A coupled cloud physics–radiation parameterization of the bulk optical properties of cirrus and its impact on the Met Office unified model global atmosphere 5.0 configuration

A new coupled cloud physics–radiation parameterization of the bulk optical properties of ice clouds is presented. The parameterization is consistent with assumptions in the cloud physics scheme regarding particle size distributions (PSDs) and mass–dimensional relationships. The parameterization is based on a weighted ice crystal habit mixture model, and its bulk optical properties are parameterized as simple functions of wavelength and ice water content (IWC). This approach directly couples IWC to the bulk optical properties, negating the need for diagnosed variables, such as the ice crystal effective dimension. The parameterization is implemented into the Met Office Unified Model Global Atmosphere 5.0 (GA5) configuration. The GA5 configuration is used to simulate the annual 20-yr shortwave (SW) and longwave (LW) fluxes at the top of the atmosphere (TOA), as well as the temperature structure of the atmosphere, under various microphysical assumptions. The coupled parameterization is directly compared against the current operational radiation parameterization, while maintaining the same cloud physics assumptions. In this experiment, the impacts of the two parameterizations on the SW and LW radiative effects at TOA are also investigated and compared against observations. The 20-yr simulations are compared against the latest observations of the atmospheric temperature and radiative fluxes at TOA. The comparisons demonstrate that the choice of PSD and the assumed ice crystal shape distribution are as important as each other. Moreover, the consistent radiation parameterization removes a long-standing tropical troposphere cold temperature bias but slightly warms the southern midlatitudes by about 0.5 K

The impact of two coupled cirrus microphysics-radiation parameterizations on the temperature and specific humidity biases in the tropical tropopause layer in a climate model

The impact of two different coupled cirrus microphysics-radiation parameterizations on the zonally averaged temperature and humidity biases in the tropical tropopause layer (TTL) of a Met Office climate model configuration is assessed. One parameterization is based on a linear coupling between a model prognostic variable, the ice mass mixing ratio, qi, and the integral optical properties. The second is based on the integral optical properties being parameterized as functions of qi and temperature, Tc, where the mass coefficients (i.e. scattering and extinction) are parameterized as nonlinear functions of the ratio between qi and Tc. The cirrus microphysics parameterization is based on a moment estimation parameterization of the particle size distribution (PSD), which relates the mass moment (i.e. second moment if mass is proportional to size raised to the power of 2 ) of the PSD to all other PSD moments through the magnitude of the second moment and Tc. This same microphysics PSD parameterization is applied to calculate the integral optical properties used in both radiation parameterizations and, thus, ensures PSD and mass consistency between the cirrus microphysics and radiation schemes. In this paper, the temperature-non-dependent and temperature-dependent parameterizations are shown to increase and decrease the zonally averaged temperature biases in the TTL by about 1 K, respectively. The temperature-dependent radiation parameterization is further demonstrated to have a positive impact on the specific humidity biases in the TTL, as well as decreasing the shortwave and longwave biases in the cloudy radiative effect. The temperature-dependent radiation parameterization is shown to be more consistent with TTL and global radiation observations

Sensitivity of simulated mesoscale convective systems over East Asia to the treatment of convection in a high-resolution GCM

Mesoscale convective systems (MCSs) downstream of the Tibetan Plateau (TP) exhibit unique precipitation features. These MCSs can have damaging impacts and there is a critical need for improving the representation of MCSs in numerical models. However, most global climate models are typically run at resolutions that are too coarse to reasonably resolve MCSs, and it is still unclear how well higher-resolution global models can reproduce the precipitation characteristics of MCSs. In this study, the sensitivity of MCSs simulated by a global high resolution (~ 10 km), atmosphere-only climate model to different treatments of convection (with and without parametrized convection, and a hybrid representation of convection) have been investigated. The results show that explicit convection (i.e., non-parameterized) can better reproduce the observed pattern of MCS precipitation over the East Asian Summer Monsoon region. In general, explicit convection better simulates the diurnal variability of MCSs over the eastern China, and is able to represent the distinctive diurnal variations of MCS precipitation over complex terrain particularly well, such as the eastern TP and the complex terrain of central-northern China. It is shown that explicit convection is better at simulating the timing of initiation and subsequent propagating features of the MCS, resulting in better diurnal variations and further a better spatial pattern of summer mean MCS precipitation. All three experiments simulate MCS rainfall areas which are notably smaller than those in observations, but with much stronger rainfall intensities, implying that these biases in simulated MCS morphological characteristics are not sensitive to the different treatment of convection

Intensification of mesoscale convective systems in the East Asian rainband over the past two decades

