Jim Kinter -- Benefits and Challenges of High Spatial Resolution in Climate Models – The webinar will highlight recent results from several different numerical experiments conducted with climate models having both moderate and enhanced spatial resolution in either the atmospheric or the oceanic component. In particular, the presentation will summarize results of Project Athena, in which a single global atmospheric model was run through a series of identical protocols with different horizontal resolutions ranging from grids nominally used for climate simulation and prediction (~100 km) to grids typically used for global numerical weather prediction (16 km). Results that are particularly relevant to climate prediction on intra-seasonal to seasonal time scales and simulation of climate change will be highlighted. Both the benefits and the challenges of using high spatial resolution will be described.
Jiming Jim -- Simulations of Lake Processes and Their Effects on Precipitation using a Coupled WRF-Lake Model -- A one-dimensional physically-based lake model was coupled with the Weather Research and Forecasting (WRF) model to improve lake-effect precipitation simulations for the Great Lakes region. This coupling work provides WRF with a capability for dynamic simulations of lake-atmosphere interactions. Initial simulations with the WRF-Lake model show that the seasonal cycle of lake surface temperature (LST) was greatly exaggerated especially for the deep lakes such as Lake Superior. It is found that the exaggerated LST seasonal cycle results from insufficiently simulated turbulent mixing in the lake. A series of sensitivity tests with the WRF-Lake model were performed to optimize the eddy diffusivity that controls water mixing in the lake scheme and is a function of surface wind and roughness length. The coupled model is able to realistically reproduce the LST seasonal cycle with the optimized eddy diffusivity. In addition, we performed multi-year simulations at 10 km resolution for the period of 2003-2008 forced with 32 km resolution North American Regional Reanalysis data to validate the coupled model. The results reveal that the simulated LSTs are in very good agreement with surface buoy observations and Moderate Resolution Imaging Spectroradiometer satellite data. The realistic LST simulations also generate more accurate lake-effect precipitation when compared with that produced by the release version (3.2) of WRF without a lake scheme.
Sarah Kapnick -- The Importance of Resolution for Modeling Global Snow -- A new high-resolution global climate model GFDL-CM2.5 and its low resolution counterpart, GFDL-CM2.1, are used to explore snow variability in the present climate and as a result of doubling atmospheric CO2. In the present climate, increasing resolution leads to the improved representation of snow in complex orographic regions and the seasonality of Northern Hemisphere snow covered area. In response to CO2 doubling in both models, global snowfall increases in the high-to-mid latitudes and decreases in the mid-to-low latitudes. However, in mid-to-low latitudes, GFDL-CM2.5 is unique in that its high resolution allows it to resolve complex mountain systems, leading to a change in sign in snowfall projections over high mountains in comparison to its predecessor.
Lucas Harris -- Two-way nested-grid climate simulations in the GFDL High Resolution Atmosphere Model -- Regional climate simulations typically use limited-area models driven by the output from a lower-resolution global model. The limited-area model may not be numerically consistent with the global model, the data may only be available at temporally-coarse intervals, and the limited-area model cannot feed back onto the coarse grid. We present a two-way nested grid version of the GFDL High Resolution Atmosphere Model (HiRAM) and demonstrate enhanced-resolution climate simulations for North America and the Maritime Continent. Two resolutions are presented: a c90 (approximately 1 degree) global grid with a factor-of-three nest, and a c192 (0.5 degree) global grid with a factor-of-two nest. We find that the nested grid does not adversely affect the global climate compared to a single-grid model. In the nested grid region topography is better resolved and orographic precipitation is better represented with more detail. Some model precipitation biases are also alleviated in the nested region, although others are unchanged.
Stefan Tulich -- Using hindcasts to improve depiction of the MJO in next-generation climate models -- Through advances in computing power, it is now practical to perform decadal climate simulations at horizontal grid spacings in the range 20-50 km, which is roughly a factor of five smaller than in previous years. Despite these advances, however, global models still suffer from a number of glaring deficiencies, including poor depiction of tropical wave variability, especially with regards to the MJO. In this presentation, a strategy will be described for addressing this issue, which involves using several different global models to perform 30-day hindcasts of the MJO. These include conventional high-resolution models, as well as a lower-resolution model with a superparameterization for convection. Results show that all of the models are generally able to capture some semblance of the MJO's eastward-propagating signals, especially during the earlier stages of the simulations. In the case of the superparameterized model, however, there is little evidence for improving model performance through increases in horizontal resolution. Rather, it appears that the largest gains can be made through subtle changes to the vertical diffusion of moisture at low levels. A similar type of result seems to carry over to at least one of the conventional high-resolution models.