On the Climate Impact of Surface Roughness Anomalies

On the Climate Impact of Surface Roughness Anomalies

DANIEL B. KIRK-DAVIDOFF

University of Maryland, College Park, College Park, Maryland

DAVID W. KEITH

University of Calgary, Calgary, Alberta, Canada

(Manuscript received 2 May 2007, in final form 12 December 2007)

ABSTRACT

Large-scale deployment of wind power may alter climate through alteration of surface roughness. Pre-vious research using GCMs has shown large-scale impacts of surface roughness perturbations but failed toelucidate the dynamic mechanisms that drove the observed responses in surface temperature. Using theNCAR Community Atmosphere Model in both its standard and aquaplanet forms, the authors haveexplored the impact of isolated surface roughness anomalies on the model climate. A consistent Rossbywave response in the mean winds to roughness anomalies across a range of model implementations is found.This response generates appreciable wind, temperature, and cloudiness anomalies. The interrelationship ofthese responses is discussed, and it is shown that the magnitude of the responses scales with the horizontallength scale of the roughened region, as well as with the magnitude of the roughness anomaly. These resultsare further elucidated through comparison with results of a series of shallow-water model experiments.

1. Introduction

The impact of topography on synoptic- and plan-etary-scale atmospheric motions has been studied care-fully in a variety of modeling contexts (e.g., Hoskins1983; Cook and Held 1992; Schär and Smith 1993;Grubišic et al. 1995; Richter and Mechoso 2004). How-ever, the effect of isolated roughness anomalies at theearth’s surface has received far less attention. The sub-ject has attracted interest recently because of the veryrapid expansion of wind turbine power generation.Wind turbines convert atmospheric kinetic energy intoelectrical energy. Since the total kinetic energy dissipa-tion in the atmosphere is less than 1 PW, and the totalhuman demand for electrical power is projected to beover 20 TW by 2100, it is not unreasonable that heavyuse of wind-generated electricity might influence atmo-spheric motions on large scales (Keith et al. 2004, here-after KEA04). Lange et al. (2003) and Rooijmans(2004) performed micro- and mesoscale modeling simu-

lations of the impacts of offshore wind farms, respec-tively, while Roy et al. (2004) reported mesoscale mod-eling of a continental wind farm. Christiansen andHasager (2005) were able to measure the reduction inwind downwind of a real offshore wind farm using sat-ellite synthetic aperture radar. In addition, other hu-man activities (deforestation, urbanization) and naturalphenomena (climate shifts) have resulted in largechanges in surface roughness characteristics, and so thequestion of how a regional change in surface roughnessmight influence atmospheric circulation patterns is ofsome general interest.

Eliminating a gigaton of carbon per year of emis-sions, a seventh of the emission cuts needed to stabi-lized emissions over the next 50 yr, or one “stabilizationwedge” (Pacala and Socolow 2004), requires a windcapacity of about 3 TW, an amount consistent with theupper end of current projections of wind power deploy-ment (European Wind Energy Association 2005). Ifwind was to provide more than one-seventh of emis-sions mitigation, or if we look more than 50 yr ahead,more wind power would be needed. The average addi-tional kinetic energy dissipation from wind powerwould be 2.3 TW, given a deployment of 3 TW of windpower capacity assuming an atmospheric efficiency of

Corresponding author address: Daniel B. Kirk-Davidoff, 3423Computer and Space Sciences, Department of Meteorology, Uni-versity of Maryland, Colellege Park, College Park, MD 20742.E-mail: [email protected]

JULY 2008 K I R K - D A V I D O F F A N D K E I T H 2215

DOI: 10.1175/2007JAS2509.1

© 2008 American Meteorological Society

JAS2509

0.4 and a capacity factor of 0.3, where “atmosphericefficiency” is the ratio of added large-scale energy dis-sipation to electric generation (KEA04) and capacityfactor is the ratio of average-to-peak electricity produc-tion. This additional dissipation is more than 1% of theclimatological kinetic energy dissipation in the bound-ary layer over land. Given the climate impacts of a 1%change in radiative forcing due to CO2, it seems rea-sonable to investigate the impacts that might arise fromperturbations to surface roughness.

KEA04 simulated the effect of large-scale windpower consumption in a set of general circulationmodel (GCM) experiments. They accomplished this byincreasing the surface roughness length (z0) over largeregions of North America, Europe, and Asia in twoGCMs. They found changes in surface temperature ofup to 1 K, and changes in surface winds of severalmeters per second. The changes were remarkably non-local. Intuitively, it might be expected that the primaryresult of an increase in surface roughness would be alocal reduction of the temperature difference betweenthe surface and the air just above, so that surface tem-perature would decrease and the surface air tempera-ture would increase. In fact, model results showed that,although significant changes in temperature on the or-der of 1°C were sometimes found in the regions whereroughness was increased, equally large changes wereobserved over regions thousands of kilometers from theperturbed regions. Temperature changes varied in signand magnitude depending on the model, the season,and the details of the spatial distribution of perturbedroughness.

