Assessing and validating NZESM using modern & historic observations
The NZ Earth System Model (NZESM) is designed to simulate how our climate will change over the coming decades. It’s highly complex, modelling everything from weather systems to changes in Antarctic sea ice, ocean temperatures to stratospheric chemistry, so is highly complex.
The defining feature of an Earth System Model (ESM) is the incorporation of multiple, interconnected Earth system processes. This is both a strength and a weakness. It is a strength because the model captures the feedback that result from interconnected processes, and a weakness because errors in a single component of the model can compromise the fidelity of the entire model. Validation of global climate models, including ESMs, is therefore essential to ensure that the model outputs will be useful and used by stakeholders.
The Deep South Challenge allocated $1.1 million to Bodeker Scientific, who collaborated with colleagues at NIWA, the University of Canterbury, the UK Met Office, NOAA and NASA, to assess and validate the NZESM.
In this project we evaluated simulations made by NZESM. This was done primarily by comparing model outputs against observations, and also by validating specific processes within the model.
We identified components of the NZESM that are essential for providing reliable projections of climate parameters of most interest to stakeholders. This included phenomena such as extreme weather events, droughts, and increased rainfall. To do this, the project 'mined' past climate data, which were used as a baseline to test and validate the NZESM against. Also, as the Deep South geographical region has an unusually short data record, we also worked with a historian to analyse pre-industrial weather observations from the 1800s.
The project was broken down into 4 main tasks:
1. ACRE Antarctica data rescue:
Critical past weather observations were 'rescued' to extend the Southern Hemisphere coverage within global reanalyses as far back as possible into the 1800s. This was done by identifying data resources, digital scanning, keying data, quality control, and archiving observations at NIWA. The augmented reanalyses were used to:
Investigate poorly understood aspects of New Zealand regional climate that are linked to high-latitude atmospheric and oceanic dynamics,
Examine daily synoptic type trends, and
Establish a baseline more representative of pre-industrial conditions against which current and future climate can be compared.
The synoptic type classification completed in (ii) was binned into multi-decadal intervals according to phases of the Interdecadal Pacific Oscillation. Rescued data were archived and made publicly available through the International Surface Pressure Databank and NIWA's database.
Key personnel for Task 1: Andrew Lorrey (NIWA); Ursula Rack (University of Canterbury).
2. Upper-air ECVs:
We generated observations-based climate data records of upper-air essential climate variables (ECVs) for calculating long-term trends, and compared them with those simulated by NZESM. Construction of these long-term upper-air climate records followed international best practice. Observational data sets were also obtained through international collaborators. Initial target data sets include a climatology of cloud fields over the Southern Ocean from the International Satellite Cloud Climatology Project (ISCCP), measurements of tropospheric aerosol time series, and, in particular, non-sulfur organic aerosols, from selected sites over southern middle and high latitudes, as well as high vertical resolution temperature profiles to determine precise boundary layer heights.
Key personnel: Greg Bodeker, Stefanie Kremser, Jared Lewis (Bodeker Scientific); Adrian McDonald (University of Canterbury).
3. Ozone in the climate system:
We have constructed a new version of the National Institute of Water and Atmospheric Research - Bodeker Scientific (NIWA-BS) total column ozone database. We are also constructing vertically resolved global ozone climate data records to:
Provide the ozone boundary conditions for ESM simulations that exclude an interactive stratosphere,
Provide a benchmark for comparisons of ozone from ESM simulations that do include an interactive stratosphere, and
Provide databases that can be used to assess whether the Montreal Protocol is working as expected.
Space-based measurements of stratospheric and tropospheric ozone are being obtained from international collaborators, while ground- and balloon-based measurements of ozone are being obtained from global monitoring networks such as the Network for the Detection of Atmospheric Composition Change (NDACC), Southern Hemisphere ADditional OZonesondes (SHADOZ) and Global Atmosphere Watch (GAW). The multiple data streams are being combined using a well-established method to generate ozone climate data records. A version of the database is being constructed in a coordinate system that is sensitive to stratospheric dynamics (indexed by equivalent latitude and tropopause height) that will allow an assessment of the importance of zonal asymmetries in the stratospheric ozone field for stratosphere-troposphere coupling. Access to ozone climate data records already produced by the project are available under the Total Column Ozone and Vertically Resolved Ozone pages.
Key personnel: Greg Bodeker, Stefanie Kremser, Jared Lewis (Bodeker Scientific); Richard Querel (NIWA).
4. Semi-empirical models:
A process-oriented validation of NZESM has been conducted by training semi-empirical models on both NZESM diagnostic variables and on observations. Derived fit parameters are compared to reveal differences between NZESM sensitivities and those occurring in reality. NZESM processes were identified that can be described by relevant equations, with fit coefficients determined semi-empirically, to validate those processes.
Key personnel: Greg Bodeker, Stefanie Kremser, Jared Lewis (Bodeker Scientific).