## Publications

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2017
Siler, N, Po-Chedley S, Bretherton CS.  2017.  Climate Dynamics. 48(305):1-12.   10.1007/s00382-017-3673-2   Abstract

Despite the increasing sophistication of climate models, the amount of surface warming expected from a doubling of atmospheric CO2 (equilibrium climate sensitivity) remains stubbornly uncertain, in part because of differences in how models simulate the change in global albedo due to clouds (the shortwave cloud feedback). Here, model differences in the shortwave cloud feedback are found to be closely related to the spatial pattern of the cloud contribution to albedo ($$\alpha$$) in simulations of the current climate: high-feedback models exhibit lower (higher) $$\alpha$$ in regions of warm (cool) sea-surface temperatures, and therefore predict a larger reduction in global-mean $$\alpha$$ as temperatures rise and warm regions expand. The spatial pattern of $$\alpha$$ is found to be strongly predictive (r=0.84) of a model’s global cloud feedback, with satellite observations indicating a most-likely value of (0.58pm 0.31\ Wm$$^{-2}$$ K$$^{-1}$$ (90% confidence). This estimate is higher than the model-average cloud feedback of 0.43 Wm$$^{-2}$$ K$$^{-1}$$, with half the range of uncertainty. The observational constraint on climate sensitivity is weaker but still significant, suggesting a likely value of 3.68 ± 1.30 K (90% confidence), which also favors the upper range of model estimates. These results suggest that uncertainty in model estimates of the global cloud feedback may be substantially reduced by ensuring a realistic distribution of clouds between regions of warm and cool SSTs in simulations of the current climate.

2016
Christian, JE, Siler N, Koutnik M, Roe G.  2016.  Journal of Climate.   10.1175/JCLI-D-16-0128.1   Abstract

Glacier mass balance provides a direct indicator of a glacier’s relationship with local climate, but internally-generated variability in atmospheric circulation adds a significant degree of noise to mass-balance timeseries, making it difficult to correctly identify and interpret trends. This study applies “dynamical adjustment” to seasonal mass-balance records to identify and remove the component of variance in these timeseries that is associated with large-scale circulation fluctuations (“dynamical adjustment” here refers to a statistical method and not a glacier’s dynamical response to climate). Mass-balance records are investigated for three glaciers: Wolverine and Gulkana in Alaska, and South Cascade in Washington. North Pacific sea-level pressure and sea-surface temperature fields perform comparably as predictors, each explaining 50–60% of variance in winter balance and 25–35% in summer balance for South Cascade and Wolverine Glaciers. Gulkana glacier, located farther inland, is less closely linked to North Pacific climate variability, with the predictors explaining roughly 30% of variance in winter and summer balance. To investigate the degree to which this variability affects trends, adjusted mass-balance timeseries are compared to those in the raw data, with common results for all three glaciers: winter balance trends are not significant initially, and do not gain robust significance after adjustment despite the large amount of circulation-related variability. However, the raw summer balance data have statistically significant negative trends that remain after dynamical adjustment. This indicates that these trends of increasing ablation in recent decades are not due to circulation anomalies and are consistent with anthropogenic warming.

Siler, N, Durran D.  2016.  Journal of the Atmospheric Sciences.   10.1175/JAS-D-15-0371.1   Abstract

Recent studies have shown that weak rain shadows in the Cascade Mountains are associated with passing warm fronts, but the specific mechanisms responsible for this connection have eluded consensus. One theory maintains that weak rain shadows are the result of enhanced precipitation over eastern slopes caused by easterly upslope flow; the other suggests that condensation is produced primarily over the western slopes, with enhanced east-slope precipitation occurring in dynamical regimes that minimize descent and evaporation east of the crest. Here these mechanisms are investigated through numerical simulations involving both real and idealized topography. Consistent with the second theory, storms with weak rain shadows are found to exhibit much weaker mountain waves in the lee of the Cascades than storms with strong rain shadows, with correspondingly weaker lee-side evaporation. The muted wave activity during weak-rain-shadow storms is found to be caused by cold, zonally-stagnant air at low levels in the lee, which precedes the warm front, and remains in place as the progression of the front is impeded by the mountains. As the front brings warmer air aloft, the static stability of the zonally-stagnant layer increases, making it more resistant to erosion by the overlying flow. This in turn allows the weak rain shadow to persist long after the front has passed. If the mid-latitude storm tracks shift poleward in a warmer climate, our results suggest there could be an increase in the strength of the rain shadow in mountainous regions astride the current storm tracks.

