Wednesday, April 16, 2014

Orographic Precipitation

Orographic Precipitation
            Precipitation enhanced by air lifting over mountain barriers is known as orographic precipitation. Smith (2006) describes mountains as a mechanism that modifies airflow to enhance or suppress precipitation events. Thus, orographic effects can contribute to a significant portion of total precipitation during a storm or can result in rain shadows. Understanding orographic effects will help improve Utah’s winter forecasts which are important to road safety, avalanche forecasts, flood warning and mitigation, and water management.
Orographic precipitation for a mountain barrier perpendicular to the prevailing winds can be explained with a simple model. As a moist air mass ascends the windward side of the mountain the water condenses and is removed by precipitation. Orographic precipitation usually stays close to the barrier where it is lifted and cooled. The removal of water from the air increases the potential temperature. As the air descends the lee side of the mountain, temperatures warm adiabatically and the air becomes warmer than it was before its windward ascent (Roe, 2005). In general, windward sides of mountains and higher elevations tend to receive more precipitation than the lee side of the mountain and adjacent valleys. This classical model, however, is overgeneralized. Orographic effects become complicated when additional factors are considered. For instance, precipitation patterns in the Intermountain West vary spatially and in intensity because of synoptic features, atmospheric stability, terrain geometry, and the Great Salt Lake-effect.
Orographic enhanced precipitation events were observed during the IPEX study (Schultz, et al., 2002). During IOP 3 heavy and widespread orographic precipitation accompanied upper-level winds perpendicular to the mountain barrier. This event resulted in almost two feet of snow at Atla ski resort in about twelve hours. Areas in the Salt Lake Valley also received exceptional snow accumulation, suggesting that stable air blocked at the base of the mountain lifted the air before reaching the mountain barrier. Other snow events during IPEX showed different precipitation patterns because of differences in atmosphere and terrain characteristics.
Atmospheric stability determines how likely an air mass will rise over a mountain barrier. Under stable conditions low-level flow is blocked (Steenburgh, 2003) or is diverted around the barrier (Roe, 2005). For the stable case, the cold air blocked at the mountain base acts as a slope for the air mass to ascend resulting in condensation and precipitation ahead of the mountain barrier. For this reason, large mountain barriers with lots of blocking tend to have more orographically influenced precipitation on the windward side and ahead of the mountain while lower mountain barriers with less blocking receive more precipitation at their peaks (Roe, 2005). When the atmosphere is neutral, flow approaching the mountain barrier is allowed to ascent the entire mountain without being blocked. For unstable conditions, convection along the mountain barrier can cause heavy precipitation (Steenburgh, 2003).
Forecasting orographic precipitation becomes more complex when lake-enhanced precipitation is considered. Sensitivity studies by Alcott and Steenburgh show that a combination of lake-effect and orographic enhancement can result in significant precipitation events such as the “Hundred-Inch Storm” of 2001. Their study also investigated the influence of upstream mountain ranges and orographically forced convergence by concave terrain.
Like all numerical weather prediction, model based orographic predictions are poor due to insufficient representation of complex topography as well as imperfect model physics. In addition, snowfall isn’t directly predicted in the models. Instead, snow accumulation is estimated by assuming a snow-to-water ratio (Schultz, et al., 2002). A ratio of 10:1 is traditionally used, but that is not necessarily a good estimate for all cases as ratios can spread from 25:1 to 5:1. Forecasters should also be cautious when analyzing radar data. Since the radar beam is aimed over the mountain peaks, low elevation orographic precipitation is not measured by radar (Schultz, et al., 2002).

Future Research
Since much of the validation of orographic precipitation models are done only during large field campaigns it would be beneficial to install or obtain additional and more accurate precipitation observations at higher elevations to verify and improve numerical weather forecasts.

