CIRRUS COUPLED
CLOUD-RADIATION EXPERIMENT
General Overview of
CIRCCREX
Climate and weather prediction models demand
understanding of how cirrus clouds, high in the troposphere (6-14km in
altitude) affect our climate. Cirrus covers up to 30% of the globe and its
effects should be accurately included in global climate models.
Clouds have two main effects; they are the main
atmospheric component in the hydrological cycle, but they also trap radiation,
both reflecting sunlight back to space (cooling the Earth's surface) and
trapping the thermal energy emitted from the surface (as they are cold,
emitting less energy to space than an equivalent cloudless sky). The balance
between the shortwave (sunlight) and longwave
(thermal radiation) effect depends on factors such as altitude and thickness of
the cloud, and the size and shape of the ice crystals that make up the cloud.
The crystals can take on myriad shapes, and the shapes existing in particular
clouds depend on conditions and on the evolutionary sequence that the particles
experience; growing, aggregating and/or dissipating over time, dependant on the
changes in temperature, humidity and meteorological environment they
experience. Different crystal sizes and shapes reflect and scatter light in
different ways. Some crystal shapes are efficient at reflecting sunlight, but
not thermal radiation and some the other way round. The net effect of a cloud
on the radiation budget depends on the microscopic shapes of the crystals
inside it. By measuring both the heat emitted by the cloud and its internal
crystal properties ('microphysics') we can determine the link between the two,
and hence the overall effect the cloud is having on the climate.
Cirrus
models have been derived that calculate the expected response of different
crystal types across the spectrum, and these are usually combined with
predicted particle size and shapes (Particle Size Distributions, PSD) found
from in-situ flight campaign measurements using cloud probes. These are
parameterised (simplified) and used in climate models and general circulation
models (GCMs), eg. in
numerical weather prediction (NWP) and climate change, but these cirrus models
have not been tested across the full spectrum. Some studies have been made of
specific radiative properties of some crystal types
in the shortwave, and of other crystal types in parts of the longwave, but there has not been a successful measurement
covering the full spectrum simultaneously measuring the precise make up of the crystal sizes and types in a cloud.
We
plan a novel flight campaign combining full spectrum radiative
measurements (125-0.3 microns) from longwave to
shortwave, with state-of-the-art measurements of crystal PSDs, the ice water
content and temperature etc. We will test scattering models and PSD
parameterisations used to describe cirrus cloud in atmospheric models, such as
the UK MetOffice (MO) Unified model Numerical Weather
Prediction (NWP) with model improvements implemented by our MO project
partners.
Our
project is possible because of NERC funded research that led to: state-of-the-art
cloud probe instruments and software tools that addressed problems of ice
crystal shattering at the inlet apertures and the great uncertainty in ice
crystal size distributions of the past; and the development of the unique
far-IR instrument TAFTS
at Imperial College (IC). The ability to measure the entire spectrum from an
aircraft, and so simultaneously measure the cirrus crystal types, sizes,
temperature and IWC, roughness etc., is a unique facility only available on the
UK FAAM aircraft. We combine radiometry in
the far-IR of IC, in mid-IR to solar of the Met Offices ARIES spectrometer, cloud
microphysics instrumentation and expertise of the University of Manchester
and state-of-the-art SID
(small ice detector) ice probes of the University of Hertfordshire, and
UKMO/FAAM with complementary cloud and atmospheric state measurements. This
will give a leap forward to cirrus modelling, our datasets allowing testing and
development of models and parameterizations used to predict the effect of
cirrus in climate models and NWP.