Thursday, October 23, 2014
In by Andrew Joros // 10:51 PM //
Dr. Seasonal or: How I learned to stop worrying and love the planet
Why should you not concern yourself with seasonal (or longer) forecasts of the Earth system? Quite simply, it all comes down to the fact that we are operating in a complex, nonlinear system that is highly dependent on the initial conditions. We know a few, but since we don’t know what each molecule of air or water vapor is doing we are just guessing.
Edward Lorenz clearly demonstrated this in the late 1960s, shortly after John von Neumann and Jules Charney produced the first numerical weather forecasts. While state of the art models have made impressive progress in forecast lead times over the past 50 years, at longer forecast times the problems of numerical error growth (floating point and truncation) results in diverging solutions as does the failure to accurately represent the physical mechanisms at various, interacting scales. As the equations of fluid motion have no analytical solution, numerical approximations must suffice and thus are susceptible to the aforementioned numerical errors.
Turbulence in the atmosphere and ocean are critical components involved in the transfer of energy both up and down scales, and are not closed mathematical systems. Unclosed mathematical systems have more unknowns in the set of equations than known equations. This problem has been haunting physicists since initial recognition of the problem in 1924 by Keller and Friedman. Approximations are the only way around this, and out the window goes long-term skill. The list of issues goes on, from poor resolution of terrain that exerts a tremendous influence on the regional and global circulations (see Broccoli and Manabe 1992) to the formation of clouds in all environments.
Figure 1: Global averaged annual sea surface temperature from the MERRA ERA-Interim reanalysis model and observational data assimilation product.
Global climate models do a phenomenal job of recreating physically reasonable features predictability with skill continues to disappoint. This is likely due to the issues listed above (turbulence, not knowning the states of all molecules) as well as significant biases in key fields, such as sea surface temperatures (SSTs; see Figures 1 and 2). Thus I see virtually no use, unless your interest is in wasting time, to make a seasonal, let alone multiple-season (i.e., extended winter NDJFMA) forecast, as an educated guess versus a random guess will likely demonstrate the same skill.
Figure 2: Global averaged annual sea surface temperature hindcast (1870-2013) and forecast (2013-2100) output from the CCSM4 global climate model. Note the ~3°C cold bias of CCSM4 and the linear increase in SSTs. Few natural systems are so linear, and this is cause for concern both in the modeling approach and the potential system responses (abrupt climate change anyone?).
Relationships between predictor variables are demonstrably nonstationary (Ramage 1983). This means that the relationships change over time. For example, for some period of time the negative phase of ENSO leads to less than average precipitation but then for some other period the opposite is true. Big trouble! The use of stationary assumptions developed over a finite period of time will only confound the prediction process. Relationships do exist, of course, but the interactive nature of the forcing terms greatly complicates the matter as these relationships change as boundary conditions change and are subject to variance in higher frequency forcing terms which also perturbs the 'known' relationships. Even if we did know all of the conditions, we still can’t solve the equations analytically!
It should be easy to see why forecasting at lead times longer than 7-10 days will only lose you money. That said, GCMs are excellent tools for examining sensitivities to the climate system, such as how the hydrologic cycle will respond to a warmer planet (warning, not good! See Held and Soden 2006 and Trenberth 2011 for depressing news). The bottom line is just wait and see! Spend your time training, visualizing, and planning for radness. This way, your season will be awesome no matter what the outcome.
Wednesday, October 22, 2014
In by Andrew Joros // 5:05 PM //
What does El Nino mean for us?
By now you have probably been inundated with talk of the “forecasted El Nino” and thoughts of record breaking snowfall and epic years come to mind. Well if you live in Northern Arizona, Southwestern Colorado or Northern New Mexico get ready, statistically, El Nino is all the rage but what about if you frequent the awe inspiring terrain of the Central Sierra. For those of us who are lucky enough to have deep blue views of Lake Tahoe in our backyards, El Nino and ENSO in general can be much less of a sure thing. In fact, except for during strong ENSO years there isn’t a strong relationship between El Nino and precipitation in the Central Sierra (see Figure 1). According to the National Weather Service’s Sacramento Office, four out of five strong El Nino’s since 1950 have brought above average precipitation to the area. Meanwhile five out of seven of the strong La Nina’s during the same time period have also brought above average precipitation. The data and plots can be found here: http://www.wrh.noaa.gov/sto/climate/ENSO.php
|Figure 1 [click for larger image]|
Figure 1 only includes strong ENSO data back to 1950 and it should also be noted that measurements prior to 1946 might be less accurate and thus the mean total snowfall and maximum snow depths may not be completely accurate.
