Editing IPCC:AR6/WGI/Chapter-10 (section)

===== 10.3.3.4.3 Coastal winds and lake effects =====

<div id="h4-10-siblings" class="h4-siblings"></div>

Simulating coastal climates and the influence of big lakes are a modelling challenge, due to the complex coastlines, the different heat capacities of land and water, the resulting wind system, and differential evaporation. The AR5 concluded that RCMs can add value to the simulation of coastal climates.

Summer coastal low-level jets off the mid-latitude western continental coasts are forced by the semi-permanent subtropical anticyclones, inland thermal lows, strong across-shore temperature contrasts in upwelling regions, and high coastal topography. They are important factors in shaping regional climate by, for instance, preventing onshore advection of humidity and thereby causing aridity in the Iberian Peninsula ( [[#Soares--2014|Soares et al., 2014]] ), or by transporting moisture towards precipitating regions as in the North American monsoon ( [[#Bukovsky--2013|Bukovsky et al., 2013]] ).

Reanalyses and most global models do not well resolve the details of coastal low-level jets ( [[#Bukovsky--2013|Bukovsky et al., 2013]] ; [[#Soares--2014|Soares et al., 2014]] ), but they are still able to represent annual and diurnal cycles and interannual variability ( [[#Cardoso--2016|Cardoso et al., 2016]] ; [[#Lima--2019|Lima et al., 2019]] ). [[#Bukovsky--2013|Bukovsky et al. (2013)]] found RCM simulations at a 50 km resolution to improve the representation of the coastal low-level jet in the Gulf of California and the associated precipitation pattern compared to the driving global models. [[#Lucas-Picher--2017|Lucas-Picher et al. (2017)]] find indirect evidence via precipitation patterns that 12 km simulations further improve the representation. [[#Soares--2014|Soares et al. (2014)]] demonstrated that an 8 km resolution RCM simulated a realistic three-dimensional structure of the Iberian coastal low-level jet, and the surface winds compare well with observations. [[#Lucas-Picher--2017|Lucas-Picher et al. (2017)]] showed that a 0.44° resolution RCM underestimated winds along the Canadian east coast, whereas a 0.11° resolution version simulated more realistic 10 metre wind speed. Also, the Etesian winds in the Aegean Sea were realistically simulated by 12 km resolution RCMs ( [[#Dafka--2018|Dafka et al., 2018]] ).

A particularly relevant coastal phenomenon is the sea breeze, which is caused by the differential heating of water and land during the diurnal cycle and typically reaches several tens of kilometres inland. Reanalyses and global models have too coarse a resolution to realistically represent this phenomenon, such that they typically underestimate precipitation over islands and misrepresent its diurnal cycle ( [[#Lucas-Picher--2017|Lucas-Picher et al., 2017]] ). RCMs improve the representation of sea breezes and thereby precipitation in coastal areas and islands. Over Cuba and Florida only a 12 km-resolution RCM is able to realistically simulate the inland propagation of precipitation during the course of the day ( [[#Lucas-Picher--2017|Lucas-Picher et al., 2017]] ). RCM simulations at 20 km horizontal resolution realistically represented the sea breeze circulation in the Mediterranean Gulf of Lions including the intensity, direction and inward propagation ( [[#Drobinski--2018|Drobinski et al., 2018]] ). Even though a coupled ocean–atmosphere simulation improved the representation of diurnal SST variations, the sea breeze representation itself was not improved.

Big lakes modify the downwind climate. In particular during winter they are relatively warm compared to the surrounding land, provide moisture, destabilize the passing air column and produce convective systems. The increase in friction when moving air reaches land causes convergence and uplift, and may trigger precipitation. [[#Gula--2012|Gula and Peltier (2012)]] found that a state-of-the-art GCM does not realistically simulate these effects over the North American Great Lakes, but a 10 km RCM better represents them and thereby simulates realistic downwind precipitation patterns, in particular enhanced snowfall during the winter season. Similar results were found by [[#Wright--2013|Wright et al. (2013)]] , [[#Notaro--2015|Notaro et al. (2015)]] and [[#Lucas-Picher--2017|Lucas-Picher et al. (2017)]] . In a convection permitting simulation of the Lake Victoria region, a too strong nocturnal land breeze resulted in unrealistically high precipitation ( [[#Finney--2019|Finney et al., 2019]] ).

There is ''high confidence'' that climate models with sufficiently high resolution are necessary for realistically simulating lake and coastal weather including coastal low-level jets, lake and sea breezes, as well as lake effects on rainfall and snow.

In regions like Fenno-Scandinavia or central-eastern Canada, very large fractions of land are covered by small and medium sized lakes. Other regions have fewer but larger lakes, such as central-eastern Africa, the eastern border between the USA and Canada, and central Asia. In these regions it has been considered essential to include a lake model in an RCM to realistically represent regional temperatures ( [[#Samuelsson--2010|Samuelsson et al., 2010]] ; [[#Deng--2013|Deng et al., 2013]] ; [[#Mallard--2014|Mallard et al., 2014]] ; [[#Thiery--2015|Thiery et al., 2015]] ; [[#Pietikäinen--2018|Pietikäinen et al., 2018]] ), as well as remote effects ( [[#Spero--2016|Spero et al., 2016]] ). The most common approach in RCMs is the two-layer lake model, including a lake-ice model, with parametrized vertical temperature profiles ( [[#Mironov--2010|Mironov et al., 2010]] ; [[#Golosov--2018|Golosov et al., 2018]] ). For the Caspian Sea, it is found that a three-dimensional ocean model simulated the SST fields better than a one-dimensional lake model when coupled to the same RCM ( [[#Turuncoglu--2013|Turuncoglu et al., 2013]] ).

There is ''medium evidence'' and ''high agreement'' that it is important to include interactive lake models in RCMs to improve the simulation of regional temperature, in particular in seasonally ice-covered areas with large fractions of lakes. There is ''medium evidence'' of the local influence of lakes on snow and rainfall as well as the importance of including lakes in regional climate simulations.

<div id="10.3.3.4.4" class="h4-container"></div>

<span id="fronts"></span>