The North Atlantic has been very quiet since Hurricane Alex in late June. Alex was the strongest storm in terms of wind speed in the month of June since Alma (1966). However, and according to NOAA’s 2010 Updated Atlantic Hurricane Season Outlook issued on 5th August, the Atlantic Basin remains on track for an active season. The three main indicators are:

- Expected continuation of tropical multi-decadal signal. Key components of this signal include an enhanced west African monsoon circulation and above average SSTs in both the lower and higher latitudes of the North Atlantic. In the tropical Atlantic and Caribbean Sea atmospheric aspects of the tropical multi-decadal signal seen since 1995 include reduced vertical wind shear, weaker easterly trade winds, and an extensive area of cyclonic shear at 700-hPa along the equatorward flank of the African Easterly Jet. These atmospheric and oceanic conditions, that are now in place, are conducive to hurricane formation;

- La Niña. It refers to a periodic anomalous cooling of SSTs in the central and eastern equatorial Pacific Ocean. This cooling affects rainfall patterns across the tropical Pacific which, in turn, alters wind patterns so as to reduce the vertical wind shear in the tropical Atlantic and Caribbean Sea. Consequently, La Niña is typically more conducive to increased Atlantic hurricane activity (Gray, 1984);

- Above average SSTs. Currently the SSTs in the tropical Atlantic and Caribbean Sea are 1-2C above average and are expected to be much above-average to near-record during August-October. This warmth is much larger than anywhere else in the global tropics, and is a further indication that climate conditions are conducive for hurricane development in the Atlantic basin.

In the next couple of weeks we should be heading into a more favorable large-scale regime for tropical cyclone formation according to the latest Madden-Julian Oscillation forecasts. Klotzbach (2010) showed that when the MJO is located in Phases 1 and 2 (convectively active over the Indian Ocean), it reduces vertical wind shear in the tropical Atlantic, thereby providing a more conducive environment for formation on a shorter time-scale basis. The GFS ensemble is hinting that the MJO may be amplifying in the Indian Ocean in the next couple of weeks and the weather pattern will also become more conducive for development so we are heading towards a busier second half of the month.

A summary of July from our atmospheric observatory is included below (courtesy of Mike Stroud):

Atmospheric observatory data from July

Notable features are the general warmth in all four temperature diagnostics consistent with the UK picture from the Met Office:

One notable feature of July was that all 31 days recorded maximum temperatures above 20 degC. This is only the second time that this particular statistic has been achieved (the first being 2006 when temperature were very high all month).

Wednesday (4^th August 2010) night, sky-watchers as far south as Denmark and northern Germany were treated to a glimpse of the aurora – unfortunately it didn’t extend far enough to be lost in the bright lights of Reading. This geomagnetic activity was the result of a pair of solar eruptions on August 1^st, recorded in detail by the new Solar Dynamics Observatory (there’s a stunning movie here. One ejection can be seen as the erupting dark material to the north of disc centre, while the other originated in the bright flaring region just left of disc centre). The ejecta were subsequently tracked out into the solar wind, ultimately to Earth, by the twin STEREO spacecraft.

Space weather prediction

Based even on early observations, forecasters were confident the ejecta were headed towards Earth and via ballistic extrapolation of the initial speed, made reasonably accurate estimates of their arrival at Earth (though with uncertainties of order +/- 1 day, i.e., +/- 30% of the travel time). Lacking, however, were quantitative physics-based predictions of the impact on the Earth’s system, in particular intensity, location or timing of aurora. If we know a “storm front” is on its way, we can measure its speed and know it will be travelling almost entirely radially from the Sun, why is space weather forecasting so difficult?

Quite a number of reasons, it turns out. In situ coverage of interplanetary space is sparse at best: The sheer volume  means events can only be monitored at one, maybe two, points, and orbital dynamics mean never at the locations optimum for forecasting. Advanced prediction is therefore based on remote observations of the solar wind, which are limited to hydrodynamic properties, notably plasma density. Solar wind disruption of the Earth’s magnetic field, however, is primarily governed by electromagnetic properties, in particular the strength and orientation of the magnetic field. Presently, the only routine way to measure the solar wind magnetic field is with spacecraft in near-Earth space, giving at most ~30 minutes warning.

