Wednesday, November 28, 2018

New ERA5 Reanalysis

One of the more exciting developments in the world of weather and climate science this year has been the release of new data sets by the European Union's Copernicus Climate Change Service.  The program is funding the free and open distribution of vast quantities of data through the Climate Data Store, so there is almost unlimited scope for new research as well as commercial development using the data.

The data set that I'm most excited about is the latest generation of reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF), which is well-known for having the most accurate global weather forecast model in the short-medium-range time frame (out to two weeks in the future).  I've often used reanalysis data from NOAA on this blog, and indeed NOAA's global reanalysis from 1948-present is heavily used worldwide and is extremely valuable.  However, the NOAA reanalysis relies on a model that is very out of date now.  Happily, the new ECMWF reanalysis - using the ECMWF's top-notch modeling capability - is now coming online via the Copernicus program; the data are currently available back to 2000, but next year we'll see the product extended back to 1950.

Here's an article about ECMWF's new ERA5 reanalysis:

Back in 2015 I did a brief comparison of NOAA's reanalysis data with real observations from Fairbanks; here's one of the figures, showing the very poor correlation of reanalysis to actual temperature and precipitation in summer.

The chart below is a similar figure using ERA5 data for the nearest grid point to Fairbanks, which happens to be located just to the south across the Tanana River (the grid spacing is about 20km).  The performance is impressive.  Now admittedly the correlations ought to be very high for temperature, because the ECMWF model uses surface observations to refine its gridded estimates of evolving weather conditions hour by hour.  However, precipitation is predicted by the model over short time intervals, so the model does not "know" how much precipitation occurred in reality; and neither ground-truth data nor radar estimates are used to improve the estimates.  Given that the ERA5 precipitation data is purely a (short-range) forecast, I think it's very impressive that the monthly correlations are as high as ~0.8 in May through July, when hard-to-predict showers and thunderstorms produce most of the rain.

Here's a look at correlations of daily rather than monthly temperatures through the year.  Daily low temperatures are generally more difficult to get right than high temperatures, because the warmest conditions of the day tend to be more closely tied to the more homogeneous, well-predicted temperatures of the free atmosphere above.

Finally, the wind speed estimates from the model are not as impressive; apparently the low-level wind regimes near Fairbanks are a challenge even for the world's best global modeling system.

In due course I will be acquiring a larger volume of the ERA5 data and will have a chance to do a more extensive analysis; and it would be fun to set up an online map catalog of ERA5 data for the Alaska domain.  If anyone has an interest in helping out with such a project, let me know - perhaps there could be a collaboration.

Wednesday, November 21, 2018

Freezing Drizzle

Residents of Fairbanks experienced one of the more unpleasant kinds of wintry weather at times last night and this morning: freezing drizzle.  At least, that's what was reported by automated instruments for a period of 7 hours at the international airport and more briefly at Fort Wainwright and Eielson AFB.

Freezing drizzle occurs when cloud droplets grow large enough to produce drizzle at the same time that (a) the cloud and underlying air temperature are below freezing, and (b) the cloud top is no colder than roughly -10°C.  The cloud and drizzle droplets are therefore supercooled, but the cloud is not cold enough to contain much or any ice.  (See this old post for a more detailed discussion.)

Here are the atmospheric profile measurements from Fairbanks airport at 3pm yesterday (top) and 3am today (bottom).  The observations fit the conceptual model for freezing drizzle formation quite nicely, with a saturated layer in the lowest few thousand feet - where the red (temperature) and green (dewpoint) lines are superimposed - but cloud-free air above that.

Here are a couple of shots from the Ester Dome webcam that normally looks out over Fairbanks.  These were taken after the freezing drizzle ended, but they show the low cloud layer below with fair weather above.

Looking at hourly ASOS data since 2000, Fairbanks tends to report freezing drizzle on just one or two days a year, on average, although the distribution is very uneven: a few winters account for most of the events.  Looking at a handful of sites across Alaska, here's the percentage of all observations (throughout the year) at which drizzle is reported along with a temperature below freezing:

Utqiaġvik/Barrow   0.39%
Bettles   0.08%
Fairbanks   0.06%
Anchorage   0.05%
Northway   0.03%
McGrath   0.03%
Tanana   0.02%
Gulkana   0.02%

I'm slightly suspicious of the data, because in more than 18 years of data there are zero reports of freezing drizzle in Eagle, Delta Junction, or Kaltag - all of which I understand to have the same ASOS instrumentation as the sites above.  Perhaps there are in fact some instrument differences that contribute to the variation in the results.

Here's the seasonal distribution of freezing drizzle reports in Fairbanks; less than 20 years is insufficient to build up a good climatology, but there does seem to be a preference for these events in early winter rather than mid-late winter.

Up in Utqiaġvik, where freezing drizzle is much more common, the majority of events occur in early winter and late spring; it is very unusual in deep winter, when moisture levels and temperatures are usually both too low to produce the phenomenon.

