System
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Measurement
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1
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Motion/navigation package†
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Motion correction for turbulence
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2
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Sonic anemometer/thermometer†
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Direct covariance turbulent fluxes
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3
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LiCor Li-7500 fast H20/CO2
sensor†
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Direct moisture and CO2 turbulent
fluxes
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4
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Sea snake SST, air
temperature/RH†
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Near-surface meteorology, bulk
turbulent fluxes
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5
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Pyrgeometer\Pyranometer
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Downward IR and Solar radiative
fluxes
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6
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Ceilometer*
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Cloud-base height, cloud fraction
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7
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Doppler Mini-Sodar*
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High-resolution turbulence profiles
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8
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Rawinsonde
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Wind, temperature, humidity prof.
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9
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0.92 GHz Doppler radar profiler*
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Wind profiles, BL microturbulence
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10
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Ozone Lidar
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BL and higher O3 profiles
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11
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mini-MOPA Lidar
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BL wind/aerosol profiles
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12
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AL fast Ozone sensor†
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Direct O3 turbulent flux
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13
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Ship’s Data System (Selected
variables logged directly on ETL
flux system)
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Navigation, meteorological
sensors, thermosalinograph
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14
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60 Ghz Scanning microwave
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Near-surface T profile
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15
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Ronald H. Brown C-band radar
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BL wind profiles, spatial structure
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