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A. Bellon and G. L. Austin

Abstract

Empirical relationships between visible and/or IR data and rainfall rate are derived by comparing gage-calibrated radar data with colocated satellite information over Montreal, Canada. The accuracy of 1739 point gage-satellite measurements from 14 sequences of summertime rainfall during daylight hours is evaluated. The absolute difference, defined as ∑ |GISi|/∑ Gi where Gi and Si are the corresponding gage and satellite estimates, is 85%. The Critical Success Index (CSI), the Probability of Detection (POD), and the False Alarm Ratio (FAR) at the 2 mm level are of the order of 50, 70 and 35%, respectively, and the cross-correlation coefficient γ is computed to be 0.56.

The relative accuracy in the rainfall estimation of four empirical methods based on point satellite readings is determined. The scores of the visible-IR and “visible only” methods am adequate (γ = 0.56 and 0.50, respectively), but the scores of the “IR only” method are judged inadequate (γ = 0.30) because of its serious overestimation. A rain/no-rain estimate scores nearly as high (γ = 0.50) as a continuous rain estimate.

It is found that satellite estimates using our objective techniques are better than gage-interpolated estimates at locations where the nearest gage is farther than 40 km. The usefulness of these statistical satellite rainfall measurements is thus limited to the data sparse regions of the world.

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A. Bellon and G. L. Austin

Abstract

Digital weather radar data have been used with a simple pattern recognition procedure to automatically generate precipitation forecasts in the zero to three hours range. Such a technique has been in real time operation for two years. The verification of the procedure has led to a preliminary “radar climatology” for the Montreal area in the form of a map of areas showing a predominant growth or decay of precipitation patterns.

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A. Bellon, S. Lovejoy, and G. L. Austin

Abstract

An algorithm yielding probability of rain from GOES visible-infrared imagery and simultaneous radar data is applied over a satellite image the size of eastern Canada. It is then mapped by means of a conic projection an a constant resolution Cartesian grid to facilitate overlay with synoptic charts. A pattern recognition technique is applied to 16 subareas of the entire map and has proved successful in tracking the displacement of the probability-of-rain contours. The potential of the system for making short-range precipitation forecasts is discussed briefly.

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O. Geoffroy, D. Saint-Martin, D. J. L. Olivié, A. Voldoire, G. Bellon, and S. Tytéca

Abstract

This is the first part of a series of two articles analyzing the global thermal properties of atmosphere–ocean coupled general circulation models (AOGCMs) within the framework of a two-layer energy-balance model (EBM). In this part, the general analytical solution of the system is given and two idealized climate change scenarios, one with a step forcing and one with a linear forcing, are discussed. These solutions give a didactic description of the contributions from the equilibrium response and of the fast and slow transient responses during a climate transition. Based on these analytical solutions, a simple and physically based procedure to calibrate the two-layer model parameters using an AOGCM step-forcing experiment is introduced. Using this procedure, the global thermal properties of 16 AOGCMs participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are determined. It is shown that, for a given AOGCM, the EBM tuned with only the abrupt 4×CO2 experiment is able to reproduce with a very good accuracy the temperature evolution in both a step-forcing and a linear-forcing experiment. The role of the upper-ocean and deep-ocean heat uptakes in the fast and slow responses is also discussed. One of the main weaknesses of the simple EBM discussed in this part is its ability to represent the evolution of the top-of-the-atmosphere radiative imbalance in the transient regime. This issue is addressed in Part II by taking into account the efficacy factor of deep-ocean heat uptake.

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O. Geoffroy, D. Saint-Martin, G. Bellon, A. Voldoire, D. J. L. Olivié, and S. Tytéca

Abstract

In this second part of a series of two articles analyzing the global thermal properties of atmosphere–ocean coupled general circulation models (AOGCMs) within the framework of a two-layer energy-balance model (EBM), the role of the efficacy of deep-ocean heat uptake is investigated. Taking into account such an efficacy factor is shown to amount to representing the effect of deep-ocean heat uptake on the local strength of the radiative feedback in the transient regime. It involves an additional term in the formulation of the radiative imbalance at the top of the atmosphere (TOA), which explains the nonlinearity between radiative imbalance and the mean surface temperature observed in some AOGCMs. An analytical solution of this system is given and this simple linear EBM is calibrated for the set of 16 AOGCMs of phase 5 of the Coupled Model Intercomparison Project (CMIP5) studied in Part I. It is shown that both the net radiative fluxes at TOA and the global surface temperature transient response are well represented by the simple EBM over the available period of simulations. Differences between this two-layer EBM and the previous version without an efficacy factor are analyzed and relationships between parameters are discussed. The simple model calibration applied to AOGCMs constitutes a new method for estimating their respective equilibrium climate sensitivity and adjusted radiative forcing amplitude from short-term step-forcing simulations and more generally a method to compute their global thermal properties.

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