Saturday, November 17, 2018

Alaska Observation Palooza

Hi, Rick T. here. In searching back through Deep Cold posts, it looks like we've never put up a review of a subject near and dear to my heart: an overview of point-based weather and climate observations in Alaska. So, I thought it might be worthwhile to lay out some of the details. Note: this piece is acronym heavy. I've included a decoder table at the end.

When it comes to weather observations: right now here in Alaska, we are living in the golden age of real time environmental observations: the good old days were definitely not always good. When I started with the NWS in Alaska in 1988, for most of the year there were less than four dozen reliable, real-time 24-hour/per weather observations in the entire state, and some of those (ones with the asterisk in the graphics below) only had temperature, wind and pressure for part, or all, of the day. Every single one of these were operated by either then NWS, FAA or DoD.

There were more part-time or irregular weather observations the 1980s. There were perhaps 20 contract aviation observations (paid for by NWS or FAA) that took 6 to 16 observations a day, e.g. Umiat, Slana and Ambler. By the late 1980s the DoD dew line stations were no longer regularly reporting 24 hours per day but sent observation on an occasional schedule. There were a tiny number of NWS operated remote stations (RAMOS) that reported temperature and winds and were still operating in places like Anaktuvuk Pass and Sitkanak (southern Kodiak Island). There were some, but not usually real-time observations from a small number of the Soil Conservation Service (now NRCS) SNOTel sites. In the summer there were even more, from Alaska Fire Services RAWS (which were deployed and then removed at the start and end of the fire season) and from relatively few river gauges which had temperature and tipping bucket sensors.

Fast forward to today: a quick count of observations in Alaska available on the MESO-WEST website for 3am Saturday morning revealed about 785 separate observations with at least one meteorlogical element, and to that you could add more than a dozen more home weather stations that are online only on Weather Underground. Here's a graphic I made up in 2017 that identifies the operators and types of observation in the eastern Interior and Copper River Basin.
This shows most of the classes of 24 hour per day weather observations that are currently deployed around Alaska, with the exception of some coastal specific observations. You'll see that most of the platforms belong to some Federal agency (e.g. NWS, FAA, BLM, NRCS) but there are some others, including state of Alaska DOT and private (the home weather stations). The stations marked as USARRAY are an interesting class. These are actually part of a temporary, high density seismic monitoring network on which weather sensors have been installed. Only a  fraction of the stations have weather stations included (the full map is here). These stations have provided never before seen real-time weather detail over the North Slope and Brooks Range, you can as see in this graphic (same 2017 project):

On the climate side of the house, it's a different story. The March 1988 edition of the NCDC publication Climatological Data had data from about 130 stations in Alaska reporting daily temperature extremes and precipitation data, and this represents the vast majority of climate observations that were made that month in Alaska. About 75 of these were NWS cooperative sites which made observations once a day and the rest NWS and FAA (including the contract observations) and DoD sites. In March 2018, there were just over 100 stations with data in that publication, and NWS cooperative sites was still number about 75. The losses are mostly from the drop off of the aviation contract and DoD sites: almost none of those became strictly cooperative stations.

The big difference beween 1988 and 2018 is that now there are multiple sources of readily available climate data that are not included in the Climatological Data publication. These include explicit daily climate data from unaugmented NWS ASOS (e.g. Kaltag and Seldovia) and the nearly two dozen NCEI flagship Climate Reference Network stations as well as derived climate data from the 100+ RAWS (mostly operated by the Alaska Fire Service and National Park Service) and four dozen NRCS SNOTel sites. Now one can question the climate value of some of these platforms, e.g. RAWS, which we know report temperatures that are too warm during high-sun, low-wind situations to due insufficient thermistor shielding, but clearly, Climatological Data is no longer the definitive source for Alaska climate data. Rather, it's perhaps the definitive source for NWS cooperative data, only some of which is reported in near real-time, and of course this does (eventually) make it into online climate sources, e.g. the NWS NOWData and scACIS.

But that's not the end of the story. Since 2010 the FAA has upgraded nearly all of the early 1990s era AWOS, the first generation automated operational weather observing systems that included visibility and ceiling height, with modern automated equipment which is functionally very similar to the NWS ASOS, as well as installing this modern equipment at at airports that previously lacked any weather observations. There are about 80 of these these FAA owned and operated systems that report temperatures and precipitation in the same way as ASOS, including sites with long histories of climate observations as well as sites that have never had climate observations before. While these systems don't handle frozen precipitation, the temperature data is reliable, but at the moment none of this is being used systematically in most climate analyzes. It's not true that it's been tossed out: the data is all archived at NCEI and other online locations, but we're not making much use of it. Here's a plot of these stations climate data we're not using:

As a sidenote, I do keep track of the daily data from several of these location that have historic climate data (Kaktovik, Unalakleet, Gambell, Ft. Yukon) and use it in my work. So while we have many, many more weather observations nowadays, climate data has not expanded, and in fact over much of northern and western Alaska we have less now than we did 30 years ago.