As one of the major producers of extreme precipitation, mesoscale convective systems (MCSs) have received much attention. Recently, MCSs over several hotpots, including the Sahel and US Great Plains, have been found to intensify under global warming. However, relevant studies on the East Asian rainband, another MCS hotpot, are scarce. Here, by using a novel rain-cell tracking algorithm on a high spatiotemporal resolution satellite precipitation product, we show that both the frequency and intensity of MCSs over the East Asian rainband have increased by 21.8% and 9.8% respectively over the past two decades (2000–2021). The more frequent and intense MCSs contribute nearly three quarters to the total precipitation increase. The changes in MCSs are caused by more frequent favorable large-scale water vapor-rich environments that are likely to increase under global warming. The increased frequency and intensity of MCSs have profound impacts on the hydroclimate of East Asia, including producing extreme events such as severe flooding

The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth System Models

The Earth system models (ESMs) that participated in the sixth Coupled Model Intercomparison Project (CMIP6) tend to simulate excessive cooling in surface air temperature (TAS) between 1960 and 1990. The anomalous cooling is pronounced over the Northern Hemisphere (NH) midlatitudes, coinciding with the rapid growth of anthropogenic sulfur dioxide (SO2) emissions, the primary precursor of atmospheric sulfate aerosols. Structural uncertainties between ESMs have a larger impact on the anomalous cooling than internal variability. Historical simulations with and without anthropogenic aerosol emissions indicate that the anomalous cooling in the ESMs is attributed to the higher aerosol burden in these models. The aerosol forcing sensitivity, estimated as the outgoing shortwave radiation (OSR) response to aerosol concentration changes, cannot well explain the diversity of pothole cooling (PHC) biases in the ESMs. The relative contributions to aerosol forcing sensitivity from aerosol–radiation interactions (ARIs) and aerosol–cloud interactions (ACIs) can be estimated from CMIP6 simulations. We show that even when the aerosol forcing sensitivity is similar between ESMs, the relative contributions of ARI and ACI may be substantially different. The ACI accounts for between 64 % and 87 % of the aerosol forcing sensitivity in the models and is the main source of the aerosol forcing sensitivity differences between the ESMs. The ACI can be further decomposed into a cloud-amount term (which depends linearly on cloud fraction) and a cloud-albedo term (which is independent of cloud fraction, to the first order), with the cloud-amount term accounting for most of the inter-model differences

Mesoscopic simulations of reaction-diffusion-advection problems

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A case study of the July 2021 Henan extreme rainfall event: From weather forecast to climate risks

During 17–22 July 2021 a rainfall event in Henan Province of China has delivered over 820 mm precipitation around Zhengzhou with hourly intensity exceeded 201 mm. This paper presents a systematic case study of its driving processes, predictability, and future climate risks. The key aspects of this event are 1) geographically stationary and 2) temporally persistent. A sustained low-level moist easterly jetstream resulting from between typhoon In-Fa and a northward displaced subtropical high and orography in the region appear to have played a major role in the event. The event was, overall, well predicted in global operational 5-day weather forecast, though the details may not be very accurate. From a climate perspective, the large-scale low-level circulation pattern was like the August 1975 catastrophic floods in the same region, but opposite to the 2020 summer anomalous extreme Meiyu situation. Both weather patterns were rare during the past 47 years, together accounting for less than 10% of low-level daily weather patterns. The orographic features around Henan make it vulnerable for floods. A risk assessment based two sets of ensemble climate model simulations suggest the probability of such events occurring in the future will increase under the IPCC RCP8.5 scenario

A bimodal diagnostic cloud fraction parameterization : part I : motivating analysis and scheme description

Cloud fraction parameterizations are beneficial to regional, convection-permitting numerical weather prediction. For its operational regional midlatitude forecasts, the Met Office uses a diagnostic cloud fraction scheme that relies on a unimodal, symmetric subgrid saturation-departure distribution. This scheme has been shown before to underestimate cloud cover and hence an empirically based bias correction is used operationally to improve performance. This first of a series of two papers proposes a new diagnostic cloud scheme as a more physically based alternative to the operational bias correction. The new cloud scheme identifies entrainment zones associated with strong temperature inversions. For model grid boxes located in this entrainment zone, collocated moist and dry Gaussian modes are used to represent the subgrid conditions. The mean and width of the Gaussian modes, inferred from the turbulent characteristics, are then used to diagnose cloud water content and cloud fraction. It is shown that the new scheme diagnoses enhanced cloud cover for a given gridbox mean humidity, similar to the current operational approach. It does so, however, in a physically meaningful way. Using observed aircraft data and ground-based retrievals over the southern Great Plains in the United States, it is shown that the new scheme improves the relation between cloud fraction, relative humidity, and liquid water content. An emergent property of the scheme is its ability to infer skewed and bimodal distributions from the large-scale state that qualitatively compare well against observations. A detailed evaluation and resolution sensitivity study will follow in Part II

