FYI
CAOS PhD Thesis DefenceTitle:
Temporal Persistence and Spatial Coherence of Tropical RainfallSpeaker: Ram Ratan
Date:
December 30, 2016 (Friday)Time:
4:00PMVenue: CAOS Seminar Hall
Abstract
This work focuses on documenting the multiscale nature of the temporal
persistence and spatial coherence of tropical rainfall. The first two
parts utilise satellite-retrieved rainfall at multiple space and time
resolutions, and the third part assesses the ability of
state-of-the-art coupled models to reproduce some of the observed
features.
In the first part of the study, which focuses on the temporal
persistence of rain, we analyze the Tropical Rainfall Measurement
Mission (TRMM) satellite-based observations to compare and contrast
wet and dry spell characteristics over the tropics (30S-30N). Defining
a wet (dry) spell as the number of consecutive rainy (nonrainy) days,
we find that the distributions of wet spells (independent of spatial
resolution) exhibit universality in the following sense. While both
ocean and land regions with high seasonal rainfall accumulation (humid
regions) show a predominance of 2-4 day wet spells, those regions with
low seasonal rainfall accumulation (arid regions) exhibit a wet spell
duration distribution that is essentially exponential in nature, with
a peak at 1 day. The behaviour that we observed for wet spells is
reversed for dry spell distributions. The total rainfall accumulated
in each wet spell has also been analyzed, and we find that the major
contribution to seasonal rainfall for arid regions comes from very
short length wet spells; however, for humid regions, this contribution
comes from wet spells of duration as long as 30 days. We also explore
the role of chance in determining the 2-4 day mode as well as the
influence of organized convection in separating reality from chance.
The second part deals with the spatial coherence of tropical rain. We
take two different approaches, namely, a global and local view. The
global view attempts to quantify the conventional view of rain, i.e.,
the dominance of the intertropical convergence zone (ITCZ), while the
local view tries to answer the question: if it rains, how far is the
influence felt in zonal and meridional directions? In both
approaches, the classical e-folding length for spatial decorrelation
is used as a measure of spatial coherence. The major finding in the
global view approach is that, at short timescales of accumulation
(daily to pentad to even monthly), rain over the Equator shows the
most dominant zonal scale. It is only at larger timescales of
accumulation (seasonal or annual) that the dominance of ITCZ around 7N
is evident. The local view quantifies the dominance of the zonal
scale in the tropical ocean convergence zones, with an anisotropy
value (ratio of zonal to meridional scales) of 3-4. Over land, on the
other hand, the zonal and meridional scales are comparable in
magnitude, suggesting that rain tends to be mostly isotropic over
continental regions. This latter finding holds true, irrespective of
the spatial and temporal resolutions at which rain is observed.
Interestingly, the anisotropy over ocean, while invariant with spatial
resolution, is found to be a function of temporal resolution: from a
value of 3-4 at daily timescale, it decreases to around 1.5-2 at
3-hourly resolution.
The final part analyses a few models from the suite of Coupled Model
Intercomparison Project (CMIP5) models, to evaluate their ability to
reproduce some of these aforementioned features. For all the strong
biases that models are known to have, some of the observed features
(e.g., 2-4 day mode in wet spells, isotropic rain over ocean) are
remarkably well reproduced.
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