In this paper, our goal is to make clear the mecha-nisms by which these patterns arise and lead to theclimate impacts discussed by KEA04. The strongly non-local effects found in that work indicated that theroughness changes were causing change in the atmo-sphere’s general circulation, leading to changes in heatand moisture fluxes. Our approach will be to first per-form experiments with isolated high–surface roughnessregions in the simplified contexts of a shallow-watermodel, and a modified aquaplanet GCM, and then touse these results to aid our interpretation of the modelruns using a more realistic representation of land, sea,and surface topography. We will show that the climateanomalies resulting from surface roughness anomaliescan best be understood as a stationary Rossby waveresponse to the roughness anomaly. The dependence ofthe climate changes on the size and distribution of theroughened regions is consistent with this understand-ing. We turn first to the shallow-water model results,which we use to develop intuition about the factors

controlling the response of a geophysical fluid to anisolated roughness anomaly.

2. Shallow-water model results

To better frame our discussion of general circulationmodel results, it is helpful first to simulate the impact ofa region of isolated roughness on shallow-water flow.We use a modified version of the Matlab codetopo_wave_1.m included in Holton (2004) that inte-grates the shallow-water vorticity equation:

��g�t

� �u � u�g��g�x

��2�

�x2�� g��g�y

��2u

�y2 � �� r��g �

��r ��

�x�

��r �u�

�y, �1�

where u and � are the background wind components; ��gis the perturbation geostrophic vorticity; u�g and ��g arethe perturbation geostrophic wind components and arediagnosed by integrating the vorticity while assumingzero perturbation wind at the boundaries; r is the vor-ticity damping rate, 4 10�6 s�1; and r � is a spatiallyvarying vorticity damping term added to represent theadditional drag of a surface roughness anomaly. Peri-odic boundary conditions are used at the east and westboundaries, while no-slip walls are imposed for the per-turbation velocities on the north and south boundaries.The variable r � is set to zero everywhere, except for aregion within a distance Lm of the origin, where it is setto a fixed value, 4 10�6 s�1, unless otherwise indi-cated. Because of the rigid lid and the lack of surfacetopography the flow is always nondivergent. In allcases, the model equilibrates within several days ofmodel time, and the results discussed below are allsteady-equilibrium solutions.

The background and anomaly damping rates im-posed in the model are large compared to values in thereal atmosphere [plotting wind stress against mean tro-pospheric winds from the Community AtmosphereModel, version 3.1 (CAM3.1) model data and using anatmospheric mass of 104 kg m�2 gives a damping rate ofabout 1.5 10�6 s�1]. Of course the damping rate inthe shallow-water model must also account for radia-tive damping, but in any case, shallow-water results areintended only to illustrate the scale and spatial depen-dence of the most conceptually simple Rossby waveresponse to a damping anomaly, rather than to make aquantitative prediction of the magnitude of the re-sponse.

Figure 1 shows contours of the meridional wind com-ponent for four different boundary conditions, after themodel has been run to equilibrium. In Fig. 1a, the back-

2216 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 65

ground wind is held constant at u � 10 m s�1 and � �0 while , the gradient of the Coriolis parameter, is setto zero. The resulting wind pattern shows strong me-ridional divergence over the region of enhanced damp-ing at the origin, due to the deceleration and conver-gence of the u wind component over the region ofanomalous damping. Mass conservation requires a re-turn flow toward the centerline; this is accomplishedover a broader region of smaller meridional velocitiesdownstream of the damping anomaly.

In Fig. 1b, has been set to 1.62 10�11 s�1, its valueat 45°N, so that the fluid now supports the propagationof Rossby waves. Now parcels that initially move northfrom the centerline must experience a decrease in theirrelative vorticity to compensate for their increasingplanetary vorticity. Thus we see a stationary pattern ofpositive and negative meridional velocity anomaliesthat radiate downstream and away from the east–west-running centerline of the domain. In Figs. 1c,d, thebackground wind u varies sinusoidally from �10 to 10m s�1 in the meridional direction with a wavelength of12 000 km. This simulates the variations of mean tro-pospheric winds from easterlies in the tropics to west-

erlies in the midlatitudes to easterlies in the polar re-gions. In Fig. 1c, u has a maximum at y � 0, so that thedamping anomaly is centered within the westerlies. InFig. 1d, u � 0 at y � 0 and has a maximum at y � 3000km, so the damping anomaly straddles a node of themean zonal wind. In each case, the meridional velocityresponse to the damping anomaly is confined to theregion of positive u winds. In Fig. 1c, the response ofmeridional velocity remains symmetric, but it is con-fined to one strong downstream maximum, and oneweaker minimum downstream of the first, with thesigns reversed on either side of the east–west center-line. In Fig. 1d, the response becomes asymmetric, sincestationary waves are only possible in the region of posi-tive u, and although mean meridional velocity is zero,the largest negative meridional velocity anomaly is�0.55 m s�1, 71% larger than the largest positiveanomaly of 0.32 m s�1.