Reading List
Alcott, T. I., & Steenburgh, W. J. (2013). Orographic Influences on a Great Salt Lake-Effect
Snowstorm. Monthly Weather Review, 2432-2450.
Roe, G. H. (2005). Orographic Precipitation. Annual Review of Earth and Planetary Science, 645-671.
Schultz, D. M., Steenburgh, W. J., Trapp, R. J., Horel, J., Kingsmill, D. E., Dunn, L. B., . . . Trainor, M. (2002). The Intermountain Percipitation Experiment (IPEX). Bulliten of the American Meteorological Society, ES1-ES30.
Smith, R. B. (2006). Progress on the theory of orographic precipitation. Geological Society of America Special Paper 398, 1-16.
Steenburgh, W. J. (2003). One Hundred Inches in One Hundred Hours: Evolution of a Wasatch Mountain Winter Storm Cycle. Weather and Forecasting, 1018-1036.




Monsoon Convection

Monsoon Convection
The North American Monsoon is a thermally driven circulation that develops as a result of strong ocean-land temperature contrasts (Higgins, et al., 2006). In summer months, monsoon circulation is characterized by ascent over the continent and descent over the ocean (Climate Prediction Center, 2004). This circulation transports moist, tropical air from the Gulf of California and the Gulf of Mexico inland towards northwest Mexico and the southwest United States. Monsoon moisture contributes to as much as 70% of precipitation in the Southwest (Adams & Comrie, 1997). The strength of the monsoon has a large societal and economic impact. If the annual monsoon is too weak agriculture and residents suffer from drought. On the other hand, if the monsoon is too strong, residents will frequently experience severe thunderstorms, dust storms, and floods. Thus, long and short-term prediction of monsoon strength, duration, and timing is necessary for drought prediction and for warning the public of severe weather. (Grantz, Rajagopalan, Clark, & Zagona, 2007).
The monsoon develops in May and June as the storm track weakens and moves north. As the season progresses, an upper-level ridge develops over the western United States causing a low-level southerly jet transporting moisture from the Gulf. Strong surges of moisture caused by synoptic features such as cyclones, easterly waves, and inverted troughs are responsible for the more severe weather events (Higgins, et al., 2006).
In July and August the monsoon circulation reaches its peak and extends as far north as Utah. However, precipitation patterns fade and become irregular towards the northern extent of the monsoon (Grantz, Rajagopalan, Clark, & Zagona, 2007). These areas may experience dry periods between surges of moisture. As shown in Adams and Comrie’s review, there is not a strong monsoon signal at Salt Lake City. The southern parts of Utah, however, are more likely to experience weak and infrequent monsoon related precipitation events during the monsoon season (Adams & Comrie, 1997). The monsoon begins to decay in September and October as the ridge over the western states and moisture transport weaken (Higgins, et al., 2006; Yao & Wang, 1997).
Monsoon related precipitation is most severe as CAPE increases with intense surface heating and advection of moisture. Localized instabilities are strongest in the afternoon and result in thunderstorm development (Higgins, et al., 2006). Thunderstorms can merge together to produce severe weather such as dust storms, lightning, flash floods, and high winds. Lightning in storms with low relative humidity can initiate wild fires (Climate Prediction Center, 2004). As storms begin to precipitate, surrounding temperatures will begin to cool as rain water evaporates (Higgins, et al., 2006).
Complex topography makes forecasting convection difficult. Numerical models in the past have done a poor job of predicting precipitation during monsoons because they cannot resolve localized convection and terrain. Convection tends to develop over mountain ranges, but terrain induced confluence also contributes to thunderstorm development (Adams & Comrie, 1997).
Monsoon strength can vary each year due to changes in the land’s surface characteristics. One example is the negative correlation between spring snowpack depth and summer monsoonal strength (Grantz, Rajagopalan, Clark, & Zagona, 2007). For winters with a deep snow pack, more energy is required to melt the snow and evaporate spring runoff. In addition, widespread snow cover has a higher albedo and reflects solar radiation. These conditions effectively reduce the surface heating over land and delay or weaken the monsoon circulation.

