Biases in the diurnal temperature range in Central Europe in an ensemble of regional climate models and their possible causes

Abstract

The study examines how regional climate models (RCMs) reproduce the diurnal temperature range (DTR) in their control simulations over Central Europe. We evaluate 30-year runs driven by perfect boundary conditions (the ERA40 reanalysis, 1961–1990) and a global climate model (ECHAM5) of an ensemble of RCMs with 25-km resolution from the ENSEMBLES project. The RCMs’ performance is compared against the dataset gridded from a high-density stations network. We find that all RCMs underestimate DTR in all seasons, notwithstanding whether driven by ERA40 or ECHAM5. Underestimation is largest in summer and smallest in winter in most RCMs. The relationship of the models’ errors to indices of atmospheric circulation and cloud cover is discussed to reveal possible causes of the biases. In all seasons and all simulations driven by ERA40 and ECHAM5, underestimation of DTR is larger under anticyclonic circulation and becomes smaller or negligible for cyclonic circulation. In summer and transition seasons, underestimation tends to be largest for the southeast to south flow associated with warm advection, while in winter it does not depend on flow direction. We show that the biases in DTR, which seem common to all examined RCMs, are also related to cloud cover simulation. However, there is no general tendency to overestimate total cloud amount under anticyclonic conditions in the RCMs, which suggests the large negative bias in DTR for anticyclonic circulation cannot be explained by a bias in cloudiness. Errors in simulating heat and moisture fluxes between land surface and atmosphere probably contribute to the biases in DTR as well.

Appendix

The airflow indices are calculated using MSLP in grid points shown in Fig. 1.

The westerly (zonal) component of the geostrophic surface wind is calculated as the pressure gradient between 45°N and 55°N and represents the westerly flow w:

$$ w = 0.5 \cdot (p[4] + p[5]) - 0.5 \cdot (p[12] + p[13]). $$

(A.1)

The southerly (meridional) component of the geostrophic surface wind represented by the pressure gradient between 10°E and 20°E is the southerly flow s:

$$ s = 1.56 \cdot (0.25 \cdot (p[13] + 2 \cdot p[9] + p[5]) - 0.25 \cdot (p[12] + 2 \cdot p[8] + p[4])). $$

(A.2)

The resultant total flow strength is

$$ STR = \sqrt {w^{2} + s^{2} }. $$

(A.3)

The direction of flow DIR is calculated as

$$ DIR = \arctan \left( \frac{w}{s} \right);{\text{ added 18}}0^\circ \;{\text{if}}\;s \ge 0,{\text{ and 36}}0^\circ \;{\text{if}}\;s < 0\; {\text{or}}\;w \ge 0. $$

(A.4)

The total shear vorticity VORT is the sum of the westerly and southerly vorticity:

$$ VORT = zw + zs, $$

(A.5)

where zw corresponds to the difference of the westerly flow between 40°N and 50°N and of that between 50°N and 60°N

$$ \begin{aligned} zw & = 1.08 \cdot (0.5 \cdot (p[1] + p[2]) - 0.5 \cdot (p[8] + p[9])) \\ &\quad - 0.94 \cdot (0.5 \cdot (p[8] + p[9]) - 0.5 \cdot (p[15] + p[16])), \\ \end{aligned} $$

(A.6)

and zs is the difference of the southerly flow between 30°E and 20°E and of that between 10°E and 0°E

$$ \begin{aligned} zs & = 1.21 \cdot (0.25 \cdot (p[14] + 2 \cdot p[10] + p[6]) - 0.25 \cdot (p[13] + 2 \cdot p[9] + p[5]) \\ &\quad - 0.25 \cdot (p[12] + 2 \cdot p[8] + p[4]) + 0.25 \cdot (p[11] + 2 \cdot p[7] + p[3])). \\ \end{aligned} $$

(A.7)

Constants used in these equations reflect differing sizes of grid cells at each latitude. DIR is expressed in degrees, while STR and VORT have units of hPa per 10° latitude at 50°N.