2015
Siler, N, Durran D.  2015.  Journal of the Atmospheric Sciences. 72:803-820.: American Meteorological Society   10.1175/JAS-D-14-0200.1   Abstract

The partial reflection of mountain waves at the tropopause has been studied extensively for its contribution to downslope windstorms, but its impact on orographic precipitation has not been addressed. Here linear theory and numerical simulations are used to investigate how the tropopause affects the vertical structure of mountain waves and, in turn, orographic precipitation. Relative to the no-tropopause case, wave-induced ascent above the windward slope of a two-dimensional ridge is found to be enhanced or diminished depending on the ratio of the tropopause height to the vertical wavelength of the mountain waves?defined here as the ?nondimensional tropopause height? . In idealized simulations of flow over both two-dimensional and three-dimensional ridges, variations in are found to modulate the precipitation rate by roughly a factor of 2 under typical atmospheric conditions. The sensitivity of precipitation to is related primarily to the depth of windward ascent but also to the location and strength of leeside descent, with significant impacts on the distribution of precipitation across the range (i.e., the rain-shadow effect). Using a modified version of Smith and Barstad?s orographic precipitation model, variations in are found to produce significant rain-shadow variability in the Washington Cascades, perhaps explaining some of the variability in rain-shadow strength observed among Cascade storms.

2014
Siler, N, Roe G.  2014.  Geophysical Research Letters. 41:2606-2613.   10.1002/2013GL059095   Abstract

Future changes in orographic precipitation will have important consequences for societies and ecosystems near mountain ranges. Here we use a simple numerical model to evaluate the response of orographic precipitation to surface warming under idealized conditions representative of the strongest orographic storms. We find an upward shift in the pattern of condensation with warming, caused by larger fractional changes in condensation at low temperature and amplified warming aloft. As a result, the distribution of precipitation shifts downwind, causing larger fractional changes in precipitation on the lee slope than on the windward slope. Total precipitation is found to increase by a smaller fraction than near-surface water vapor, in contrast to expected changes in other types of extreme precipitation. Factors limiting the increase in orographic precipitation include the pattern of windward ascent, leeside evaporation, and thermodynamic constraints on the change in condensation with temperature.

2013
Siler, N, Roe G, Durran D.  2013.  Journal of Hydrometeorology. 14:122-139.: American Meteorological Society   10.1175/JHM-D-12-045.1   Abstract

Washington State's Cascade Mountains exhibit a strong orographic rain shadow, with much wetter western slopes than eastern slopes due to prevailing westerly flow during the winter storm season. There is significant interannual variability in the magnitude of this rain-shadow effect, however, which has important consequences for water resource management, especially where water is a critically limited resource east of the crest. Here the influence of the large-scale circulation on the Cascade rain shadow is investigated using observations from the Snowfall Telemetry (SNOTEL) monitoring network, supplemented by stream gauge measurements. Two orthogonal indices are introduced as a basis set for representing variability in wintertime Cascade precipitation. First, the total precipitation index is a measure of regionwide precipitation and explains the majority of the variance in wintertime precipitation everywhere. Second, the rain-shadow index is a measure of the east?west precipitation gradient. It explains up to 31% of the variance west and east of the crest. A significant correlation is found between the winter-mean rain shadow and ENSO, with weak (strong) rain shadows associated with El Niño (La Niña). The analysis is supported by streamflow data from eastern and western watersheds. A preliminary review of individual storms suggests that the strongest rain shadows are associated with warm-sector events, while the weakest rain shadows occur during warm-frontal passages. This is consistent with known changes in storm tracks associated with ENSO, and a variety of mechanisms likely contribute.