Figure 2 shows the dependence of the meridionalvelocity on the diameter of the region of enhanceddamping, for two values of (spatially constant) u. In theupper panel, the maximum meridional velocity isshown for u � 10 m s�1 and u � 20 m s�1. In either

FIG. 1. Equilibrium meridional (�) wind component for rigid-lid, shallow-water model simu-lations of flow over an isolated rough patch. In each figure, the region within 1000 km of theorigin (outlined in red) has an elevated damping rate. Here x and y distances are noted on theaxes in thousands of kilometers. Positive contours are thick, negative contours are fine, witha contour interval of 0.2 m s�1, starting at 0.1 m s�1. (a) Constant background zonal flow (10m s�1) on an f plane. (b) Constant background zonal flow on a plane. (c) Background zonalwind varies sinusoidally in y with a maximum (10 m s�1) at y � 0, and a wavelength of 12 000km, shown plotted against y, with the magnitude in m s�1 on the x axis. (d) As in (c), but thebackground wind is shifted to have U0 � 0 at y � 2000 km.


Fig 1 live 4/C

case, meridional velocities increase approximately lin-early with the scale of the damping region until a peakis reached, at a scale that depends on u. This depen-dence of the peak scale on u arises because the wave-length of a stationary Rossby wave scales as L �2��u/. That is, the magnitude of the response to thedamping anomaly increases as the horizontal scale ofthe roughened region increases, until the upstream andthe first downstream meridional velocity anomalies ofthe opposite sign are separated by approximately L/2.For u � 10 m s�1, L/2 � 2470 km, while for u � 20m s�1, L/2 � 3490 km; the peak of the scale dependenceof the maximum meridional velocity occurs when theseparation of the upstream and downstream anomalyexceed these values by about 500 km. As noted in theupper panel, a good rule of thumb is that the maximumresponse occurs at an anomaly radius equal to L/3. Thelower panel shows the fractional area of the domainwith meridional velocities greater than 0.25 m s�1. Foru � 10 m s�1, this area increases monotonically with thediameter of the damping anomaly, while for u � 20m s�1, the area decreases beyond the peak in maximumvelocity.

Thus, our shallow-water model experiments suggestthat the response to a large region of enhanced surfaceroughness should scale with the size of the roughnessanomaly, until a saturation size is reached at a lengthscale that depends on the mean wind. They show that inthe presence of a meridionally varying background

zonal wind, the response to the roughness anomalyshould remain confined to the region of positive zonalwinds, and that this constraint also limits the down-stream extent of the anomaly. The Rossby wave re-sponse also results in a stronger anomaly downstreamof the roughened region than upstream. We will showthat these results are reflected in results from morecomplex models of the atmosphere.

3. General circulation model runs

The model experiments discussed below were run onthe National Center for Atmospheric Research(NCAR) CAM3.1, a freestanding component of theCommunity Climate System Model (Collins et al.2006). Simplified geometry runs were performed inaquaplanet mode.

a. Aquaplanet runs

The aquaplanet runs allow us to explore the physicalmechanisms discerned from the results of the shallow-water model at an intermediate level of realism. In par-ticular, we wished to identify the characteristic patternof climate changes for a single roughness anomaly, pre-venting the influence of one roughness anomaly on theflow approaching the next roughness anomaly down-stream, and avoiding zonal asymmetries in the flow. InCAM3.1’s aquaplanet mode, the globe is covered witha fixed-temperature ocean. The temperature distribu-

FIG. 2. Dependence of meridional wind anomaly on the horizontal scale of the dampinganomaly. (a) Maximum meridional wind anomaly. One-third of a stationary Rossby wave-length for each background wind speed is indicated with text and arrows. (b) Fractional areaof domain with meridional wind greater than 0.25 m s�1.


Fig 2 live 4/C

tion is zonally uniform, and varies in latitude from amaximum of 297.5 K at the equator to a minimum of270.8 K at the poles. The aquaplanet experiments wereperformed in perpetual spring mode, so the solar incli-nation at noon stays fixed in time. In the standard aqua-planet mode supplied by CAM3.1 surface roughness iscontrolled by the Charnock relation, so that Cdn �0.0027/U10 � 0.000142 � 0.0000764U10, where Cdn is theaerodynamic drag coefficient for neutral conditions,and U10 is the magnitude of the wind at 10 m above thesurface (m s�1). Since Cdn � k2/[ln(z/z0)]2, where k �0.4 is the von Kármán constant, z � 10 m is the refer-ence height, and z0 is the surface roughness length,

z0 � �10 m� exp

�� k2

0.0027�U10 � 0.000142 � 0.0000764U10�.

