Tropical Cirrus Cloud Macrophysical Properties over Darwin
from CALIPSO, the ARM MPL and the ARM Raman Lidar
Tyler Thorsen1, Qiang Fu1 and Jennifer Comstock2
1
Department of Atmospheric Sciences, University of Washington
2
Pacific Northwest National Laboratory
Transparent Cloud Fraction
Introduction
I In the tropical atmosphere thin and sub-visible cirrus clouds occur frequently.
I Cirrus clouds are important for tropical tropopause layer (TTL) radiative heating
and constrain the energy budget for various atmospheric processes.
I Characterizing the vertical structure of all cirrus clouds requires lidar observations.
I Statistics of cirrus properties from the Cloud-Aerosol Lidar and Infrared Pathfinder
Satellite Observations (CALIPSO) and the ARM micropulse lidar (MPL)
observations were compared by Thorsen et al. JGR [2011] (T11 ) at the three
ARM TWP sites: Manus, Nauru and Darwin.
I This work uses the new ARM Raman Lidar (RL) to revisit the study of T11 and
help evaluate the performance of all three lidars in observing tropical cirrus clouds.
Previous Work and Objectives
CALIPSO and MPL differences:
I The frequency of occurrence of cirrus in the MPL observations is significantly
smaller, even for transparent profiles [T11 ].
I The MPL signal is more frequently completely attenuated, particularly during the
daytime. During the daytime the MPL detects very little cirrus clouds [T11 ].
I The lack of MPL sensitivity and large amount of attenuated MPL profiles (relative
to CALIPSO) causes very large differences in the associated radiative heating rate
profiles [Thorsen et al., JGR, in review, 2012].
Does the RL improve the set of ground-based cirrus observations
relative to the MPL? How does the sensitivity of the RL compare to
CALIPSO?
CALIPSO and MPL diurnal cycles:
I Cirrus occur more frequently at night [T11 ].
I Cirrus clouds are geometrically thicker at night [T11 ].
I It is possible these effects are real and/or due to increased noise during the
daytime.
The RL elastic channel operates at 355 nm (compared to 532 nm for
CALIPSO/MPL), so the impact of the solar background is less. Do RL
cirrus cloud properties show the same diurnal cycles?
Figure 1: Cloud fraction for transparent profiles. The total number of profiles is given in the upper right of each panel with two
numbers for the ground-based datasets: the first for the dataset collocated in time and the second for all available profiles.
I MPL detects far less cirrus than CALIPSO.
I The RL and CALIPSO show good agreement.
I The RL shows a large improvement over the MPL.
I
I
I
I
Figure 2: Diurnal cycle of transparent cloud fraction.
CALIPSO/MPL: both show a similar diurnal cycle, with the MPL’s larger.
RL/CALIPSO: the maximum in cirrus occurrence (about 15-16 km) increases at night in both observations.
RL/CALIPSO: Below 15-16 km, the RL diurnal cycle is smaller or opposite sign relative to CALIPSO.
When considering all available RL profiles the diurnal cycle is small, with slightly more clouds during the daytime.
Cloud Layer Geometrical Thickness
Datasets
MPL:
I 30 m vertical resolution, averaged to 2 minutes.
I Cloud mask using backscatter (β) following Wang and Sassen, JAM [2001].
I Attenuation determined by comparing above cloud β to that expected from
molecular scattering.
RL:
I 30 m vertical resolution, averaged to 2 minutes.
I Data currently limited to altitudes below ∼ 16.5 km.
I Cloud mask defined as a depolarization ratio (δ) greater than 3% with a random
error less than 20%.
I Profiles where the mean δ over 16–16.5 km falls below 1.5% are considered
attenuated (clear-air δ is calibrated to 2%).
CALIPSO:
I L2 5 km cloud layer product (v3): 30 m/60 m vertical resolution below/above
8 km, multiple horizontal averaging (5, 20, 80 km).
I Cloud mask using β [Vaughan et al., JTECH, 2009]
I Attenuation determined by the presence/lack of surface signal.
Figure 4: Hourly anomaly in median cloud thickness. Sunrise occurs at ∼ 6
and sunset is denoted by a dashed line.
Figure 3: PDFs of cloud thickness for transparent profiles. The median cloud thickness is given in the upper right of each
panel with two numbers for the ground-based datasets: the first for the dataset collocated in time and the second for all
available profiles.
I In general, PDFs agree well within uncertainty intervals but both CALIPSO/MPL suggest clouds are
thicker at night while the RL does not.
Comparison Method
I CALIPSO data taken from a 5◦ by 5◦ domain centered on the ARM Darwin site.
I Two periods of data:
(1) June 2006 through August 2011: MPL and CALIPSO.
(2) December 2010 through April 2012: RL and CALIPSO.
I Two sets of ground-based data:
(1) All available profiles.
(2) Profiles collocated to within ± 4 hours of CALIPSO overpasses.
I Sampling uncertainty calculated using moving block bootstrap resampling method.
Total Cloud Fraction
Dec. 2010 - Apr. 2012
CALIPSO
21.85
RL
4.83
RL, ± 4 hr
5.85
Table 1: Ratio of the daytime background signal to the nighttime background signal.
I Solar background has an order of magnitude larger impact on the MPL relative to
CALIPSO.
I The impact of the solar background on the RL is non-negligible, but is about 4-5
times less than CALIPSO.
http://www.atmos.washington.edu/∼tylert
Conclusions
I The impact of the solar background on the RL is an order of
magnitude less than the MPL and 4-5 times less than CALIPSO.
I The RL shows a large improvement in cirrus cloud detection over
the MPL and agrees well with CALIPSO for transparent profiles.
I The RL observations show a smaller diurnal cycle of cirrus cloud
occurrence relative to CALIPSO/MPL observations.
I Thicker cirrus at night in CALIPSO/MPL observations is likely
non-physical and is due to the impact increased noise during the
daytime.
I Future work includes evaluation of CALIPSO/MPL/RL optical
depth retrievals.
Solar Background
Jun. 2006 - Aug. 2011
CALIPSO
12.34
MPL
195.48
MPL, ± 4 hr 145.41
I RL/MPL: Bad sampling around/at noon.
I Monte-Carlo method used to simulate RL’s monthly
sampling.
I RL suggests that CALIPSO/MPL diurnal cycle is due to
the effect of the solar background.
Figure 5: Cloud fraction for all profiles. The total number of profiles is given in the upper right of each panel with two
numbers for the ground-based datasets: the first for the dataset collocated in time and the second for all available profiles.
We thank Rob Newsom for help calculating the RL solar background. This
research was supported by the Office of Science (BER), U.S. Department of
Energy, grant DE-FG02-09ER64769
I A non-trivial amount of cirrus is missed by the RL due to complete attenuation.
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