How will precipitation change in extratropical cyclones as the planet warms? Insights from a large initial condition climate model ensemble

Abstract

The extratropical precipitation response to global warming is investigated within a 30-member initial condition climate model ensemble. As in observations, modeled cyclonic precipitation contributes a large fraction of extratropical precipitation, especially over the ocean and in the winter hemisphere. When compared to present day, the ensemble projects increased cyclone-associated precipitation under twenty-first century business-as-usual greenhouse gas forcing. While the cyclone-associated precipitation response is weaker in the near-future (2016–2035) than in the far-future (2081–2100), both future periods have similar patterns of response. Though cyclone frequency changes are important regionally, most of the increased cyclone-associated precipitation results from increased within-cyclone precipitation. Consistent with this result, cyclone-centric composites show statistically significant precipitation increases in all cyclone sectors. Decomposition into thermodynamic (mean cyclone water vapor path) and dynamic (mean cyclone wind speed) contributions shows that thermodynamics explains 92 and 95% of the near-future and far-future within-cyclone precipitation increases respectively. Surprisingly, the influence of dynamics on future cyclonic precipitation changes is negligible. In addition, the forced response exceeds internal variability in both future time periods. Overall, this work suggests that future cyclonic precipitation changes will result primarily from increased moisture availability in a warmer world, with secondary contributions from changes in cyclone frequency and cyclone dynamics.

Appendix: Construction of cyclone-centric radial grids

The radial grids used in our study are centered on cyclone centers and consist of equally-spaced shells on the sphere with each shell consisting of a desired number of points (Fig. 14).

Fig. 14

Illustration of a cyclone-centric radial grid. The green points represent model grid points. The black dot is a sample cyclone center. The red points represent the radial grid

Such radial grids have previously been used in Bengtsson et al. (2009) for cyclone compositing purposes. Here we give direct formulas for the construction of such grids.

Consider a cyclone center C identified on the globe at latitude $\phi$ and longitude $\uplambda$ where the angles are expressed in radians. The unit vectors $\overrightarrow {{e_{1} }}$, $\overrightarrow {{e_{2} }}$, $\overrightarrow {{e_{3} }}$ at C in the zonal, meridional and radial directions (assuming the center of the Earth to be the origin) respectively are given by

$$\overrightarrow {{e_{1} }} = - \sin \left(\uplambda \right)\widehat{i} + \cos \left(\uplambda \right)\widehat{j}$$

$$\overrightarrow {{e_{2} }} = - \sin \left( \phi \right) \cdot \cos \left(\uplambda \right)\widehat{i} - \sin \left( \phi \right)\sin \left(\uplambda \right)\widehat{j} + \cos \left( \phi \right)\widehat{k}$$

$$\overrightarrow {{e_{3} }} = \cos \left( \phi \right) \cdot \cos \left( \lambda \right)\widehat{i} + \cos \left( \phi \right)\sin \left( \lambda \right)\widehat{j} + \sin \left( \phi \right)\widehat{k}$$

where $\widehat{i}$, $\widehat{j}$, $\widehat{k}$ are the unit vectors of a fixed right-handed coordinate system with origin at the center of the globe.

Let $s$ be the desired great-circle distance between the shells of the radial grid centered on C and $m$ be the desired number of shells. Let the desired number of points on each shell be n. Then, the projections $x$, $y$, $z$ of the radial vector joining the origin to the $q$th point on the $p$th shell are given by

$$x\widehat{i} + y\widehat{i} + z\widehat{k} = R\sin \left( {\frac{ps}{R}} \right)\cos \left( {\frac{2\pi q}{n}} \right)\overrightarrow {{e_{1} }} + R\sin \left( {\frac{ps}{R}} \right)\sin \left( {\frac{2\pi q}{n}} \right)\overrightarrow {{e_{2} }} + R\cos \left( {\frac{2\pi q}{n}} \right)\overrightarrow {{e_{3} }}$$

where $R$ is the radius of the Earth, $p$ can range from 1, 2, 3,…, $m$ and $q$ from 1,2,3,… $n$

The latitude and longitude $\phi_{p,q}$ and $\uplambda_{p,q}$ of the point are then given by

$$\phi_{p,q} = \left( {\frac{\pi }{2} - \arccos \left( {\frac{z}{{\sqrt {x^{2} + y^{2} + z^{2} } }}} \right)} \right)\frac{180}{\pi }$$

$$\uplambda_{p,q} = \left( {\left( {Arg\left( {x + iy} \right) } \right)mod \left( {2\pi } \right)} \right)\frac{180}{\pi }$$

where $mod$ is the modulus operator, $Arg$ stands for complex argument and $i$ is the imaginary unit. $\phi_{p,q}$ and $\uplambda_{p,q}$ are output in degrees, and lie in [−90, 90] and [0, 360) respectively. By iterating $p$ and $q$ from 1 to $m$ and 1 to $n$ respectively, the latitudes and longitudes of all radial grid points can be obtained.