Implementing a process-based contrail parametrization in the Unified Model

&amp;lt;p&amp;gt;&amp;amp;#160;&amp;amp;#160;&amp;amp;#160;&amp;amp;#160; The global aviation fleet modifies cloudiness through contrail formation and their subsequent competition with natural cirrus for ambient water vapor, along with enhanced ice-nuclei concentrations from aircraft soot emissions. Contrails form in the upper troposphere at temperatures below 233 K and pressures below 300 hPa, when plume gases from jet engines, having appreciable water vapor content, saturate with respect to liquid water (Schmidt-Appleman Criterion, SAC). Realistic assessments of the aviation-induced modifications to global cloud cover demand improved representation of contrails and their interaction with background cloudiness in climate models. We have implemented a process-based parametrization of contrail cirrus, that applies to both young (&amp;amp;#8804; 5 h) and aged contrails, in the UK Met Office Unified Model, version 12.0. Contrail cirrus is introduced as a new prognostic cloud class, forming in the parametrized, fractional ice supersaturated area which then undergoes advection, depositional growth, sublimation and sedimentation. The proxy for the fractional supersaturated area is calculated using the same total water PDF as used for natural cirrus but with a different critical relative humidity, r&amp;lt;sub&amp;gt;cc&amp;lt;/sub&amp;gt; - a value at which part of the model grid box is at least ice-saturated. The persistence of contrails being allowed in the ice supersaturated areas, the simulated coverage is not confined to flight corridors, but is advected to air traffic free zones as well. The simulated annual mean global coverage due to young contrails is 0.13%, with the main traffic areas of Europe and North America having the maximum coverage. Similar to natural cirrus, the contrail ice particles reflect the solar short-wave (SW) radiation and trap outgoing long-wave (LW) radiation, thereby modifying the radiative balance of the Earth&amp;amp;#8217;s atmosphere. Contrail cirrus is radiatively active in the model with forcing studies enabled via a &amp;amp;#8216;double radiation call&amp;amp;#8217; approach, wherein parallel runs of the radiation scheme &amp;amp;#8216;with&amp;amp;#8217; (prognostic) and &amp;amp;#8216;without&amp;amp;#8217; (diagnostic) the contrail radiative effects isolates the contrail-induced perturbations. Contrails are seen to induce a short-wave cooling and long-wave warming and the net (SW+LW) direct top-of-atmosphere radiative forcing by young contrails amounts globally to 0.5 mWm&amp;lt;sup&amp;gt;-2&amp;lt;/sup&amp;gt;, with the peak forcing seen along the main air traffic areas of North America, Europe and East Asia. The implementation of this process-based parametrization in the UM enables the simulation of the life cycle of persistent contrails, and can provide valuable insights to the aviation-induced modifications to the global cloud cover.&amp;lt;/p&amp;gt;</jats:p

Understanding sources of contrail cirrus radiative forcing uncertainty using a new diagnostic contrail scheme for the UK Earth System Model

&amp;lt;p&amp;gt;Condensation trails (contrails) are aircraft-induced line-shaped high clouds, which may persist and grow to form contrail cirrus in ice supersaturated regions. Contrail cirrus is the largest known component of aviation radiative forcing, with a large uncertainty associated with troposphere water budgets and contrail radiative properties. In addition, due to the limited number of climate models able to simulate contrail cirrus, the uncertainty in the global contrail cirrus radiative forcing cannot be estimated statistically.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The aim of this work is to implement a contrail cirrus parameterisation in the UK Earth System Model, therefore providing a new independent estimate of the contrail cirrus radiative forcing to be used in future assessments of aviation climate impacts. The new diagnostic scheme is based on the processes governing contrail formation (Schmidt-Appleman Criteria) and persistence (ice supersaturation). Persistent contrails are then added to the model cloud fields, where they interact with and evolve alongside natural clouds.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We use ensemble runs of both nudged and free running model experiments to estimate the contrail cirrus cover and effective radiative forcing. Comparisons with a similar contrail scheme implemented in the NCAR Community Atmospheric Model (CAM) indicate the importance of the host climate model via (i) the host&amp;amp;#8217;s cloud microphysics scheme (e.g. single vs. double moment) and (ii) its ability to simulate realistic ice supersaturated regions. By providing a new independent assessment of the contrail cirrus radiative forcing, our work contributes to improving our understanding of aviation climate impacts and therefore potential mitigation strategies for current and future generation aircraft.&amp;amp;#160;&amp;lt;/p&amp;gt;</jats:p