The recent events in context

Although the geoeffectiveness of these recent eruptions was difficult to forecast, it’s useful to look at them in retrospect. The solar wind speed, V, and its magnetic field strength in opposition to that of the Earth, -B_Z, determines the rate at which energy is injected into the terrestrial system. The figure above compares the hourly occurrence of the solar wind “driving parameter,” -VB_Z, over the last solar cycle to the values observed in these recent events. Clearly, events of this magnitude are fairly common over the course of a typical solar cycle (though enhanced hourly -VB_Z values are highly clustered in time). So why was there such widespread media interest? The significance of these aurora is their timing – after nearly half a decade of anomalously quiet solar activity, they are a reminder that the Sun is finally starting a new cycle.

My prediction for aurora over Reading: 2012. Give or take a year.

Recently much attention has been paid in the news media to heavy monsoon rainfall and consequential devastating flooding over Pakistan, particularly over the northern-most provinces.  Owing to the complex geography of the region, particularly that the huge Indus river travels down Pakistan from north to south, flood warnings have already been issued for far southern regions such as Sindh.

Most of the heaviest rainfall fell over three days in late July (28th-30th), particularly in the Gilgit Baltisan / Azad Kashmir and Khyber Paktunkhwa regions.  The Northwest Frontier Province (NWFP) recorded July rainfall totals of 179.5% above normal.  Even recording stations in northern Punjab received heavy rainfall, with some parts of Islamabad totalling more than 250mm on 30th July.  It is important to realise that Pakistan expects to receive monsoon rains during the months of July and August, however the quantity and persistence of this rainfall has been most unusual.

So what have been the causes?  La Nina cold events over the tropical Pacific Ocean, such as the one currently developing, can often be associated with heavier than normal monsoon rains over the South Asia region, however there is no reason to blame it for this particular event just yet.  Near to the surface, the only clue in dynamical fields is given by the lower tropospheric winds, which suggest anomalous easterlies extending from the north side of the monsoon trough at the Himalayan foothills and extending well into northern Pakistan.  This suggests an extension of monsoon conditions over India into Pakistan.

More insight can be gained by examining behaviour at upper levels.  In the theta on PV2 animation of 23rd-30th July shown below, there is evidence suggesting a persistent upper level trough located over northern Pakistan, particularly during the latter stages of that week.

Figure 1: Potential temperature on PV2 surface animation: 23/7/2010-30/7/2010

By examining the streamfunction at this level, an interesting picture emerges.  The suggestion is one of an omega-shaped (Ω) blocking pattern centred near the Ural Mountains in Russia, implicated in recent soaring temperatures and wildfires during the persitant heatwave in Moscow and other regions.  A pair of troughs is noted to the south-west and east, appearing as perturbations to the stationary wave running along the 35-45°N latitude band.

Figure 2: Weekly mean stremafunction at 250hPa: 23/7/2010-30/7/2010

Further analysis (not shown) demonstrates that this pattern has existed since early July, although in a weaker form.

The current outlook suggests further heavy monsoon rains over northern and some southern parts of Pakistan over the next few days.  However, even in the absence of further rains, the downstream effects of this accumulation will likely continue to cause devastation.

With thanks to Mike and Brian for useful discussions.

After a relatively quiet start, the tropics are beginning to show some signs of life in the far-eastern Atlantic. An elongated cluster of showers and thunderstorms over the southeastern Atlantic is slowly organizing as it moves generally west to west-northwest. Dry air remains to the north as the system is experiencing little in the way of unfavorable wind shear. Sea surface water temperatures are warm in the system’s path. According to Accuweather.com meteorologist Joe Bastardi there are three reasons why development is likely to occur:

1) This system, which may be two combining, is large. Large means that there is plenty of energy to be bundled and so while development make take longer, it could end up stronger. Like Alex, this is developing in a way that if you watch typhoons, you see quite a bit, large systems with more than one center that eventually combine, but that can, if focused, summon a lot of energy;

2) Negatively titled surface trough with it. The surge east of stratocumulus well north of the system, not into and through the system, conserves low level vorticity in the path of the system, while destroys it north of the jet axis. This means high pressure tries to self build north of the storm with lower pressure in the path of it. Negatively tilted systems in the tropics tend to self develop faster since the strong east stream north of the system means the reverse eddy south of the stream is cyclonic, anticyclonic north of the stream, and this helps maintain the system. When the system is embedded in strong easterlies (trades further south) pressures tend to lower southwest of the system, rise on to and north and one gets a fast moving wave that shears itself. A visit to a fast moving stream will help you understand how this works;

3) Outflow signature southwest and south of the system. It already has a channel developed aloft so you are ventilating properly.