So what does it mean for this winter???
Well in reality, it just means that we have no way of really knowing what is going to happen this winter in the Central Sierra. In case this isn’t enough proof check out the October-March precipitation in Reno, NV (Figure 2, compliments of WRCC) and Blue Canyon, CA (Figure 3, compliments of NWS Sacramento) based on ENSO. There is absolutely no relationship between ENSO and precipitation amount in Reno.
Meanwhile in Blue Canyon when the October-March is broken into fall and winter, there is more precipitation in winter during El Nino and more precipitation in fall during La Nina.
|Figure 3 [click for larger image]|
Bottom Line: The lack of signal between ENSO and precipitation makes it extremely difficult to predict this or any winter's snowfall in the Sierra/Tahoe region.
Tuesday, October 21, 2014
In by Andrew Joros // 4:08 PM //
How are NOAA’s seasonal climate outlooks developed?
The Climate Prediction Center (CPC), a branch of NOAA, issues the operational seasonal climate and drought outlooks for the US. Figure 1 shows the official US seasonal drought outlook that is valid through the end of January, 2015. This map shows the likelihood of drought development, removal, or persistence. Pretty sad looking for much of CA and NV. However, these forecasts must be taken with a grain of salt, as skill beyond ~two weeks remains quite low. Remember, these are climate outlooks, and not weather forecasts. The CPC is not predicting individual storm events, but rather departures from mean climate states. So, how are these maps developed? Well, there is one dynamical model (Climate Forecast System Version 2 (CFSv2); based on atmospheric physics and weather dynamics), and a variety of statistical methods that use historical conditions and patterns to predict the future (If you are interested in the technical details, follow this link: http://www.cpc.ncep.noaa.gov/products/predictions/90day/tools.html). Statistical models can be just as reliable as dynamical models at long lead times because the atmosphere is a nonlinear, chaotic system that is not perfectly predictable by any means. A limited set of variables are used for seasonal prediction, and typically include temperature, precipitation, soil moisture, and sea surface temperature.
Figure 1. CPC US seasonal drought outlook. Source: http://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.html
Experimental Seasonal Forecasts
Multiple dynamical models (as opposed to just CFSv2) can also be used to reduce the uncertainty of a single model. The average forecast is taken from several different models, and is termed an ensemble forecast. Studies have found the ensemble approach to typically be more accurate than one model, but skill at seasonal lead times remains low. The North American Multi-Model Ensemble (NMME) is an experimental project that provides individual forecasts and an ensemble forecast for eight different models, including CFSv2. Figure 2 shows the November average precipitation anomaly (mm/d) from the NMME ensemble and three members of the ensemble. Red indicates abnormally dry and green indicates abnormally wet. If you just look at the ensemble, NMME shows slightly above normal precipitation for CA. But look at how contrasting the three individual members are! CFSv2 is extremely dry, CMC2 is extremely wet, and the NASA model splits the state of CA about 50/50, take your pick! The ensemble basically smooth’s out the contrast found amongst the eight members.
Figure 2. November 2014 precipitation anomaly forecast from NMME ensemble and 3 individual
members. For all ensemble members and more forecast see: http://www.cpc.ncep.noaa.gov/products/NMME/monanom.shtml.
Come back to this post at the end of November and see which model wins! There are an awful lot of resources out there to look at seasonal predictions, but none of them are reliable at this point. One thing that can help seasonal prediction skill is if a model is initialized (start date of the model run) during an ENSO event (El Niño or La Niña). Unfortunately, we are ENSO neutral right now, so the current seasonal predictions may end up being quite poor.
