Sensitivity of orographic precipitation to evolving topography

Transcription

1 Sensitivity of orographic precipitation to evolving topography Joseph Galewsky Department of Earth and Planetary Sciences University of New Mexico Albuquerque, NM, galewsky@unm.edu ABSTRACT The co-evolution of topography and precipitation in active mountain belts is a central component of the coupling between climate and tectonics. However, there are few theoretical constraints on the sensitivity of orographic precipitation to evolving topography. Using a fully nonlinear atmospheric model with an idealized two-dimensional topography and simplified atmospheric initial conditions, it is found that orographic precipitation is extremely sensitive to even small changes in topography. In the model configuration used here, 800 m of ridge uplift (representing both an increase in surface elevation and in relief) produced a 10-fold increase in simulated 12 hour precipitation over the uplifting ridge, and 200 m of ridge uplift doubled the precipitation over the uplifting ridge. Furthermore, the 800 m of ridge uplift produced a near-total rain shadow over a downstream ridge, and 200 m of ridge uplift cut the precipitation over the downstream ridge in half. The results of the model thus provide strong theoretical support for the hypothesis that rock uplift in some mountain belts may be balanced by erosion and suggest that the

2 22 23 effects of downstream aridification associated with tectonic uplift may be triggered by relatively small changes in topography INTRODUCTION Precipitation, erosion, and tectonics appear to be linked in many orogenic systems (Hodges et al., 2004; Meigs and Sauber, 2000; Reiners et al., 2003), but some studies indicate that current theories of these links are inadequate (Burbank et al., 2003; Molnar, 2003). In an active orogen, precipitation is expected to increase as relief and surface elevation increase. The increased precipitation can enhance erosion, thereby potentially balancing rock uplift (Brozovic et al., 1997; Mitchell and Montgomery, 2006; Willett, 1999). In addition, an uplifting ridge may produce a rain shadow with an associated geological signature of downstream aridification (Kleinert and Strecker, 2001; Vandervoort et al., 1995), a reduction of exhumation, and the potential for increased surface uplift (Sobel and Strecker, 2003). The co-evolution of precipitation and topography in active mountain belts is thus a central component of the coupled climatictectonic orogenic system, but there are few theoretical constraints on the sensitivity of orographic precipitation to changes in topography. The aim of this study is to provide such constraints. Previous studies of the geological effects of orographic precipitation have relied on precipitation parameterizations based upon the concept of saturated upslope flow (Alpert, 1986; Masek et al., 1994; Roe et al., 2002). In the simplest form of this model (Smith, 1979), the rain rate R is directly proportional to the vertical velocity, w, which is simply vh/l, where v is the horizontal wind speed flowing over a two-dimensional barrier

3 of height h over a distance l. Since w is constant with height in this formulation, the rain rate R can be expressed as: where ρ (h) is the saturation vapor density. vs R = v h ρ (h) (1) l This relation is approximately valid for saturated air flow (i.e. one in which the relative humidity is 100%) over simple topographic barriers, but is inapplicable to unsaturated flows and may not be appropriate for complex terrain. The major difficulty for developing a theory of orographic precipitation over complex terrain is that the winds, temperature, and moisture content at any point over the interior of an orogen are dependent on the history of upstream orographic flow modification, which is not generally known a priori. Orographic flows are characterized by a highly nonlinear switching mechanism in which the dynamics of saturated air parcels are quite different from the dynamics of unsaturated air parcels (Barcilon et al., 1979). This is due to the effects of the latent heat released as water vapor condenses to form cloud water, and is irreversible if the condensed water falls out as precipitation. Even very small changes in water vapor content, temperature, or winds may be enough to de-saturate the atmosphere, with far-reaching influences on the air flow and precipitation (Miglietta and Rotunno, 2005). Because of these nonlinearities, small changes in topography may thus exert a large influence on precipitation. The goal of this study is to use a fully nonlinear atmospheric model to asses the sensitivity of orographic precipitation to changing topography, with a particular emphasis on understanding the potential for upstream precipitation enhancement and downstream rain shadow development associated with surface uplift and increasing relief. The extent vs