A large dome of high pressure will steer the low westward through the week, as it approaches the Leeward and Windward islands by late week.

This was certainly a case of right place, right time. I was enjoying a drink with a friend on the grass outside the union on Friday the 4th when I witnessed something pretty spectacular. Spotting a circular motion of leaves over to my right, I didn’t think much of it since this is quite a common occurrence. But within a few seconds, it had moved towards me and developed into a thin funnel extending about 10 metres up into the air having picked up grass and debris. Then moving right behind me, it quickly dissipated after reaching the union building with the only marks of its occurrence being a few squeals from some students and the shocked faces on my friend and myself. Unfortunately, my friends of the non-weather-geeks variety I saw over the next couple of days failed to share my enthusiasm and instead could only resort to the cheeky remark: “So you then saw a unicorn next, right?”

Being so pleasantly surprised at witnessing something rather cool, I tried to seek some answers as to why this occurred. Strong solar heating is an important driving factor. The weather conditions that day were dominated by a ridge of high pressure extending over the UK (see synoptic chart) , resulting in a Tropical Continental airmass bringing warm conditions (maximum temperature of 26.6°C), clear skies and light winds. Therefore, strong solar heating of the ground would have led to strong convective activity in the form of turbulent eddies and rising hot air.


An important driving factor in the whirlwind formation is the turbulent motion due to the convection driven by the warm surface; any resulting spinning motion can get intensified due to vortex stretching effects (by conservation of angular momentum) and these can lead to substantial coherent vortices. An important factor in the visibility of the vortex near the surface is the effect of friction. With the main balance of the pressure gradient force pulling air towards the centre and the centrifugal force pushing air away from the centre, this produces a stable vortex.  But friction at the surface will retard the flow and thus reduce the centrifugal force.  Now the balance is lost: the air, along with dust and debris, will start to rush in towards the centre of the low pressure area, feeding the whirlwind.

Turbulence clearly goes on in the boundary layer all the time, particularly during calm sunny conditions that enhance convection and so invisible whirlwind motions are commonly present in the boundary layer. I was just fortunate to see one of these turbulent whirlwinds reaching the earth’s surface made visible by the present of grass and debris. Maybe this phenomena happens a lot more often than we think.

Thanks to Maarten Ambaum for his help with this. Any further comments/corrections are welcome.

After all the recent controversy over hacked emails, IPCC errors and the British public apparently growing more skeptical about climate science and scientists, it is perhaps interesting to take a step back and look at why we believe that emissions of various gases can influence our climate. There are many comprehensive sources of the historical background to modern climate science, and many of the classic papers are freely available. This is a very brief summary of some of the those first ideas.

Early stages

In the 1820s and 1830s scientists such as Joseph Fourier and Claude Pouillet had questioned what set the temperature of the Earth. They realised that something like the greenhouse effect might exist (Fourier’s 1824 paper talks of trapping ‘non-luminous’ heat, i.e. infra-red radiation) , especially from experiments by Horace-Benedict de Saussure who carried an early ’solar oven’ (essentially a blackened cork box with various plates of glass as insulators) to the top of Alpine peaks, but they did not yet understand the radiative effects fully.

Understanding ice ages

In 1837, Louis Agassiz, had established that the planet had previously been in an ‘ice age’, and many scientists wanted to know why (Milutin Milankovic did not publish his theories on orbital changes until 1920). It was trying to understand these issues which led John Tyndall to perform his pioneering experiments, published in 1861. Using apparatus such as that pictured below, Tyndall attempted to measure the absorption of various gases (including CO₂, N₂O and C₂H₄) and concluded that,

“… an almost inappreciable admixture of any of the hydrocarbon vapours would produce great effects on the terrestrial rays and produce corresponding changes of climate.”

The work of Tyndall, and others, gave Svante Arrhenius the data needed to make the first climate prediction. In a paper published in 1896, he used simple radiative calculations (done by hand!) to estimate the temperature change caused by changing the constituents of the atmosphere, and concluded that if you doubled the amount of CO₂ in the atmosphere you would increase the global temperature by about 5°C (modern estimates are around 3°C). Arrhenius also predicted a polar amplification of any change in climate and an increased land-sea contrast, agreeing with modern evidence, but he had ignored the effect of clouds in his calculations.