Monday, October 20, 2014
In by Andrew Joros // 4:12 PM //
Over the next four days, Powdiction will bring you the inaugural Anti-Seasonal Outlook, Seasonal Outlook: A Four Part Series. Each day, a different aspect of seasonal outlooks will be discussed hopefully in a coherent manner. I know you are thinking we may have gone all Peter Jackson with splitting this up and dragging it out. However, we hope this will inform our readers about the current state of the climate (spoiler: not good), what could change it, how it’s forecasted, and the difficulties in doing so.
The Sierra Nevada mountain range is in extreme to exceptional drought, which are the two highest categories. Essentially all of California and Nevada are drought stricken with it being the worst in recorded history in some areas of California. This drought started way back in 2011, after we had an epic powder year. Then the drought progressively worsened each year.
Some areas in the Sierra had above average precipitation over the summer, but it barely made a dent into the drought. That is due to the low monthly precipitation averages during the summer (June-August), which makes it easier to be above normal. For example, Tahoe City averages 0.30” of precipitation in August, and this August Tahoe City received 0.89” of precipitation, which is nearly three times the average. Furthermore, Tahoe City averages 5.55” of precipitation in December. So 300% of average precipitation in August is only about 16% of normal December precipitation.
Most precipitation in December falls as snow, which acts as a water reservoir through the spring and into the summer. Compared to summer thunderstorms, which are usually short, intense rainfall events, snowmelt is preferable due to the increased duration and the gradual nature of the water moving into the soil, plants, lakes, rivers etc. One caveat this year is the rain event we had associated with a tropical disturbance moving along the Sierra August 4th and 5th that brought mostly steady rain. Bottom line, we need a good winter to bust this drought as the below graphic eloquently illustrates.
The Sierra depends heavily on cold season precipitation. The majority of the precipitation falls November through March. For example, Tahoe City and Truckee receive more than 70% of their annual precipitation in these five months. So what’s on tap for this winter?
For the past 29 months, the El Niño-Southern Oscillation (ENSO) has been neutral. This represents the 3rd longest ENSO neutral period since 1950. And examining ENSO’s history, each instance where ENSO has been neutral for more than 13 months, an El Niño has followed.
Well there has been a forecast of El Niño for the past several months. While the Southern Oscillation Index (SOI) has been borderline El Niño since July, the sea surface temperature (SST) anomaly has been stuck in neutral. The SST anomaly is much closer to an El Niño than La Niña currently and has been since the spring.
The reason I am discussing ENSO is it can be a strong influence on the weather and climate, and for us to bust the drought; we need a change in the atmosphere. Ridges control the weather and they tend to establish over areas of drought. That does not bode well for us. So a change in ENSO or other atmospheric-oceanic circulations needs to occur to break this rut.
In the subsequent posts, we will discuss ENSO, some of the other atmospheric-oceanic circulations, seasonal forecast guidance, the complexity of seasonal forecasting, and how it all affects the Sierra and Tahoe. Stay tuned for Part 2 of Powdiction’s inaugural Anti-Seasonal Outlook, Seasonal Outlook: A Four Part Series!
Monday, May 5, 2014
In by B.Hatchett // 12:47 PM //
After a bad week for freezing temperatures at lower and upper elevations, the recent cold front has brought the recipe for perfect corn conditions to the region. Higher elevations will now have had a solid freeze since the last snowfall and middle elevations will have finally experienced sustained cold temperatures at night for several days in a row. This means excellent spring skiing is ahead! As if it wasn't already pretty good in select locations, now it will be super duper everywhere once the wind abates.
Tuesday will have continued cool temperatures, cloud cover, and some light precipitation but by late Wednesday we will be transitioning back into seasonal conditions. For Wednesday, blustery northerly winds and slightly lower than normal temperatures will keep the corn window cracked but only with wise choices of terrain and aspect. Thursday onward should be straight up awesome. Get some!
Tales of coverage and quality remain great given the lackluster totals this season. Saddlebag Lake road should open any day with many fun steep runs to be enjoyed. Solstice, Ripper, Chute Out, and others are skiing well and should have sufficient coverage for a short while. Shasta's south side is still in play, the east side should be coming into awesomeness now that a few warm days helped transition to spring, and the north side has lots of walking. Lassen is likely still a ton of fun with some Devastated walking required (not sure on the south side entrance situ, should be open anytime if not already). Tahoe is just about dun-dilly unless you don't mind a long stroll.