4 to which these nonlinear effects influence orogenesis is not known, but these model results provide theoretical constraints on the sensitivity of the system to evolving topography and may provide guidance into some features to be looked for in field observations and in more complex models MODEL DESCRIPTION Atmospheric models typically consist of two main components: a dynamical core that numerically solves the equations of atmospheric motion (which are a version of the Navier-Stokes equations), and a physics package that simulates subgrid scale and external processes such as radiation, cloud microphysics, moist convection, and land surface fluxes. These models can be used in several different configurations. For forecasting weather or for simulating historical meteorological events, the models can be used with a full suite of physics parameterizations and initialized with observations of real atmospheric conditions (e.g., Galewsky and Sobel, 2005). Alternatively, the models can be used with idealized representations of topography and atmospheric conditions and with fewer physics parameterizations in order to study fundamental processes (e.g., Colle, 2004). This study takes the latter approach. The calculations presented here were computed with an idealized, two- dimensional version of the Weather Research and Forecast model (WRF version 2), an atmospheric model that solves the fully nonlinear, nonhydrostatic equations of atmospheric motion (Michalakes et al., 2004; Skamarock et al., 2005). The model topography consists of a series of 4 ridges of elevation h n, each with half-width a, and peak-to-peak spacing of x p, prescribed by:

5 91 h h = (2) 2 x )) 2 / a 3 n n= 0 1+ ( x (1.5 x p n p For the results described below, a = 20 km, x = 30 km, and h = 1500m, except for the third ridge ( n = 2 in the equation above), where h n is systematically varied from 1100 m to 1900 m in 200 m increments, representing tectonic uplift. In this formulation of topography, increasing h n represents a combination of surface uplift and increased relief, both of which can influence precipitation. In this study, we do not attempt to separate the effects of relief from the effects of surface uplift. Furthermore, the topography in these simulations is simply prescribed; there are no erosional feedbacks between the topography and the atmospheric model. The model domain is 1200 km wide (in the x direction) and 25 km high (in the z direction) with grid spacing of Δx = 2 km and Δz =.625 km. For all solutions, a representative mid-latitude Coriolis force was used p n 102 ( f = s ). Experiments with different domain sizes and grid spacing (both horizontal and vertical) did not yield significantly different results from those presented here. The domain configuration is summarized in Figure 1. Precipitation processes were parameterized using the Kessler cloud microphysics parameterization (Emanuel, 1994), which simulates the condensation of water vapor to form cloud water, the finite formation time of rain from cloud water, and the advection of rain water as it falls to the surface. The Kessler scheme does not simulate the role of ice phases in cloud or precipitation processes. Additional experiments (not shown) were performed with more complex microphysics schemes that include ice phases. While there were some quantitative differences between the results from these more complex

6 schemes and the simpler Kessler scheme, the main results discussed here were not significantly affected. The initial atmospheric state of the model was defined by a uniform vertical profile of temperature and humidity prescribed at fixed vertical levels. The temperature and humidity were chosen so that the initial moist static stability (a measure of the gravitational resistance encountered by air as it tries to flow over a topographic 118 barrier) was uniform at all levels ( N m = 0.007s 1 ), and were computed using the method 119 of Miglietta and Rotunno (2005), with an initial surface temperature t = 16 C and 120 vertically uniform winds U = 10ms 1. The initial humidity of the atmosphere upstream of the topography is 100%, but evolves into locally unsaturated regions as the winds interact with the topography. The results presented here are from 12 hours of model integration for each elevation increment. Model sensitivity was evaluated by running approximately one hundred simulations with a wide range of model domain configurations and initial atmospheric states. The quantitative variability among the simulations was as much as 40% in some cases, but there were several key features of the precipitation dynamics that were robust across all of the model configurations. These features are discussed below, where our focus here is not on the absolute values of precipitation, but on the relative changes in precipitation in response to evolving topography MODEL RESULTS Figure 2 shows the elevation (a) and 12-hour accumulated precipitation (b) for five increments of ridge uplift. We focus on the precipitation variations in three zones:

