Project Summary:
Present-day springtime high-latitude surface albedo as a predictor of simulated climate sensitivity
Levis, S., G.B. Bonan, and P.J. Lawrence
Geophysical Research Letters., 34, L17703, doi:10.1029/2007GL030775.
Abstract
Simulations by the Community Atmosphere Model (CAM) and 15 other climate models suggest that climate sensitivity is linked to continental middle to high latitude present-day springtime albedo. We compare 1 × CO2 and 2 × CO2 CAM simulations against similar simulations with snow cover fraction purposely increased. Greater snow cover fraction leads to higher albedo and lower temperatures at 1 × CO2 but has less influence at 2 × CO2 when little snow remains due to global warming. This makes the simulation with higher albedo at 1 × CO2 more sensitive to increased CO2, in agreement with past work. We show that the wide variation in simulated snow-albedo feedbacks and climate sensitivities among 15 other models correlates well with variations in the continental middle to high latitude present-day springtime albedo, in agreement with our CAM results. The development of more accurate snow and albedo parameterizations should improve model estimates of climate sensitivity.
An urban parameterization for a global climate model: 1. Formulation and evaluation for two cities.
Oleson, K.W., G.B. Bonan, J. Feddema, M. Vertenstein, and C.S.B. Grimmond
Journal of Applied Meteorology and Climatology, in press
Abstract
Urbanization, the expansion of built-up areas, is an important yet less studied aspect of land use/cover change in climate science. To date, most global climate models used to evaluate the effects of land use/cover change on climate do not include an urban parameterization. Here, we describe the formulation and evaluation of a parameterization of urban areas that is incorporated into the Community Land Model, the land surface component of the Community Climate System Model. The model is designed to be simple enough to be compatible with structural and computational constraints of a land surface model coupled to a global climate model, yet complex enough to explore physically-based processes known to be important in determining urban climatology. The city representation is based upon the 'urban canyon' concept which consists of roofs, sunlit and shaded walls, and canyon floor. The canyon floor is divided into pervious (e.g., residential lawns, parks) and impervious (e.g., roads, parking lots, sidewalks) fractions. Trapping of longwave radiation by canyon surfaces and solar radiation absorption and reflection is determined by accounting for multiple reflections. Separate energy balances and surface temperatures are determined for each canyon surface. A one-dimensional heat conduction equation is solved numerically for a ten-layer column to determine conduction fluxes into and out of canyon surfaces. Model performance is evaluated against measured fluxes and temperatures from two urban sites. Results indicate the model does a reasonable job of simulating the energy balance of cities.
An urban parameterization for a global climate model: 2. Sensitivity to input parameters and the simulated urban heat island in offline simulations
Oleson, K.W., G.B. Bonan, J. Feddema, and M. Vertenstein
Journal of Applied Meteorology and Climatology, in press
Abstract
In a companion paper (Oleson et al. 2007), we presented a formulation and evaluation of an urban parameterization designed to represent the urban energy balance in the Community Land Model. Here we test the robustness of the model through sensitivity studies and evaluate the model's ability to simulate urban heat islands in different environments. Findings show that heat storage and sensible heat flux are most sensitive to uncertainties in the input parameters within the atmospheric and surface conditions considered here. The sensitivity studies suggest that attention should be paid to not only accurately characterizing the structure of the urban area (e.g., height to width ratio), but also to the input data reflecting the thermal admittance properties of each of the city surfaces. Simulations of the urban heat island show that the urban model is able to capture typical observed characteristics of urban climates qualitatively. In particular, the model produces a significant heat island that increases with height to width ratio. In urban areas, daily minimum temperatures increase more than daily maximum temperatures resulting in a reduced diurnal temperature range compared to equivalent rural environments. The magnitude and timing of the heat island vary tremendously depending on the prevailing meteorological conditions and the characteristics of surrounding rural environments. The model also correctly increases the Bowen ratio and canopy air temperatures of urban systems as impervious fraction increases. In general, these findings are in agreement with those observed for real urban ecosystems. Thus, the model appears to be a useful tool for examining the nature of the urban climate within the framework of global climate models.
Rapid vegetation responses and feedbacks amplify climate model response to snow cover changes
Cook, B.I., G.B. Bonan, S. Levis, and H.E. Epstein
Climate Dynamics, submitted
Abstract
We investigate the response of a climate system model to two different methods for estimating snow cover fraction. In the control case, snow cover fraction changes gradually with snow depth; in the alternative scenarios (one with prescribed vegetation and one with dynamic vegetation), snow cover fraction initially increases with snow depth almost twice as fast as the control method. In cases where the vegetation was fixed (prescribed), the choice of snow cover parameterization resulted in a limited model response. Increased albedo associated with the high snow caused some moderate localized cooling (3-5"C), mostly at very high latitudes (>70"N) and during the spring season. During the other seasons, however, the cooling was not very extensive. With dynamic vegetation the change is much more dramatic. The initial increases in snow cover fraction with the new parameterization lead to a large-scale southward retreat of boreal vegetation, widespread cooling, and persistent snow cover over much of the boreal region during the boreal summer. Large cold anomalies of up to 15"C cover much of northern Eurasia and North America and the cooling is geographically extensive in the northern hemisphere extratropics, especially during the spring and summer seasons. This study demonstrates the potential for dynamic vegetation within climate models to be quite sensitive to modest forcing. This highlights the importance of dynamic vegetation, both as an amplifier of feedbacks in the climate system and as an essential consideration when implementing adjustments to existing model parameters and algorithms. |
Publications:
Gochis, D. J., G. Bonan, E. Brandes, F. Chen, D. Lenschow, M. LeMone, R. Rasmussen, T. T. Warner, M. Ek, K. Mitchell, 2007: A ten-year vision for advancing coupled land-atmosphere prediction. Water Resources Research. (Submitted)
Cook, B. I., G. B. Bonan, S. Levis, H. E. Epstein, 2007: Rapid vegetation responses and feedbacks amplify climate model response to snow cover changes. Clim. Dyn., doi: 10.1007/s00382-007-0296-z. (In Press)
Oleson, K. W., G. B. Bonan, J. Feddema, 2007: An urban parameterization for a global climate model. 1. Formulation and evaluation for two cities. J. Appl. Meteor. Climat.. (In Press)
Oleson, K. W., G. B. Bonan, J. Feddema, M. Vertenstein, 2007: An urban parameterization for a global climate model. 2. Sensitivity to input parameters and the simulated urban heat island in offline simulations. J. Appl. Meteor. Climat.. (In Press)
Cook, B. I., G. B. Bonan, S. Levis, H. E. Epstein, 2007: Rapid vegetation responses and feedbacks amplify climate model response to snow cover changes. Clim. Dyn.. (Submitted)
Levis, S., G. B. Bonan, P. J. Lawrence, 2007: Present-day springtime high-latitude surface albedo as a predictor of simulated climate sensitivity. Geophys. Res. Lett., 34, L17703, doi: 10.1029/2007GL030775.
Lawrence, D. M., P. E. Thornton, K. W. Oleson, G. B. Bonan, 2007: The partitioning of evapotranspiration into transpiration, soil evaporation, and canopy evaporation in a GCM: Impacts on land-atmosphere interaction. J. Hydrometeorol., 8, 862-880, doi: 10.1175/JHM596.1. |
