Climate processes

Abstract:

<p><strong>Abstract.</strong> The relationship between changes in integrated water vapour (IWV) and precipitation can be characterized by quantifying changes in atmospheric water vapour lifetime. Precipitation isotope ratios correlate with this lifetime, a relationship that helps understand dynamical processes and may lead to improved climate projections. We investigate how water vapour and its lifetime respond to different drivers of climate change, such as greenhouse gases and aerosols. Results from 11 global climate models have been used, based on simulations where CO₂, methane, solar irradiance, black carbon (BC), and sulphate have been perturbed separately. A lifetime increase from 8 to 10&amp;thinsp;days is projected between 1986&amp;ndash;2005 and 2081&amp;ndash;2100, under a business-as-usual pathway. By disentangling contributions from individual climate drivers, we present a physical understanding of how global warming slows down the hydrological cycle, due to longer lifetime, but still amplifies the cycle due to stronger precipitation/evaporation fluxes. The feedback response of IWV to surface temperature change differs somewhat between drivers. Fast responses amplify these differences and lead to net changes in IWV per degree surface warming ranging from 6.4&amp;plusmn;0.9&amp;thinsp;%/K for sulphate to 9.8&amp;plusmn;2&amp;thinsp;%/K for BC. While BC is the driver with the strongest increase in IWV per degree surface warming, it is also the only driver with a reduction in precipitation per degree surface warming. Consequently, increases in BC aerosol concentrations yield the strongest slowdown of the hydrological cycle among the climate drivers studied, with a change in water vapour lifetime per degree surface warming of 1.1&amp;plusmn;0.4&amp;thinsp;days/K, compared to less than 0.5&amp;thinsp;days/K for the other climate drivers (CO₂, methane, solar irradiance, sulphate).</p>

Abstract:

Posit numbers, a recently proposed alternative to floating-point numbers, claim to have smaller arithmetic rounding errors in many applications. By studying weather and climate models of low and medium complexity (the Lorenz system and a shallow water model) we present benefits of posits compared to floats at 16 bit. As a standardised posit processor does not exist yet, we emulate posit arithmetic on a conventional CPU. Using a shallow water model, forecasts based on 16-bit posits with 1 or 2 exponent bits are clearly more accurate than half precision floats. We therefore propose 16 bit with 2 exponent bits as a standard posit format, as its wide dynamic range of 32 orders of magnitude provides a great potential for many weather and climate models. Although the focus is on geophysical fluid simulations, the results are also meaningful and promising for reduced precision posit arithmetic in the wider field of computational fluid dynamics.

Abstract:

<p><strong>Abstract.</strong> The global aerosol-climate model ECHAM6.3-HAM2.3 (E63H23) and the previous model versions ECHAM5.5-HAM2.0 (E55H20) and ECHAM6.1-HAM2.2 (E61H22) are evaluated using global observational datasets for clouds and precipitation. In E63H23 low cloud amount, liquid and ice water path and cloud radiative effects are more realistic than in previous model versions. E63H23 has a more physically based aerosol activation scheme, improvements in the cloud cover scheme, changes in detrainment of convective clouds, changes in the sticking efficiency for accretion of ice crystals by snow, consistent ice crystal shapes throughout the model, changes in mixed phase freezing and an inconsistency in ice crystal number concentration (ICNC) in cirrus clouds was removed. Biases that were identified in E63H23 (and in previous model versions) are a too low cloud amount in stratocumulus regions, deep convective clouds in the Atlantic and Pacific oceans form too close to the continents and there are indications that ICNCs are overestimated.</p> <p>Since clouds are important for effective radiative forcing due to aerosol-radiation and aerosol-cloud interactions (ERF_ari+aci) and equilibrium climate sensitivity (ECS), also differences in ERF_ari+aci and ECS between the model versions were analyzed. ERF_ari+aci is weaker in E63H23 (&amp;minus;1.0&amp;thinsp;W&amp;thinsp;m^&amp;minus;2) than in E61H22 (&amp;minus;1.2&amp;thinsp;W&amp;thinsp;m^&amp;minus;2) (or E55H20; &amp;minus;1.1&amp;thinsp;W&amp;thinsp;m^&amp;minus;2). This is caused by the weaker shortwave ERF_ari+aci (new aerosol activation scheme and sea salt emission parameterization in E63H23, more realistic simulation of cloud water) overcompensating the weaker longwave ERF_ari+aci (removal of an inconsistency in ICNC in cirrus clouds in E61H22).</p> <p>The decrease in ECS in E63H23 (2.5&amp;thinsp;K) compared to E61H22 (2.8&amp;thinsp;K) is due to changes in the entrainment rate for shallow convection (affecting the cloud amount feedback) and a stronger cloud phase feedback.</p>