  • ASOS  Automated Surface Observing System: the NWS standard, requires commercial power
  • AWOS Automated Weather Observing System: the first generation FAA station, did not report climate data, required commercial power 
  • NRCS: Natural Resources Conservation Service: Successor to Soil Conservation Service, part of the US Department of Agriculture. 
  • RAWS: Remote Automated Weather Station: the workhorse of fire weather. Run on solar or wind power, often installed at higher elevations 
  • SNOTel: Snow Telemetry: NRCS station widely in the West and Alaska primarily for to measuring snow pack but now includes in a variety of meteorological parameters. 
  • USARRAY: US Array: a 15-year program to place a dense network of permanent and portable seismographs across the continental United States. Installed in Alaska 2015-17, scheduled for removal starting in 2019. 

Thursday, November 15, 2018

Warm Winter Ahead?

Today the U.S. Climate Prediction Center came out with their seasonal forecast for winter, i.e. December through February, and to probably no-one's surprise they are showing a rather high chance of above-normal temperatures in Alaska in the coming months.

By way of reminder, the CPC forecast is probabilistic and aims to predict the chances of each of the three terciles of the 1981-2010 distribution; so a 50-60% chance of "above normal" (as for most of Alaska above) means that the probability of the upper tercile is 50-60% rather than the baseline normal probability of 33%.  This is a big shift in the probability distribution and a very strong warm signal; it's the first time that CPC has shown such a large area of 50+% warmth in Alaska for Dec-Feb.

Here's the corresponding precipitation forecast.

My blog co-author Rick will be presenting much of the rationale behind the forecast in his regular monthly webinar tomorrow, and listeners are guaranteed to learn something even if they're already familiar with the complexities of seasonal forecasting.

Not to steal Rick's thunder, but I'll suggest just a few reasons why a very warm forecast seems reasonable if not inevitable.  First, sea surface temperatures to the west and south of Alaska have been extremely warm in recent months, so regardless of how the weather pattern plays out, there is a lot of residual heat available.  The map below (click to enlarge) shows that October SSTs were more than 3 standard deviations above the 1981-2010 normal near Alaska, according to NOAA's ERSST data.  This is a pretty extraordinary warm anomaly.

Second, El Niño has developed in the tropical Pacific Ocean.  As we speculated back in July (see here), this El Niño episode is focused in the central rather than eastern tropical Pacific, so it's a so-called Modoki El Niño.  It's interesting to consider the different implications for Alaska of having a Modoki rather than East Pacific El Niño; I've found that the results are a bit sensitive to the definition that's used, but the overall message seems to be that Modoki episodes favor warmth more widely across the Bering Sea - Alaska region.  East Pacific El Niño's are more closely linked to warmth in eastern and southeast Alaska than in the rest of the state.  But in any case, an El Niño winter is most definitely a warm signal overall.

Third, the long-range computer model forecasts are showing pronounced warm signals for the upcoming winter.  Here's a sample.

The NMME multi-model ensemble mean anomaly: note the +3-4°C anomaly near the Bering Strait.
 The NMME (non-calibrated) tercile probability forecast:

The UK Met Office ensemble mean anomaly:

And the Japanese seasonal model - see below.  It's a little hard to see the coastlines, so I've circled Alaska in red.  The top left panel shows 500mb height, indicating a southerly flow over Alaska; the middle left panel shows 850mb temperature - note that the model expects the warmest conditions in the Northern Hemisphere (relative to normal) over Alaska.  The bottom left panel shows sea-level pressure - note the Bering Sea trough.

Finally, the map below shows a statistical forecast based on sea surface temperatures that I developed at work recently; this is an ensemble mean forecast from a number of models that have undergone cross-validated historical testing to determine the optimal predictors.  Note that the baseline is the 1950-2017 trend, so the forecast for Alaska would be even warmer relative to the standard 1981-2010 climatology.  The skill of these statistical forecasts is modest at best, but they're worth having because they provide independent guidance to complement the computer models.

So is there any contradictory guidance showing a cold or even a normal winter?  Having looked at a very broad collection of predictors in the past couple of weeks, the answer is "almost none".  The only hint I found was in looking at past years in which late summer and early autumn produced strong and persistent high pressure ridging over the northern North Pacific, similar to this year.  The subsequent winters had a modest tendency for unusual high pressure over the Bering Sea, which is a cold pattern for Alaska (see below); but the signal is not very striking, and in any case most of the "analog" years are taken from earlier decades that were colder to begin with.

Tune in to Rick's talk tomorrow for much more detail and considerably more expertise related to the winter forecast!

Tuesday, November 6, 2018

North Pacific Blog Post

It's been a while since I did one of these, but I've just posted an update on the North Pacific "blob tracker" blog.

I'll aim to add some interpretation for the Alaska climate scene on here in the coming days.