Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX)
The CICCREX project was funded by the Natural Environment Research Council (NERC) with the grant references - NE/K015133/1 and NE/K01515X/1. It was lead by Dr Juliet Pickering (Imperial College London) and Dr Keith Bower (The University of Manchester) between 2013-2016.
The aim of the CICCREX project was to understand the link between evolving ice cloud microphysical properties and the resulting radiative signatures of the cirrus, at the macrophysical scale, as seen from a remote sensing platform.
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 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.
This project carried out a novel flight campaign which combined 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. The project tested 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.
The project was 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. They combined radiometry in the far-IR of IC, in mid-IR to solar of MO, cloud microphysics instrumentation and expertise of Manchester and Hertfordshire Universities, and UKMO/FAAM with complementary cloud and atmospheric state measurements. This project provides a leap forward to cirrus modelling, the datasets allowing for testing and development of models and parameterizations used to predict the effect of cirrus clouds in the high troposphere.
The project's aim was to understand the link between evolving ice cloud microphysical properties and the resulting radiative signatures of the cirrus, at the macrophysical scale, as seen from a remote sensing platform. This objective was achieved through a ground breaking cirrus coupled cloud-radiation airborne campaign across the Arctic and Mid-latitudes, which obtained first time radiation measurements across the electromagnetic spectrum (visible to sub-mm wavelengths) together with state-of-the-art cloud microphysics measurements. This exploited the recent leap in advances in microphysical and radiance data quality. The project will use these unique datasets to test and facilitate improvements to cirrus scattering models and parameterizations for climate and NWP models. The goal of the project was for an accurate parameterisation of cirrus optical properties in global climate modelling and NWP.
Through a radiative closure experiment, and testing of cirrus scattering models throughout the LW and SW for the first time, they provided the evidence for a more direct coupling between cloud physics and radiation, and to show that such schemes can simulate the measured radiation fields correctly, through a direct link between GCM prognostic variables and the cirrus optical properties. Such schemes will represent a paradigm shift in GCM parameterization, as current operational GCMs rely on linking radiation to cloud physics through diagnosed quantities only.
The objectives of this project were as follows (priority here relates to the order in which the objectives will be met):
1. A radiative closure cirrus cloud-radiation experiment in northern and mid latitudes.
2. Obtain a well calibrated set of high resolution radiance measurements throughout the infra-red and visible spectrum, 0.3-125 microns, from above and below and within an extensive layer of well developed cirrus.
3. Characterise the atmospheric column above and below the cloud layer in terms of humidity distribution, temperature structure and other key radiatively-active species.
4. Map the ice crystal particle size distribution, habit types and crystal complexity (including roughness, concavity etc) within the cloud layer, to provide an accurate, well-constrained, consistent description of the microphysical state.
5. Use derivatives of the macrophysical and microphysical state, including ice water content and temperature, as input into state-of-the-art scattering model codes and cirrus parameterisations, whose output (via a radiative transfer model) will be critically validated against the radiance measurements throughout the sub-mm, infrared and visible spectrum. Sensitivity of the predicted radiance to PSD, habit types, aggregate and ensemble models, crystal complexity will be investigated by reference to the microphysical datasets.
6. Exploit campaign datasets and constraint of cloud microphysical and radiative uncertainties in case studies to facilitate improvement of cirrus scattering models allowing a self-consistent and physically-based parameterisation of the ice crystal scattering properties.
7. Through case studies using northern and mid-latitude campaign datasets, test the ice crystal scattering models and the coupled cloud-radiation parameterization by running the high-resolution version of the MO Unified Model (UM) at 1.0km resolution, with a view to incorporating the new parameterisations into the widely used UM.
The results have impacted cirrus modelling in GCMs, both for numerical weather prediction (NWP) and climate change, and in remote sensing.
In addition further objectives were to:
8. Conduct a study of the moderating effect of cirrus on far-IR heating rates by comparing derived heating rates directly from the far-IR data, and comparing these to models using the newly-derived scattering properties.
9. Study the spatial variability of the far-IR cirrus radiative signal as a function of the cloud structure, and determine the impact on precision of ((cirrus cloud modelling???.))
****The dataset contains 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. 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. ****
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