Variability of Cloud Base Heights in Yampa River Valley during a Wintertime Snow Event Croix Christenson Undergraduate Student University of Wisconsin-Madison ABSTRACT The common understanding is that NW flow is ideal for Steamboat Springs, Colorado to experience orographic clouds and snow. The hypothesis that was tested in this experiment is how accurate the NW assumption is in relation to Steamboat Springs. The findings of this experiment disagree and agree with the understanding that NW is ideal for Steamboat Springs, Colorado. First, the agreement that NW on the synoptic scale is essential for the moisture to reach Steamboat and not be deposited on upstream mountains such as the San Juan's, Sierra Nevada's and the mountain ranges of Utah. However, on the mesoscale, NW flow is not ideal, where W to WNW is best for Steamboat Springs to experience moderate to heavy amounts of snow and experience low cloud bases causing Storm Peak Laboratory to be in cloud. If the flow is greater then 315 degrees, Steamboat is in the shadow of Elk Head Mountain thus reducing the amount of snow and orographic clouds experienced. Introduction example, if the flow is from the SW, the In the winter months of North San Juan and Flattops should expect America a phenomenon known as heavy snow. Westerly winds will favor upslope precipitation occurs, producing the mountains near Salt Lake City and heaving snow in mountainous regions. resorts in Colorado such as Copper Upslope precipitation is important not Mountain and Vail. only ski resorts but the southwest United Northwesterly flow is perfect for States. The deep snow pack that forms producing “champagne powder” at over the higher elevations melts every Steamboat Springs, Colorado the spring, providing water for drinking, location of this experiment. The recreation, entertainment and agriculture combination of NW flow, moisture and to the drier southwest. Water struggles typically cold 700mb temperatures have persisted for decades, and likely produces a very light (ie low density) will continue well into the future. snow perfect for skiing and Upslope precipitation is when flow snowboarding. The purpose of this collides with a barrier, in this case a experiment is determining a range of mountain range, which often lifts the wind directions for which Steamboat entire layer of air. As the air rises it sees orographically enhanced or purely cools and condenses forming clouds and orographic snowfall and more precipitation with sufficient moisture specifically clouds. This will be looked present. 700mb streamlines and relative at from an observational standpoint as humidity is a good proxy for well as using a high resolution determining which mountain ranges will simulation. It is commonly known that be favored for significant snowfall. For NW winds at 700mb is favorable for orographic snow and low cloud bases at moisture, temperature and wind speed. Steamboat, however there has been little One example is that if the mountain done to investigate what range of wind width were to be increased this would directions and moisture produce snow allow for an increase in the advective and low cloud bases. This experiment timescale increasing the efficiency of the will test the hypothesis that NW flow is hydrometeor formation (Jiang & Smith). ideal on both the synoptic and mesoscale It has been concluded that orographic for producing low cloud bases and heavy clouds are highly variable and can begin snow at Steamboat Springs, Colorado. with timescales on the order of hours to days and quickly become convectively Wintertime Snow in Steamboat unstable and be on the order of seconds Springs, Colorado to minutes. If the clouds are of a longer timescale a more homogeneous Steamboat Springs is known for precipitation total will result. Compare its “Champagne Powder” a very light this to if the flow becomes unstable and and fluffy snow. Studies have been has embedded convection. Then the conducted in Colorado and Wyoming to snow will be highly variable over just a explore this claim and have found that few kilometers. Steamboat Springs does indeed have dry, The final ingredient in light fluffy snow. It was found that understanding orographic clouds and Steamboat Springs occasionally has precipitation is the cloud top snow with exceptionally low density and temperatures. It has been discussed in of the six sites in the experiment the past that the cloud top temperature consistently experienced low density directly influences precipitation rates snow (Judson & Doesken 2000). This is and characteristics (Rauber 1987). As a not however surprising as if you result it can be concluded that compare the San Juan Mountains with precipitation rates and amounts are the NW facing slopes that Steamboat dependent on the timescale of the Springs are located. When the San Jaun orographic clouds, moisture, advective Mountains experience large orographic