�2�

In our runs, we eliminated the connection betweenwind speed and the aerodynamic drag coefficient. In-stead, a constant value of 10 m s�2 was substituted forU10 in the CAM3.1 subroutine flxoce, except for anisolated circular region of enhanced roughness, whereU10 was set to 80 m s�1. These values of U10 correspondto z0 values of 86 m and 6.4 cm, respectively, and Cdn

values of 0.0012 and 0.0063, respectively. We ran threeexperiments: case A has a rough patch with a radius of20° of latitude and longitude, centered at 40°N and180°W; case B has a rough patch with a radius of 10°centered at 40°N and 180°W; case C has a rough patchwith a radius of 10° centered at 30°N and 180°W. Inaddition, for all runs, the meridional band from 0° to10°E was slightly roughened, using a wind speed of 20m s�1 for the Charnock relation, producing a surfaceroughness of 0.81 mm in that band. This is done so thatthe roughened circular patch will not be the only fea-ture breaking the symmetry of the planet. All runs areintegrated for 16 yr.

b. Full-model runs

KEA04 performed simulations using both theCAM2.0 and the Geophysical Fluid Dynamics Labora-tory (GFDL) atmospheric model, using two represen-tations of the wind turbine drag: by increasing the sur-face roughness length, or by adding an explicit dragterm into the boundary layer momentum equations.The results were generally similar for both forms offorcing, and in both models, so we will confine ourdiscussion to the CAM2.0 runs using surface roughnesschanges, which are most directly comparable with the

CAM3.1 aquaplanet runs. They included runs in whichthe surface roughness in a number of regions represent-ing high population density and energy consumptionwas increased to represent increasing usage of windturbines. The roughness enhancement was designed tobe of comparable magnitude to that which would resultif the present global electrical demand were met en-tirely with wind power. Of course, this is a much higherfraction than anyone anticipates, but if wind powerwere to supply 25% of carbon dioxide mitigation by theend of the century, it would be generating the equiva-lent of our present total consumption (Pacala and So-colow 2004).

In addition, runs were performed in which all landregions were roughened. In all these runs, land surfacetemperatures were allowed to vary, while sea surfacetemperatures were held fixed. Here we consider runs inwhich there are three regions of enhanced surfaceroughness length. All runs enhance roughness in thesame regions (over central North America, western Eu-rope, and eastern Asia), but with varying roughnesslengths. There are seven enhanced roughness runs,each 15 yr long. There are four control runs, each also15 yr long. Enhanced roughness runs and control runsare separately averaged together; the differences of thetwo ensembles are presented below.

4. Results and discussion

a. Aquaplanet runs

Figures 3–10 show the results of the aquaplanet simu-lations. The aquaplanet model differs from the shallow-water model in several obvious ways: there is no rigidlid, and the flow is compressible, so divergence andvertical motions are possible; evaporation of water va-por and release of latent heat in the atmosphere arepossible; radiation can damp temperature anomalies;and surface fluxes include sensible and latent heating,as well as momentum damping. Nevertheless, as we willshow, the results of experiments with regions of en-hanced surface roughness are qualitatively quite similarto the results for the shallow-water model.

In the difference panels, significance is indicated bycrosshatching, and determined by breaking the datainto three equal time blocks, taking the standard devia-tion over the three chunks for each grid square, andrequiring, for significance, that the differences at agiven latitude be larger than the maximum standarddeviation for that latitude. That is, at each crosshatchedpoint, the difference between the experiment and thecontrol run is larger than the variability of this differ-ence at any point at that latitude. The background state,


shown in the top panel of Fig. 3, is simple, with bands ofeast and west zonal winds at the surface. The zonalwind bands correspond to bands of equatorward andpoleward meridional winds forced by surface friction(not shown). The lower panels of Fig. 3 show a clearreduction in the magnitude of the zonal flow in the(circled) regions of enhanced surface roughness.

Taking the differences between the control case andthe three cases with enhanced roughness, we see thisstrong reduction in surface winds confirmed in Fig. 4.Maximum reductions in surface zonal winds are 5.5m s�1 for case A, 4.5 m s�1 for case B, and 4.9 m s�1 forcase C, from control values of 9.4 m s�1 in heart of thezonal wind belt. Zonal winds to the north and south of

the roughened patches appear enhanced in Fig. 4, but itshould be noted that these represent reductions in windspeed in regions where the surface winds were easterlyin the control case. The magnitude of the meridionalcomponent of the surface wind (Fig. 5) is also reducedin the region of enhanced roughness, but to a lesserdegree than the zonal wind, so that the angle of thewind direction in the southern half of the roughenedregion, where winds are southwesterly, increases from198° to 205°. This represents the expected increase inthe cross-isobar component of the wind as friction in-creases.

As for the shallow-water model results above, thesereductions in wind speed in the region of enhanced

FIG. 3. Aquaplanet 2-m zonal wind. (a) Results from the control run; (b) results for a20°-wide circular area of high roughness centered at 40°N; (c) results for a 10°-wide rough areacentered at 40°; (d) results for 10° centered at 30°N.


Fig 3 live 4/C

surface roughness require changes in both meridionaland vertical wind speeds outside the roughened regionto satisfy mass conservation and momentum balance. Ineach case, inspection of the region immediately south-west of the rough patch shows convergence in the zonalwind and divergence in the meridional wind. At theeastern edge of the rough patch, the zonal wind is di-vergent and the meridional wind is convergent. Thus,there are anomalously southerly winds to the northwestand southeast of rough patches, and anomalouslynortherly winds to the southwest and northeast of therough patches. As in the shallow-water simulations, the

meridional velocity anomalies are not symmetric, butrather extend farther to the east of the rough patchesthan to the west. Further, the strongest meridionalanomalies occur within the belt of westerly mean winds.The different results for cases A and B confirm theshallow-water model prediction that meridional veloc-ities scale in both size and magnitude with the lengthscale of the rough patch, so that a larger rough patchresults in both spatially larger and more intense merid-ional and zonal wind anomalies. To test whether resultsshown here can be fully attributed to the stationarywave response, we can examine the stationary and tran-

FIG. 4. Aquaplanet 2-m zonal wind anomalies. (a) Difference between the run with a20°-wide circular area of high roughness centered at 40°N and the control run; (b) differencefor a 10°-wide rough area centered at 40°; (c) difference for 10° centered at 30°N. Cross-hatching shows points where changes are significant, according to the standard explained inthe text.