Future Research
Most of the current research on Monsoon circulation and precipitation is focused in Arizona and northwest Mexico where the monsoon signal is strongest. However, much like atmospheric rivers, the inland penetration of the North American Monsoon is not well understood. Future documentation and research on how monsoon circulation penetrates inland into Utah would improve our ability to predict the infrequent, but significant, monsoon convection in Utah.
Reading List
Adams, D. K., & Comrie, A. C. (1997). The North American Monsoon. Bulletin of the American Meteorological Society, 2197-2213.
Climate Prediction Center. (2004, August). The North Amercian Monsoon. Retrieved from Reports to the Nation on our Changing Planet: http://www.cpc.ncep.noaa.gov/products/outreach/Report-to-the-Nation-Monsoon_aug04.pdf
Grantz, K., Rajagopalan, B., Clark, M., & Zagona, E. (2007). Seasonal Shifts in the North American Monsoon. Journal of Climate, 1923-1935.
Higgins, C. V., Amador, J., Ambrizzi, T., Garreaud, R., Gochis, D., Gutzler, D., . . . Zhang, C. (2006). Toward a Unified View of the Americna Monsoon Systems. Journal of Climate- Special Section, 4977-5000.
Yao, Y., & Wang, X. L. (1997). Influence of the North American Monoon System on the U.S. Summer Precipitatino Regime. Journal of Climate, 2600-2622.



Atmospheric Rivers

Atmospheric Rivers
Atmospheric rivers (ARs) are narrow bands of enhanced water vapor extending from the tropics toward the midlatitudes. ARs are responsible for most of the moisture transport between the equator and poles in the lower troposphere. A significant percentage of total precipitation on the west coast has been shown to come from ARs between the months November and April (Rutz & Steenburgh, 2012).
Neiman et al. (2008) identified ARs from SSM/I satellite measurements and defined them as regions with concentrated water vapor (integrated water vapor greater than 20 mm) that are at least 2,000 km long, but are less than 1,000 km wide. ARs can make landfall anywhere along the west coast from British Columbia to the Baja Peninsula, but their frequency varies throughout the year. For instance, ARs in the northwest tend to occur more frequently during November while ARs in the southwest occur more frequently during January (Rutz, Steenburgh, & Ralph, 2014). Moisture from these ARs can cause precipitation in Utah if that moisture propagates beyond the mountain barriers. Forecasters should be aware of ARs because they can cause heavy precipitation and flooding if an AR persists for a long period of time. For this reason, forecasting an AR’s location and duration should influence short-term precipitation and water management forecasts.
Since ARs correspond with transported tropical moisture, ARs are typically found in the low-level jet ahead of cold fronts in the warm sector of an extratropical cyclone (Ralph, Neiman, Kiladis, & Weickmann, 2011). ARs are also located between a cyclonic pressure trough and an anticyclone ridge when circulation in the trough extends into the moist tropics and transports water vapor northward. ARs tend to make landfall in the Pacific Northwest when the longwave trough is positions over the Pacific Ocean with a ridge over the western United States. When the trough axis is closer to the coast ARs will make landfall in southern California or cross the Baja Peninsula (Rutz, Steenburgh, & Ralph, 2014; Neiman, Ralph, Wick, Lundquist, & Dettinger, 2008).
The effects of atmospheric rivers are most apparent along the west coast where they first interact with the topography. As ARs advance inland the amount of AR precipitation decays because orographic lifting causes the moisture laden air to condense and precipitate. Consequently, ARs that interact with the High Sierras are less likely to penetrate inland (Rutz, Steenburgh, & Ralph, 2014). When an AR intersects the High Sierras, most of the moisture is rained out before that moisture can reach Utah. This rain shadow effect is evident in Rutz and Steenburgh’s (2014) analysis of AR frequency. Because of the rain shadow, the Great Basin is the most unlikely region in the Intermountain West to experience AR related precipitation.
ARs will more likely penetrate inland if they pass over smaller mountains or through mountain gaps. For example, Rutz and Steenburgh (2014) show that AR moisture can extend as far as Idaho and Montana when transported through the low elevations of the Columbia River Basin and Snake River Plain. AR moisture can extend into Utah when moisture is channeled around the High Sierras. Southern Utah is mostly influenced by ARs that cross the Baja Peninsula. Although ARs in the Southwest do not occur as often they tend to have a longer duration (Rutz, Steenburgh, & Ralph, 2014).
During winter months precipitation from ARs tend to increase the snow-water equivalent in the snowpack. This is because AR moisture originates from the tropics and is warmer than the winter storms coming from the northwest. These warmer storms often have higher freezing levels and bring valley rain (Neiman, Ralph, Wick, Lundquist, & Dettinger, 2008). Heavy precipitation from AR moisture is enough for flood conditions, but flooding can become more severe by the potential melting of the snowpack by warm rains (Neiman, Ralph, Wick, Lundquist, & Dettinger, 2008).

