Measurements made by the AVHRR (Advanced Very High Resolution Radiometer)
on board of five NOAA polar orbiting satellites were used to retrieve cloud
optical depth (τ) and cloud droplet effective radius (r[sub eff]) for marine boundary layer
clouds over the Pacific Ocean west of California and over the Southern Ocean near
Tasmania. Retrievals were obtained for 21 days of data acquired between 1987 and
1995 from which over 300 subscenes ~ 256 km x 256 km in size were extracted. On
this spatial scale cloud fields were found to have mean τ between 8 and 32 and mean
r[sub eff] between 6 and 17 μm. The frequency distribution of τ is well approximated by
a two parameter gamma distribution. The gamma distribution also provides a good
fit to the observed r[sub eff] distribution if the distribution is symmetric or positively
skewed but fails for negatively skewed or bi-modal distributions of r[sub eff] which were
also observed.
The retrievals show a relationship between τ and r[sub eff] which is consistent with
a simple "reference" cloud model with reff ~ r[sup 1 / 5]. The proportionality constant
depends on cloud droplet number concentration N and cloud subadiabaticity β
through the parameter N[sub sat] = N/ [sq rt. Β]. Departures from the reference behaviour
occur in scenes with spatially coherent N[sub sat] regimes, separated by a sharp boundary.
AVHRR imagery is able to separate two N[sub sat] regimes if they differ by at least 30%
in most cases.
Satellite retrievals of τ and r[sub eff] were compared with in situ aircraft measurement
near Tasmania. The retrievals overestimated r[sub eff] by 0.7 to 3.6 μm on
different flights, in agreement with results from earlier comparison studies. The
r[sub eff] overestimation was found to be an offset independent of τ. The reference cloud
model and the N[sub sat] retrieval were tested on aircraft data and yield results consistent
with direct in situ measurements of N and 8.
Spectral and multifractal analyses of the spatial structure of cloud visible
radiance, τ and r[sub eff] fields in 34 satellite scenes revealed scale breaks at 3 to 2 km in all analysed scenes in agreement with some earlier observations (Davis et al.
(1996a)) but in contrast with other work (Lovejoy et al. (1993)). The nonstationarity
H(1) and intermittency C(1) parameters were computed for the 34 scenes, stratified
using the reference cloud model and according to mean τ and r[sub eff]. Similar values
of H(1) and C(1) were found in all these categories.
These measurements of the frequency distribution and spatial variability of τ,
r[sub eff], liquid water path (Iwp), and N[sub sat] can be used to place constraints on mesoscale
models of layer clouds.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/8647 |
| Date | 05 1900 |
| Creators | Szczodrak, Malgorzata |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Relation | UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/] |
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