properties, mountain width and cloud top snow falls the flow is from the southwest temperature. bringing moisture but also warmer temperatures from the Pacific, producing Data heavy snow but also a more dense snow For this experiment numerous fall. Compared to Steamboat Springs, sources of data were used, beginning where the moisture is still from the with the University of Wisconsin- Pacific but is often associated with cold Madison's Nonhyrdostatic Modeling air advection (CAA) on the back side of System. As described by Greg an upper trough producing heavy snow, Tripoli(1992), the model uses nonlinear but also a lower density snow. theory without making Boussinesq The timescale of orographic approximations. This allows for non- precipitation is highly variable that geostrophic processes on the mesoscale produces snow at Steamboat Springs. to be modeled, such as upslope Simplified box models allow for a precipitation in mountainous regions. In connection to be made based on multiple this particular simulation the emphasis factors such as mountain width, was to locate moisture in the Yampa River Valley in an attempt to draw Based on observations some conclusions about moisture, wind interesting conclusions can be made direction and cloud base heights. It was from the orographic snow event on initialized off of the GFS 18z analysis Thursday April 1st and Friday April 2nd. and a high resolution topography map, to The storm had three significant wind capture the orographic's that produce shifts and will be the focus for this snow and clouds at Steamboat Springs, section. At 2:13am local standard time Colorado. Run at 3km resolution, this (LST) the wind shifted from SE to W simulation was initiated at 1800 UTC 31 and was directly correlated with a APR 2010 and run out to 48 hours to decrease of the number of particles being capture the full duration of the snow counted by the SMPS machine. The event. The actual model simulation wind was highly variable in direction figures were plotted using Vis5d, a between 2:15am LST and 2:35 LST program that is customized for which can be seen in the SMPS data, overlaying parameters in a cross section suggesting there is a direction which or horizontal plots for UW-NMS output. Steamboat is no longer blocked by Multiple loops of the 48 hour simulation higher terrain to the west. Looking at the were also created though not depicted two data outputs every 5 minutes and using Vis5D. comparing, a westerly threshold between Scanning Mobility Particle Sizer 265-270 degrees seems to exist. (SMPS) data was acquired using Storm The wind shifts back to SSE at Peak Laboratory (SPL) archive page 5:25am LST but the SMPS data is well which is courtesy of Western Regional behind and does not increase until Climate Center. This data was used as a 6:53am LST. This is likely due to the proxy for when the lab is in or out of light and variable winds present at SPL cloud. In addition figures were created between 5:25am and 6:53am. At 6:45am using Surface plots, though not depicted, the winds increase from ~12mph to over were created using the National Weather 30mph at 6:50am LST and scour out the Service Hydrometeorological Prediction moisture and cloud present at SPL, Center archive surface maps page to which is clear in the SMPS data at locate surface features, such as the 8:21am LST. From an observational embedded surface cyclone and the front standpoint, visibilities were under a ½ that brought a quick burst of snow early mile at 6:30am LST while at 7:15am Thursday Morning. Upper air plots were LST the valley was visible. The third created using the National Weather major wind shift was at 8:20am LST and Service's Storm Prediction Center to matches with the SMPS data indicating locate the upper trough, vorticity at that SPL was in cloud. This is also 500mb, vertical motion fields at 700mb evident as the lab was not visible from and 700mb moisture and winds. Finally, the top of Storm Peak Express lift at sounding data was extracted from the 9:45 LST with small triangular grauple University of Wyoming's archive falling and significant rhiming sounding data page, to better understand occurring. Interestingly, the SMPS data the vertical structure of the atmosphere was highly variable and while some of March 31st-April 3rd, 2010. the variability does correlate to wind direction changes, some appears Orographic Cloud Experiment unexplained. A hypothesis is that, in the post frontal, colder air mass, there was data there is a southern threshold. This less total moisture then in the southerly, threshold, approximately 270 degrees, prefrontal air. As a result fewer aerosols where anything greater then 270 degrees were being used as CCN. This is strictly Steamboat is in cloud, while less then a hypothesis, and requires further 270 degrees Steamboat is blocked investigation, and an entirely new upstream by the San Juan and Flattop experiment.