Fig 4 live 4/C

sient eddy components of the anomaly in meridionalflux of zonal momentum, � u and ��u�. These fluxes areshown for the lowest-model-level winds in Fig. 6. Thestationary wave component of the momentum fluxanomaly is larger than the transient component by anorder of magnitude. Similar results are found at differ-ent model levels and for fluxes of sensible heat (notshown).

Because sea surface temperatures are held fixed inthe aquaplanet runs, air temperature changes aresmaller and, presumably, of different geographic distri-bution than they would be in a fully coupled model.Figure 7 shows the surface air temperature anomaly.

The warm anomaly in the roughened patch is a straight-forward consequence of the increased roughnesslength, which results in a 5-times-larger heat flux trans-fer coefficient, and thus substantially larger sensibleheat fluxes from sea to atmosphere even for reducedwinds. The temperature anomalies outside the roughpatch are clearly correlated with the meridional windanomalies via temperature advection: northerly windanomalies yield cold surface air temperature anomalies,while southerly meridional wind anomalies yield warmsurface air temperature anomalies.

Both the meridional wind and the surface air tem-perature anomalies scale with the size of the roughened

FIG. 5. Aquaplanet 2-m meridional wind anomalies. (a) Difference between the run with a20°-wide circular area of high roughness centered at 40°N and the control run; (b) differencefor a 10°-wide rough area centered at 40°; (c) difference for 10° centered at 30°N.


Fig 5 live 4/C

region. Thus, for case A, the largest meridional surfacewind anomaly is �2.1 m s�1, and the largest tempera-ture anomaly outside the rough patch itself is �0.4 K,while for cases B and C, where the roughened region ishalf the width of the roughened region in case A, thelargest surface wind anomalies are �1.2 and �1.1m s�1, respectively, while the largest temperatureanomalies are �0.2 K for both cases B and C. Thus themeridional wind and surface temperature anomalies(outside the roughened region) scale linearly with thelength scale of the region for these experiments.

Figure 8 shows anomalies of 850-hPa meridional �(the vertical velocity in pressure coordinates, positivedownward), cloud fraction, downward-directed solarradiative flux, and upward-directed infrared radiativeflux for case A (the other cases show similar but weakerpatterns). There is a consistent pattern of decreasedcloud fraction on the northeast flank of the roughenedregion, with increased cloud fraction on the northwestflank and directly to the east. These changes are con-sistent with, though not directly proportional to,changes in vertical velocity, which in turn are related totemperature advection by the meridional wind anoma-lies. In addition, the positive (poleward) meridionalsurface wind anomaly to the south of the roughenedregion results in surface divergence that weakens up-ward vertical velocity in the aquaplanet ITCZ. This re-

sults in a sharp increase in the surface solar flux, and asignificant increase in the surface IR cooling along aline just north of the equator, south of the roughnessanomaly. Despite the strong radiative flux changes,cloud fraction in this band does not change noticeably.Rather, radiative changes result from reductions incloud optical depth. Because, in general, increasingcloud fraction is related to both decreasing downwardsurface solar flux and decreasing upward infrared flux,of approximately equal magnitudes, the sum of infraredand solar flux is unrelated to cloud fraction.

These relationships are shown as scatterplots in Fig.9 for case A. Grid points within the roughened regionare shown with a red circle around the data point. Cor-relations are somewhat different in the roughened re-gion and outside that region. In particular, cloud frac-tion is positively correlated with poleward meridionalwind speed outside the roughened region, but notwithin it, while upward IR and sensible and latent heatfluxes are negatively correlated with poleward windspeed outside the roughened region, but poorly corre-lated within it. This difference is due to the direct in-fluence of the roughness changes on surface fluxeswithin the rough region, which overwhelms the second-ary effect of wind changes. We conclude that in regionsoutside the roughened patch at least a third (r2 � 0.37)of the point-by-point change in temperature can be at-

FIG. 6. Aquaplanet lowest-model-level momentum fluxes. (a) Stationary wave flux. (b)Transient eddy flux.


Fig 6 live 4/C

tributed to surface wind direction changes due to theroughness change. A small fraction of the variability iscaused by climate noise (about 0.1 K). The remainingfraction of the variability must be caused by moresubtle changes in the flow that lead to radiative forcingof the surface temperature. Note that cloud fractionanomaly is positively correlated with meridional windanomaly, but that the impact on surface temperature ismuted, since the cloud fraction increases are accompa-nied by approximately canceling reductions in down-ward solar flux and upward IR flux.