7 (1) directly over the uplifting ridge; (2) downstream of the uplifting ridge; (3) upstream of the uplifting ridge. The precipitation directly over the uplifting ridge increased by a factor of 10 over the entire 800 m ridge uplift, and increased by a factor of 2.5 for only 200 m of ridge uplift (from h=1640 m to h=1840 m). The precipitation at the downstream ridge sharply decayed from 10 mm to much less than 1 mm as a result of upstream ridge uplift, and dropped by more than half for only 200 m of ridge uplift (from h=1640 m to h=1840 m). Over the ridge immediately upstream of the uplifting ridge, precipitation increased by 50% due to atmospheric blocking effects associated with the 800 m of ridge uplift. When water vapor condenses to form cloud water, latent heat is released; as described above, this latent heat release is part of the fundamental nonlinearity of orographic precipitation. Identification of the zones of condensational heating within the model can provide some insight into the precipitation dynamics associated with uplift (Figure 3). Condensational heating (and thus the production of cloud and rain water), is not uniformly distributed across the orogen. Instead, condensation is organized into narrow zones extending above the ridges. The structure of these zones is dependent on several factors, including the vertical and horizontal winds, the temperature, and the humidity. Over complex terrain, no single factor can be used to predict where condensation will occur, however. The structure and intensity of condensational heating is directly related to the precipitation at the surface. For h=1440 m (Figure 3a), the zone of condensation above the uplifting ridge is about 1.5 km above the surface. During the course of the uplift (Figures 3b and 3c), this zone descends toward the surface and broadens and intensifies at low levels. Similarly, the condensation zone upstream of the

8 uplifting ridge also broadens and intensifies at low levels, indicating the potential for uplift-induced precipitation more than 200 km upstream. In contrast, the condensation zone above the downstream ridge rapidly ascends during uplift, thereby cutting off the precipitation to the surface. A very small change in water vapor content was sufficient to significantly reduce the rainfall in the rain shadow region. The total column water vapor (TCWV) is an integrated measure of the water vapor content of the atmosphere above a point on the surface. For all of the cases presented here, the initial (upstream) TCWV was 43.4 mm. As the air passes over the orogen, water vapor is removed from the system by precipitation. The 200 m of uplift from h=1640 m to h=1840 m reduced the TCWV over the downstream ridge by less than 1% (from 23 mm to 22.8 mm), but was sufficient to desaturate the atmosphere and largely cut off the supply of precipitation to the downstream zone DISCUSSION The results presented here indicate that orographic precipitation is extremely sensitive to even small changes in topography. The sensitivity illustrated here is likely to represent a near-maximum potential sensitivity to changing topography because several additional processes may blur these direct orographic effects. Such processes include storm-to-storm variability in atmospheric parameters, the effects of three-dimensional topography, and a range of near-surface effects such as land surface enthalpy fluxes, radiative effects, and planetary boundary layer processes. Nevertheless, there are at present very few constraints on the potential sensitivity of orographic precipitation to

9 changing topography, and these results have some intriguing implications for understanding how the atmosphere and solid earth may interact during orogenesis. In general, erosion may be expected to increase with increasing precipitation, although the precise links between erosion and precipitation in mountain belts remain enigmatic (Burbank et al., 2003; Reiners et al., 2003). Many models of fluvial bedrock erosion rely on a stream power formulation in which the erosion rate is a power law function of discharge (Howard and Kerby, 1983). To the extent that river discharge increases with increasing precipitation, fluvial bedrock erosion will also increase with precipitation. The mechanics of glacial erosion are less well understood, but higher snow accumulation rates can increase ice thickness and associated erosion rates (Mitchell and Montgomery, 2006). The rapid increase in precipitation associated with the development of topography shown here suggests that erosion may, in general, also increase quite rapidly with surface uplift and increased relief. If rock uplift remains constant while erosion increases due increased precipitation, then erosion may balance rock uplift, producing a quasi-steady-state topography (Willett, 1999). These model results thus provide a strong theoretical justification for hypothesized links between mountain building, erosion, and climate (Brozovic et al., 1997; Meigs and Sauber, 2000; Mitchell and Montgomery, 2006). Downstream of a tectonically uplifting ridge, the development of an orographic rain shadow can lead to aridification (Kleinert and Strecker, 2001; Vandervoort et al., 1995) and reduced erosion and bedrock exhumation. If rock uplift remains constant, surface uplift rates can thus increase in the rain shadow (Sobel and Strecker, 2003). There are very few constraints on the magnitude of topographic change required to