Future Research
ARs have been a focus of study in the Northwest where they occur regularly and cause extreme floods. However, more research is needed to show how ARs affect precipitation in the Intermountain West. Rutz and Steenburgh suggested their analysis is smoothed because of poor topography resolution. Using higher resolution topography to analyze ARs may result in a better understanding of how broken and decayed ARs affect precipitation patterns in Utah.


Reading List
Guan, B., Molotch, N. P., Waliser, D. E., Fetzer, E. J., & Neiman, P. J. (2010). Extreme snowfall
 events linked to atmospheric rivers and surface air temperature via satellite measurements. Geophysical Research Letters, 1-6.
Neiman, P. J., Ralph, F. M., Wick, G. A., Lundquist, J. D., & Dettinger, M. D. (2008). Meteorological Characteristics and Overland Precipitation Impacts of Atmospheric Rivers Affecting the West Coast of North America Based on Eight Years of SSM/I Satellite Observations. Journal of Hydrometeorology, 22-26.
Ralph, F. M., Neiman, P. J., Kiladis, G. N., & Weickmann, K. (2011). A Multiscale Observational Case Study of a Pacific Atmospheric River Exhibiting Tropical–Extratropical Connections and a Mesoscale Frontal Wave. Monthly Weather Review, 1169-1189.
Rutz, J. J., & Steenburgh, W. J. (2012). Quantifying the role of atmospheric rivers in the interior western United States. Atmospheric Science Letters, 257-261.
Rutz, J. J., Steenburgh, W. J., & Ralph, F. M. (2014). Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States. Monthly Weather Review, 905-921.


Friday, April 11, 2014

KSLC Temperature Fixed

In my forecasting class we make weather forecasts for the Salt Lake City International Airport. Early in our class, everyone realize that the temperature at the airport was always several degrees warmer than any of the surrounding stations. We got in the habit of making a forecast for what we thought the temperature would be, and then we would add 5 degrees to our forecast. I'll call this "temperature inflation," when it's warmer than what you think it should be. Two days ago the National Weather Service discovered the temperature sensor was bad and finally had it replaced.

Our overall weather pattern hasn't changed to much in the last few days, but you can see in the MesoWest temperature plot below that temperatures were in the high 70's a few days ago. The the senor was changed midday on April 9th (where the temperature spikes looks unreal). You can see the temperatures were high before noon, and then dropped after the sensor was changed. Now the temperatures agree with the surrounding stations.


The weather station at the airport (KSLC) is really close to the station at the National Weather Service Office (SLCBY).
 As shown in the observations below, before installing the new sensor the airport reported almost 5 degrees warmer than the weather office. After the replacement on April 9th, the temperature at both sites were fairly close to one another.

Saturday, March 29, 2014

BYU Merit Badge PowWow: Weather

The BYU merit badge PowWow for the Boy Scouts occurs twice a year. Scouts come to classes on two Saturdays to complete the requirements for different merit badges. I taught the weather class and helped about 150 scouts earn the weather merit badge. It was a lot of fun teaching and hope I can do it again next year. You can find my power-point slides and worksheet by clicking the picture of the weather merit badge in the top corner of this blog or going to http://nathangblaylock.com/wxMeritBadge.html.


 After the PowWow my brother and I picked up a pizza and played some Frisbee golf at Provo's Bicentennial Park. The trees have blossomed, so it must be spring!