Wind anomaly patterns for case A at 850- and 500-hPa pressure surfaces (Fig. 10) are similar to those atthe surface, except that the patterns become more sym-

metric as pressure decreases, and a clear global station-ary wavenumber 5 pattern becomes apparent in themeridional wind anomaly at 500 hPa. The penetrationof the wind anomaly to the middle troposphere isstrongly dependent on horizontal scale. For case A, theratio of the negative wind anomaly over the rough re-gion at 500 hPa to the anomaly at the surface is 1.07,while for cases B and C (not shown) the ratios are 0.75and 0.60.

In summary, the imposition of a patch of increasedsurface roughness on an otherwise smooth world resultsin a slowing of winds directly over the roughened re-gion. This deceleration forces a stationary wave re-sponse, trapped within the band of westerly zonal wind.

FIG. 7. Aquaplanet 2-m air temperature anomalies. (a) Difference between the run with a20°-wide circular area of high roughness centered at 40°N and the control run; (b) differencefor a 10°-wide rough area centered at 40°; (c) difference for 10° centered at 30°N.


Fig 7 live 4/C

This response is largely barotropic, extending throughthe troposphere, for a sufficiently large roughened re-gion; the ratio of wind anomaly aloft to surface anomalyis smaller for a smaller roughened region. Vertical ve-locity anomalies are largely associated with tempera-ture advection associated with the meridional windanomalies. The meridional velocity anomalies are asso-ciated with advective temperature changes, while thevertical velocity anomalies are associated with changesin cloud fraction and precipitation (not shown). Thecloud fraction changes result in surface solar and infra-red flux changes that in this case almost perfectly can-

cel, but in principle could result in either heating orcooling, depending on the details of the mean cloudfield.

b. Full-model runs

As discussed in KEA04, the imposition of additionalsurface roughness in three large regions (NorthAmerica, Europe, and eastern Asia) resulted in a num-ber of statistically significant changes in climate. Globalmean temperature change was very small, but localchanges in temperature of up to 2 K were found invarious locations (see Fig. 11). In this section we will

FIG. 8. (a) Difference in vertical wind in pressure coordinates between the run with a20°-wide circular area of high roughness centered at 40°N and the control run; (b) differencein cloud fraction for the same run as in (a); (c) difference in downward surface solar radiativeflux; (d) difference in upward surface infrared radiative flux.


Fig 8 live 4/C

discuss the surface energy budget changes, in order togive a physical explanation for the changes in tempera-ture. We also compare results for the full-model runswith three roughened regions with results from a set ofruns with a single roughened region in North Americaof two different sizes.

Inspection of Fig. 11 shows that 2-m air temperatureanomalies are not related in a geometrically simple way

to the roughness forcing (note that SSTs were heldfixed in these runs, so surface air temperatures do notchange substantially over the oceans). Over NorthAmerica, there is a region of strong warming centeredon, but extending beyond on all sides, the region wheresurface roughness was increased. In Europe, there is aslight cooling in the region where roughness was in-creased, but there is strong cooling northeast of the

FIG. 9. Scatterplots of aquaplanet temperature and surface energy flux anomalies plottedagainst meridional surface wind anomalies for case A (rough region centered at 40°N, with aradius of 10° latitude and longitude). Red circled data points lie within the roughness anomaly,while blue crossed data points lie outside the roughness anomaly. Correlation coefficients foreach set of points are indicated in the legends, and linear regression fit lines are shown for eachsubset of data points: (a) surface air temperature, (b) cloud fraction, (c) surface solar flux, (d)surface IR flux, (e) surface sensible heat flux, and (f) surface latent heat flux.


Fig 9 live 4/C

FIG. 10. Aquaplanet tropospheric wind anomalies. (a) Difference in 850-hPa zonal windbetween the run with a 20°-wide circular area of high roughness centered at 40°N and thecontrol run; (b) 500-hPa zonal wind difference; (c) 850-hPa meridional wind difference; (d)500-hPa meridional wind difference.

FIG. 11. Full-model 2-m air temperature anomalies. Grid points with the symbol * indicatelocations of increased surface roughness intended to simulate wind turbine installations.


Fig 10 live 4/C Fig 11 live 4/C

roughened region, and moderate warming south andeast of the roughened region. In East Asia, there isagain slight cooling where the roughness was increased,and again strong cooling to the north and east. Warm-ing to the south and east is precluded since sea surfacetemperatures are held fixed.

To understand the causes of this pattern of tempera-ture change, we turn first to the wind anomalies. Figure12 shows the surface zonal and meridional winds in thecontrol runs, and the anomalies when the mean of thecontrol runs is subtracted from the mean of the runswith wind turbines. As in the aquaplanet runs, the re-sults are straightforward: slowing of the mean windover the roughened regions, with accompanying pat-terns of divergence and convergence at the upstream

and downstream boundaries of the roughened regions.Figure 13 shows that, as in the aquaplanet experiments,the response is largely barotropic: wind anomalies at850 and 500 hPa are similar in magnitude and distribu-tion to those at the surface. As in the aquaplanet case,the zonally varying response to the roughness changesis dominated by the stationary wave response, with thetransient eddy response negligible by comparison (notshown). The zonal mean temperature response shows anet cooling at high latitudes in both hemispheres ofabout 0.15 K, and calculations of the zonal mean me-ridional energy transport show slight, but statisticallysignificant reductions of poleward meridional energyflux in both hemisphere of 0.02 PW. These small zonalmean reductions in meridional heat flux are equally

FIG. 12. Full-model 2-m winds (m s�1). (a) Control zonal wind; (b) zonal wind anomaly; (c)control meridional wind; (d) meridional wind anomaly.


