Land Surface Roughness Effects on Lake Effect Precipitation

The land use of the Great Lakes region has changed significantly during historical times, and continues to change. As a preliminary step in investigating the overall effect that this might have on climate, attention is focused here on one forcing factor and one effect—land surface roughness length and lake effect precipitation, respectively—that are anticipated to be particularly sensitive pieces of the land use-climate interaction. On both a monthly basis and in an individual case of lake effect precipitation, a reduction of land surface roughness reduces the total amount of lake effect precipitation. It also reduces the degree to which the precipitation is focused on the area closest to the lakeshore. The largest reductions occur immediately adjacent to the lakeshore in an area smaller than the overall lake effect zone. In the individual lake effect event that is investigated here, precipitation increases in some places farther inland when surface roughness is reduced. Because this increase in precipitation farther inland appears to be associated with significant topography, this result is most valid for lake effect zones where there is a high topographic relief, such as near southeastern Lake Erie (the main focus of this study), and to the south and east of Lake Ontario. This displacement in location of precipitation is particularly crucial where the boundary of the drainage basin is near the shoreline, and can indicate a flux of moisture out of the Great Lakes drainage basin and into another basin. INDEX WORDS: Lake effect precipitation, land use, atmospheric modeling, *Corresponding author. [email protected] INTRODUCTION Lake effect precipitation (both snow and rain) plays an important role in the weather and hydrology of many regions near the shores of the Laurentian Great Lakes. The heaviest lake effect precipitation generally falls during the fall and early winter on the eastern or southeastern shore of each lake, where the prevailing winds are onshore. Niziol et al. (1995) summarize some of the key conditions needed for the production of lake effect and some sub-types of lake effect. This paper will be concerned with what Niziol et al. (1995) refer to as Type I and Type II lake effect events. Type I has wind blowing along the long axis of a lake. It tends to develop a single strong band of precipitation parallel to the wind, dropping precipitation on the lake and also at the shoreline. Type II lake effect has wind along the short axis of a lake and has several more diffuse bands of precipitation. Again, these can deposit precipitation both over the lake and on the shore. The lake effect events happen during cold air outbreaks, generally in the fall to early winter, when the lakes are considerably warmer than the air overlying them. This helps to fulfill one of the major conditions for lake effect precipitation—the lower atmosphere must be unstable. According to Holroyd (1971), the required threshold is that lake surface temperatures must be at least 13°C warmer than the air at 850 mb. This quantity corresponds to the dry adiabatic lapse rate, making the lower part of the air column statically unstable, and enabling the heat from the lake to erode and eliminate any inversion that may exist below the 850 mb level, as often occurs over land in a subsiding cold air mass. This erosion of the inversion creates a thick layer with a near-neutral buoyancy profile. Hjelmfelt (1990) carried out numerical simulations of scenarios of lake effect precipitation, and investigated the impact of a variety of factors on the amount of precipitation that falls in a given event. Parameters for which sensitivity was tested include lake-land temperature difference, static stability of the air, ambient wind speed, wind direction, humidity over the land, friction, and Coriolis effect. In general, lake effect precipitation occurred when 840 Brent M. Lofgren there was sufficient lake-land temperature difference and low enough static stability. In these simulations, there was sometimes seen to be an optimum wind speed of intermediate value. The wind direction had an important effect, mainly attributed to its influence on fetch over the lake. The influx of moisture from the land near the lake effect zone, as affected by the relative humidity of the air over land, was shown to significantly affect the quantity of lake effect precipitation. Of the various combinations of surface roughness on lake and land tested by Hjelmfelt (1990), the highest amount of precipitation occurred when both the land and the lake had high surface roughness. Low Coriolis effect, corresponding to latitudes closer to the equator, results in greater amounts of lake effect precipitation. Hjelmfelt (1992) investigated the effect of including vs. excluding orography, and found that orography, i.e., the forcing of air upward as it reaches shore, enhances lake effect precipitation. Laird and Kristovich (2004) further investigated the influence of wind fetch over the lake, comparing the numerical modeling results of Laird et al. (2003a, b) to observations. They found that the quantity U/L (wind speed divided by length scale) is important in determining the quantity of precipitation in near-shore snow-band types (Types I and II of Niziol et al. 1995). The value of L (the denominator of U/L) is highly dependent on the wind direction, i.e., whether the wind is blowing along the long axis or short axis of the lake. They found U/L to be less important in the midlake and mesoscale vortex type lake effect events (Types IV and V of Niziol et al. 1995). U/L has units of inverse seconds, and is a measure of the (inverse) amount of time that air parcels spend over the lake. Besides wind (for Types I and II lake effect) and unstable air, forced upward motion is an important ingredient in lake effect precipitation. Three major mechanisms can lead to upward motion: 1. Thermally-forced motion. This is most important in Type IV and Type V lake effect precipitation, as defined by Niziol et al. (1995). Passarelli and Braham (1981) highlight the importance of thermally forced land breezes leading to low-level convergence over the lake, resulting in snow bands parallel to the shore. 2. Orographic uplift, as reported by Hjelmfelt (1992). 3. Motion forced by frictional convergence at low levels near a shoreline. The thermal influence of the Laurentian Great Lakes as a group on the atmosphere in a cold-air outbreak environment can result in a surface low pressure system at the meso-α scale, i.e., on the order of 1,000 km, and thus incorporating in a significant way both geostrophic and ageostrophic motion (Sousounis and Fritsch 1994). Further evidence of this is present in the charts of 850-mb height from aggregates of lake-effect events presented by Liu and Moore (2004). This lake-aggregate vortex and the accompanying thermal effects on the atmosphere can alter the environment at the spatial scale corresponding to the group of Great Lakes and thus influence the formation and intensity of snowfall on the scale of individual lakes (Sousounis and Mann 2000). Motivated by historical changes in land use in the Great Lakes region (Cole et al. 1998), this paper will investigate some of the possible effects of land use change on the regional climate system. It focuses on the model simulation of one of the primary anticipated interactions—that between land surface roughness and lake effect precipitation. The next section describes the model that is used here. The section after that describes the experimental design using that model. The results section presents changes in precipitation during December 1993 resulting from changes in land surface roughness and then concentrates on a shorter time period on 11 December 1993, diagnosing the circulation and resulting precipitation. Finally, some concluding remarks are included in the last section.