Christopher Moseley

Climate model bias correction and the role of timescales

Abstract. It is well known that output from climate models cannot be used to force hydrological simulations without some form of preprocessing to remove the existing biases. In principle, statistical bias correction methodologies act on model output so the statistical properties of the corrected data match those of the observations. However, the improvements to the statistical properties of the data are limited to the specific timescale of the fluctuations that are considered. For example, a statistical bias correction methodology for mean daily temperature values might be detrimental to monthly statistics. Also, in applying bias corrections derived from present day to scenario simulations, an assumption is made on the stationarity of the bias over the largest timescales. First, we point out several conditions that have to be fulfilled by model data to make the application of a statistical bias correction meaningful. We then examine the effects of mixing fluctuations on different timescales and suggest an alternative statistical methodology, referred to here as a cascade bias correction method, that eliminates, or greatly reduces, the negative effects.

Large eddy simulation using the general circulation model ICON

ICON (ICOsahedral Nonhydrostatic) is a unified modeling system for global numerical weather prediction (NWP) and climate studies. Validation of its dynamical core against a test suite for numerical weather forecasting has been recently published by Zangl et al. (2014). In the present work, an extension of ICON is presented that enables it to perform as a large eddy simulation (LES) model. The details of the implementation of the LES turbulence scheme in ICON are explained and test cases are performed to validate it against two standard LES models. Despite the limitations that ICON inherits from being a unified modeling system, it performs well in capturing the mean flow characteristics and the turbulent statistics of two simulated flow configurations—one being a dry convective boundary layer and the other a cumulus-topped planetary boundary layer.

Response of Karakoram-Himalayan glaciers to climate variability and climatic change: A regional climate model assessment

The Karakoram and the Himalayan mountain range accommodate a large number of glaciers and are the major source of several perennial rivers downstream. To interactively describe to response of glaciers to climate change, a glacier parameterization scheme has been developed and implemented into the regional climate model REMO. The scheme simulates the mass balance as well as changes of the areal extent of glaciers on a subgrid scale. The parameterization scheme is for the first time applied to the region. A regional glacier inventory is compiled and is used to initialize glacier area and volume. Over the highly complex and data sparse region, the simulated mass balance largely agrees with observations including the positive Karakoram anomaly. The simulated equilibrium line altitude is well captured although a systematic underestimation is apparent. REMO simulates the glacier-climate interaction reasonably well; it has clear potential to be used for future climate assessments.

Probing the precipitation life cycle by iterative rain cell tracking

intensities over Germany. We then combine this data set with surface temperature observations and synoptic observations to group tracks according to convective and stratiform conditions. Convective tracks show clear life cycles in intensity, with peaks shifted off-center toward the beginning of the track, whereas stratiform tracks have comparatively featureless intensity profiles. Our results show that the convective life cycle can lead to convection-dominating precipitation extremes at short time scales, while track-mean intensities may vary much less between the two types. The observed features become more pronounced as surface temperature increases, and in the case of convection even exceeded the rates expected from the Clausius-Clapeyron relation.

Intensification of convective extremes driven by cloud–cloud interaction

Convective precipitation may change in a changing climate. Large eddy simulations of convection with a realistic diurnal cycle suggest that interactions between convective systems and precipitation extremes are influenced by temperature.

Statistical precipitation bias correction of gridded model data using point measurements

It is well known that climate model output data cannot be used directly as input to impact models, e.g., hydrology models, due to climate model errors. Recently, it has become customary to apply statistical bias correction to achieve better statistical correspondence to observational data. As climate model output should be interpreted as the space-time average over a given model grid box and output time step, the status quo in bias correction is to employ matching gridded observational data to yield optimal results. Here we show that when gridded observational data are not available, statistical bias correction can be carried out using point measurements, e.g., rain gauges. Our nonparametric method, which we call scale-adapted statistical bias correction (SABC), is achieved by data aggregation of either the available modeled or gauge data. SABC is a straightforward application of the well-known Taylor hypothesis of frozen turbulence. Using climate model and rain gauge data, we show that SABC performs significantly better than equal-time period statistical bias correction.

Precipitation onset as the temporal reference in convective self-organization†

In a dry convective boundary layer, convective patterns of typical scales spontaneously develop, qualitatively similar to those in a fluid which is placed between two horizontal plates and sufficiently heated from below. As soon as precipitating cumulus clouds form, this pattern is disturbed and a transition to a different state occurs. Here we use idealized large-eddy simulations to explore how the horizontal scale of convection is modified during this transition in the course of a diurnal cycle. Before onset of precipitation cells with relatively constant diameter self-organize, with diameters roughly on the scale of the atmospheric boundary layer height. We find that the onset of precipitation then signals an approximately linear increase in horizontal scale with time. For our transient simulations, this scale increase progresses at a speed which is relatively insensitive to modifications in mean surface temperature, modifications in the rate at which surface temperature changes, or the initial lapse rate. When exploring the strength of the spatial correlations, we find that precipitation onset causes a sudden disruption of order and a subsequent decline of organization — until precipitation eventually ceases. We discuss possible implications for the development of extreme precipitation events.

Erratum to: EURO-CORDEX: new high-resolution climate change projections for European impact research

Daniela Jacob • Juliane Petersen • Bastian Eggert • Antoinette Alias • Ole Bossing Christensen • Laurens M. Bouwer • Alain Braun • Augustin Colette • Michel Deque • Goran Georgievski • Elena Georgopoulou • Andreas Gobiet • Laurent Menut • Grigory Nikulin • Andreas Haensler • Nils Hempelmann • Colin Jones • Klaus Keuler • Sari Kovats • Nico Kroner • Sven Kotlarski • Arne Kriegsmann • Eric Martin • Erik van Meijgaard • Christopher Moseley • Susanne Pfeifer • Swantje Preuschmann • Christine Radermacher • Kai Radtke • Diana Rechid • Mark Rounsevell • Patrick Samuelsson • Samuel Somot • Jean-Francois Soussana • Claas Teichmann • Riccardo Valentini • Robert Vautard • Bjorn Weber • Pascal Yiou

Correlations in systems of complex directed macromolecules

An ensemble of directed macromolecules on a lattice is considered, where the constituting molecules are chosen randomly with N different colours. Molecules of the same colour experience a hard-core (exclusion) interaction. We study the robustness of the macromolecules with respect to breaking and changing the colour of a constituting molecule, using a 1/N expansion. The properties depend strongly on the density of macromolecules. In particular, the macromolecules are robust against breaking and changing the colour at high densities but fragile at intermediate densities.