Fig 12 live 4/C

distributed between transient and zonal mean eddyfluxes.

We next examine changes in surface heat fluxes. Fig-ure 14 shows changes in solar, infrared, sensible, andlatent heat at the surface, due to increased surfaceroughness. Figure 15 shows changes in vertical pressurevelocity (w), cloud fraction, and precipitation. We seethat changes in individual components of the surfaceheat budget are somewhat more consistent from oneroughened region to another than are changes in tem-perature. For example, North America and east Asia,despite having oppositely signed temperature anoma-lies, both show reductions in downward solar flux in theroughened regions, reductions in upward surface infra-red radiative flux and sensible heat flux, while both

show increases in latent heat flux and in the meridionalwind component. Thus, oppositely signed changes insurface air temperature over these regions must resultfrom differing magnitudes of surface energy fluxes thatare consistent in sign in each of the roughened regions.

The surface roughness changes cause changes in thesurface energy fluxes through several distinct mecha-nisms. First, an increase in surface roughness directlycauses an increase in the aerodynamic drag coefficient.This means that for a given temperature difference be-tween the surface and the 2-m reference height, thesensible and latent heat fluxes will be larger; over landregions where the surface energy budget must be inequilibrium over long time scales, this means that the2-m temperature will be closer to the surface tempera-

FIG. 13. Full-model tropospheric wind anomalies (m s�1). (a) 850-hPa zonal wind; (b)500-hPa zonal wind; (c) 850-hPa meridional wind; (d) 500-hPa meridional wind.


Fig 13 live 4/C

ture (and thus generally cooler), so that the sensibleand latent heat fluxes can remain in balance with thesum of solar and infrared radiative flux, whose changeis relatively small. Second, a change in surface rough-ness may result in a change in surface wind, and inwinds aloft, if these changes result in a change in tem-perature advection. If temperatures aloft are warmed,this will tend to reduce the net upwelling infrared ra-diative flux. Warm advection will also tend to reduceupward sensible and latent heat fluxes by reducing thetemperature difference between the ground and the air.Finally, increasing surface roughness causes localchanges in vertical velocity that partially compensatefor divergence and convergence in the zonal wind,

while changes in meridional wind result in temperatureadvections that are also associated with vertical windanomalies. These in turn cause cloud changes thatcause changes in both the downwelling infrared radi-ance and the surface solar radiation flux.

These relationships are shown in Fig. 16, where, as inFig. 9, the relationship between meridional windanomaly and surface air temperature, cloud fraction,and surface energy fluxes is shown. Grid points withinthe roughened regions are indicated by a red circle,land points (where surface temperatures are allowed tovary freely) outside the roughened region are shown asblack dots, while ocean points (where SST is held fixed,as in the aquaplanet runs) are marked with blue

FIG. 14. Full-model results for the surface energy budget (W m�2). (a) Downward surfacesolar radiative flux. (b) Upward surface IR radiative flux. (c) Upward sensible heat flux. (d)Upward latent heat flux.


Fig 14 live 4/C

crosses. These plots are generally consistent with thosein Fig. 9, although the correlations are generallyweaker, especially over land. Weaker correlations overland are reasonable, since both the surface-to-air tem-perature difference and the surface fluxes can indepen-dently adjust to changes in temperature advection.Weaker correlations even over ocean regions may bedue to the greater unforced variability in the full-modelsimulations. The meridional wind anomaly is, as for theaquaplanet results, positively correlated with the sur-face air temperature anomaly. This association occursbecause of the increased advection of heat associatedwith poleward meridional wind anomalies and becauseof the enhanced downward infrared radiation flux as-sociated with warm advection aloft. Cloud fraction in-creases with increasing meridional wind as well, but, aswith the aquaplanet simulation, its global impact issmall because of the cancellation of its impacts on thesolar radiative flux (cooling) and the infrared radiativeflux (warming). In contrast with the aquaplanet runs,the correlation of cloudiness and meridional wind holdsonly over the roughened regions themselves (where r �0.51); the correlation is very small outside the rough-ened regions (r � 0.12). The difference may be due to

the relative smallness of the meridional wind anomaliesoutside the roughened regions, which prevents thecloud impacts from rising above the climate noise,while within the rough regions the meridional windanomalies are larger, and the impact on clouds moresignificant.