10 trigger these downstream effects. In the model presented here, 200 m of ridge uplift produced a 50% reduction in downstream precipitation, suggesting that downstream aridification may be triggered by remarkably small changes in topography. Of course, factors other than precipitation, such as the exposure of less erodable basement rocks (Schlunegger et al., 2001; Sobel et al., 2003; Sobel and Strecker, 2003), are also potentially important in reducing erosion in mountain belts. Nevertheless, the model results suggest that geological effects related to downstream aridification may be more sensitively coupled to the atmosphere than previously recognized SUMMARY The aim of this study was to provide theoretical constraints on the sensitivity of orographic precipitation to the changing topography of tectonically active orogens. The main result is that even very small changes in topography can produce significant precipitation enhancement immediately over and upstream of an uplifting ridge and marked aridity downstream from the ridge. Specifically, in the model presented here, 800 m of ridge uplift produced approximately an order of magnitude increase in the maximum precipitation over the uplifting ridge, up to 50% precipitation enhancement extending more than 200 km upstream, and a nearly total shutoff of downstream precipitation in the rain shadow. Furthermore, even a modest 200 m of ridge uplift produced a doubling of maximum precipitation over the uplifting ridge and cut the precipitation in the rain shadow by half. These results thus provide theoretical support for the hypothesis that rock uplift and erosion may balance in some mountain belts, leading to a topographic steady state.

11 Furthermore, the results suggest that the downstream geological effects of rain shadow development may be triggered by relatively small changes in upstream topography. Finally, this study illustrates how the application of atmospheric science techniques to problems in solid earth geoscience can provide a framework for improved understanding of the links between atmospheric and tectonic processes during orogenesis ACKNOWLEDGEMENTS The WRF model is a joint project of the National Center for Atmospheric Research (NCAR), which is supported by the National Science Foundation, and the National Center for Environmental Prediction (NCEP). I thank Alex Densmore for fruitful discussions and Bill Skamarock for assistance with the WRF model REFERENCES CITED Alpert, P., 1986, Mesoscale indexing of the distribution of orographic precipitation over high mountains: Journal of Climate and Applied Meteorology, v. 25, p Barcilon, A., Jusem, J.C., and Drazin, P.G., 1979, On the two-dimensional hydrostatic flow of a stream of moist air over a mountain ridge, Geophys. Astrophys. Fluid Dynam., Volume 13, p Brozovic, N., Burbank, D.W., and Meigs, A., 1997, Climate limits on landscape development in the northwestern Himalaya: Science, v. 276, p Burbank, D.W., Blythe, A.E., Putkonen, J.L., Pratt-Situala, B.A., Gabet, E.J., Oskin, M.E., Barros, A.P., and Ohja, T.P., 2003, Decoupling of erosion and climate in the Himalaya, Nature, Volume 426, p