It is instructive to compare the results for multipleroughened regions and for a single roughened region.Again, the NCAR CAM3.1 model is used, with fixedsea surface temperatures. Figure 17 shows the zonalwind anomaly at three levels of the atmosphere for asingle roughened region, either 10° of latitude and lon-gitude on a side, or 20°. The roughened region is lo-cated over North America, and is indicated in the fig-ures by a black square. As with the aquaplanet results,the penetration of the surface wind anomaly to the freetroposphere depends strongly on the scale of the rough-ened region. The anomalies due to the smaller rough-ened region shown in the left-hand panels are onlyspottily significant at the 95% confidence level at asmall fraction of grid points, indicating a lack of overallsignificance to the pattern, except for the reduction oflowest-model-level westerlies directly over the rough-ened region in Fig. 17a. For the larger roughened re-

FIG. 15. Full-model results for vertical winds, clouds, and precipitation. (a) Vertical wind(�) (hPa s�1). (b) Cloud fraction. (c) Precipitation (m yr�1).


Fig 15 live 4/C

gion simulated in Figs. 17b,d,f, the anomaly patternshows coherent statistically significant results extendingfar downstream and penetrating upward through the500-hPa level.

The regional pattern of wind change confirms thebroad pattern found in the other runs—reduction ofwinds over the roughened region and larger anomaliesdownstream than upstream—but it also reveals the

strong dependence of the details of these effects on thesize and placement of the roughened region. The strongincrease in the westerly wind component over the At-lantic in Figs. 17b,d,f, but not seen in the KEA04 simu-lations, shows a strong dependence of the results on theexact location of the roughened region: in the singlelarge roughened region case, its eastern boundary ex-tends to 84°W, while in the KEA04 results, the North

FIG. 16. As in Fig. 9, but for full-model results. Temperature and surface energy fluxanomalies plotted against meridional surface wind anomalies. Red circled data points liewithin the roughness anomalies, black dotted data points are land data points lying outside theroughness anomalies, and blue crossed data denote ocean data points. Correlation coefficientsfor each set of points are indicated in the legends, and regression lines are plotted for eachsubset of data points.


Fig 16 live 4/C

American roughened region extends only to 92°W.Comparing wind changes over Scandinavia, we see thata single North American rough region causes a strongreduction in winds, while the addition of a Europeanroughened region shifts the region of negative zonalwind anomalies down over the roughened region (seeFig. 13), and causes a weak positive anomaly over Scan-dinavia. These results show that the stationary waveresponse to changes in surface roughness is stronglymodulated by the details of the background flow (thusthe differences between the aquaplanet results andsingle North American rough region results), but sug-gest that the response of the flow to multiple windfarms may be understood as a linear superposition ofthe response to individual rough regions. Further ex-

periments will be needed to convincingly demonstratethis linearity.

5. Conclusions

The results of our model experiments demonstratethat the addition of surface roughness anomalies canhave a noticeable impact on model surface climate.This impact occurs as a consequence of changes in thesurface and tropospheric wind fields. Slowing of thezonal wind over the roughened region yields stationarywave patterns of divergence and convergence that areassociated with meridional and vertical wind anomaliesthat in turn affect temperature advection and cloud

FIG. 17. Full-model results for a single rough patch (roughened region is indicated with a square). Zonal wind anomalies are shownat the model sigma coordinate levels equivalent to approximately (a), (b) 992; (c), (d) 867; and (e), (f) 511 hPa. In (a), (c), and (e) resultsare shown for a relatively small (�10° longitude and latitude) rough region, while in (b), (d), and (f) results are for a relatively large(�20°) rough region.


Fig 17 live 4/C

fraction. These changes in turn affect the surface heatbudget, resulting in the observed temperature anoma-lies. In addition to these explanatory findings, we havealso shown that the climate impact of the roughnessanomalies scales with their horizontal extent as well aswith their roughness. This scaling occurs both becausethe amplitude of the barotropic response scales with thehorizontal scale of the wind farm, and because the pen-etration of the wind anomaly from the surface increaseswith horizontal scale.

Our results demonstrate the temperature changespointed out by KEA04 are the result of straightforwardphysical processes that are likely, at the level of indi-vidual components of the surface energy budget, to bereplicated in any GCM. Thus we can have some confi-dence that the predicted changes in these individualterms would actually occur were such synoptic-scaleroughness anomalies ever to be created via wind tur-bine construction. However the net surface tempera-ture changes are the result of delicate balances amongthese energy budget changes, and so may be expectedto depend on details of model parameterizations, espe-cially of cloud processes. In ongoing work, we are per-forming simulations with a slab ocean model, whichallows a larger temperature response to the circulationchanges over the ocean, and may be expected to alterthe spatial pattern of both wind and temperatures. Inaddition, we are exploring the impact of time-varyingchanges of roughness, including the implications forweather modification on the synoptic scale.

Acknowledgments. This research was made possiblethrough support from the Center for Integrated Studyof the Human Dimensions of Global Change. This Cen-ter has been created through a cooperative agreementbetween the National Science Foundation (SBR-9521914) and Carnegie Mellon University. Additionalsupport was provided via National Science FoundationGrant ATM-0457515. We thank two anonymous re-viewers for helpful criticisms and suggestions.

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On the Climate Impact of Surface Roughness Anomalies

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