12 Colle, B.A., 2004, Sensitivity of orographic precipitation to changing ambient conditions and terrain geometries: An idealized modeling perspective, J. Atmos. Sci., Volume 61, p Emanuel, K.A., 1994, Atmospheric Convection, Oxford, 580 p. Galewsky, J., and Sobel, A.H., 2005, Moist dynamics and orographic precipitation in Northern and Central California during the New Year's flood of 1997, Monthly Weather Review, Volume in press. Hodges, K.V., Wobus, C., Ruhl, K., Schildgen, T., and Whipple, K., 2004, Quaternary deformation, river steepening, and heavy precipitation at the front of the Higher Himalayan ranges: Earth and Planetary Science Letters, v. 220, p Howard, A.D., and Kerby, G., 1983, Channel changes in badlands: GSA Bulletin, v. 94, p Kleinert, K., and Strecker, M.R., 2001, Climate change in response to orographic barrier uplift: Paleosol and stable isotope evidence from the late Neogene Santa Maria basin, northwestern Argentina: GSA Bulletin, v. 113, p Masek, J.G., Isacks, B.L., Gubbels, T.L., and Fielding, E.J., 1994, Erosion and tectonics at the margins of continental plateaus: Journal of Geophysical Research, v. 99, p. 13,941-13,956. Meigs, A., and Sauber, J., 2000, Southern Alaska as an example of the long-term consequences of mountain building under the influence of glaciers: Quaternary Science Reviews, v. 19, p Michalakes, J.J., Dudhia, J., Gill, D.O., Henderson, J., Klemp, J.B., Skamarock, W.C., and Wang, W., 2004, The Weather Research and Forecast Model: Software

13 Architecture and Performance, in Mozdzynski, G., ed., 11th ECMWF Workshop on the Use of High Performance Computing In Meteorology: Reading, UK, p. in press. Miglietta, M.M., and Rotunno, R., 2005, Simulations of moist nearly neutral flow over a ridge: Journal of the Atmospheric Sciences, v. 62, p Mitchell, S.G., and Montgomery, D.R., 2006, Influence of a glacial buzzsaw on the height and morphology of the Cascade Range in central Washington State, USA: Quaternary Research, v. 65, p Molnar, P., 2003, Nature, nurture and landscape, Nature, Volume 426, p Reiners, P.W., Ehlers, T.A., Mitchell, S.G., and Montgomery, D.R., 2003, Coupled spatial variations in precipitation and long-term erosion rates across the Washington Cascades, Nature, Volume 427. Roe, G.H., Montgomery, D.R., and Hallet, B., 2002, Effects of orographic precipitation variations on the concavity of steady-state river profiles, Geology, Volume 30, p Schlunegger, F., Melzer, J., and Tucker, G.E., 2001, Climate, exposed source-rock lithologies, crustal uplift and surface erosion: a theoretical analysis calibrated with data from the Alps/North Alpine Foreland Basin system: Int. J. Earth Sci., v. 90, p Skamarock, W.C., Klemp, J.B., Dudhia, J., Gill, D.O., Barker, D.M., Wang, W., and Powers, J.G., 2005, A description of the Advanced Research WRF Version 2: Boulder, NCAR, p. 100.

14 Smith, R.B., 1979, The influence of mountains on the atmosphere, Advances in Geophysics, Volume 21, p Sobel, E.R., Hilley, G.E., and Strecker, M.R., 2003, Formation of internally drained contractional basins by aridity-limited bedrock incision: J. Geophys. Res., v. 108, p. ETG ETG Sobel, E.R., and Strecker, M.R., 2003, Uplift, exhumation and precipitation: tectonic and climatic control of Late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina: Basin Research, v. 15, p Vandervoort, D.S., Jordan, T.E., Zeitler, P.K., and Alonso, R.N., 1995, Chronology of internal drainage development and uplift, southern Puna plateau, Argentine central Andes: Geology, v. 23, p Willett, S.D., 1999, Orogeny and orography: The effects of erosion on the structure of mountain belts, J.G.R., Volume 104, p. 28, ,

15 FIGURE CAPTIONS Figure 1: Detail of model domain. Model domain covers a horizontal distance of 1200 km and an altitude of 25 km. Idealized topography, shown by the solid line, was calculated with Eq. (2). Initially saturated winds flow over topography from left to right. The third ridge from the left is uplifted in 200 m increments Figure 2: (a) Model topography for each of 5 ridge uplift increments; (b) simulated 12 hour accumulated precipitation for each of the ridge uplift increments in (a) Figure 3: Condensational heating across model orogen (K/h) at 12 hours for ridge elevations of (a) 1440 m, (b) 1840 m, and (c) 2240 m. Gray zones indicate where water vapor condenses to form cloud water and, ultimately, rain water.

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