In a subsequent review article in 1901, Nils Ekholm concluded that CO₂ was the primary cause of geological changes in climate, and discussed European climate variability over the previous few hundred years. He also postulated that atmospheric concentration of CO₂ could increase over the coming millennium due to the burning of coal, which would warm the Earth and potentially stop the ice ages, concluding,

“It is too early to judge of how far Man might be capable of thus regulating the future climate. But already the view of such a possibility seems to me so grand that I cannot help thinking that it will afford Mankind hitherto unforeseen means of evolution.”

First detection of modern climate changes

However, Arrenhius’ work remained controversial as arguments over the relative roles of water vapour and CO₂ in atmospheric absorption went on. Then, in 1938, the potential influence on global climate was revisited by Guy Callendar. He compiled the first global temperature reconstructions (see figure below) and estimated the part of the changes observed which might be caused by changes in CO₂ (the dashed lines). He noted that the recent changes were larger than that predicted, and even considered whether the urban heat island effect was responsible. [We now believe that a mixture of causes (solar changes, CO₂ and natural variability) are responsible for this early 20th century warming.]

Beyond 1940

Hindsight may allow us to pick out these key scientists now, but it cannot have been easy at the time to distinguish between competing theories, particularly with the lack of observations and relatively primitive measurement equipment available. After Callendar, the subsequent few decades brought little consensus, although Gilbert Plass and Roger Revelle made important steps forward. In the 1950s and 1960s, concerns about particulates from burning fossil fuels causing a cooling were significant, but not overwhelming. Charles Keeling began measuring the quantity of CO₂ in the atmosphere in 1958, but by the 1980s, the Clean Air Acts had reduced the levels of particulates and the warming began to emerge again. In 1988 the IPCC was born and will publish its 5th Assessment Report in 2013.

It is with great admiration that one reads these early papers, particuarly the obvious dedication with which they approach devoting years to tedious calculations or designing and building new equipment. It is recommended reading for all!

Any comments and corrections are very welcome.

While in Oklahoma this academic year, I thought I had discovered something peculiar. Frequently, when discussing the weather back at home in Stockton-on-Tees with my parents via Skype, there was often a coincidence of weather type, even a swift change on the same day, e.g. the passage of a front. Of course, I was wary of psychological effects here – I believe they call it confirmation bias – or some positive version of Sod’s Law (“oh typical, it’s sunny at home and it’s 35ºC here!”). While trying to attempt to explain this to myself with recently-learned meteorological theory, I stumbled upon the fact that the distance between Norman, OK and Stockton-on-Tees, UK (7238 km) was rather close to the wavelength of a Rossby, or planetary, wave (6-7000 km, with some assumptions and a sinusoidal wave equation). Troughs in the planetary waves are associated with cyclonic (positive) vorticity advection downstream, and thus depressions, rain, wind etc., while ridges are manifested at the surface downstream by calm, anticyclonic conditions, thanks to widespread sinking motion under anticyclonic (negative) vorticity advection. Could this account for an occasional perceived parallel weather pattern?

It seems even the layman (or laywoman in this case) may inadvertently acknowledge this effect. After visiting my Grandma, a retired parcel van driver in Motherwell, Scotland, over the Christmas break, she remarked how she had foreseen a couple of the more severe snow days in the UK, because the US coast had been hit by a vicious nor’easter the day before. Her theory was that the low pressure moved across the Atlantic and hit the UK the next day. Should her perception be sound, a much more likely (and realistic) explanation is that troughs in the Rossby wave had been located over the eastern US and western Europe simultaneously.

I don’t think a quick delve into weather archives on randomly selected days would do justice to this ‘theory’, as I could easily pick three or four days to support my case and ignore the other times where it’s been hot in the Mid-West and, well, British in Britain. Instead, I throw this out to those much better qualified than myself: does this blog’s content sound reasonable to you? Have there been any papers on the matter? In the meantime, assuming my hypothesis holds, I shall continue to harbour pity for Americans in the Central Time Zone who are by my reckoning getting some pretty serious drizzle.

Another week of WCD and another fascinating selection of pretty pictures – of phytoplankton blooms in particular. But the image that inspired me most was the soil moisture anomaly of Europe, shown by Pete Inness when talking of the Polish floods (link). But my eye was drawn rather closer to home, seeing a 2-3 (unknown normalised units) dry anomaly across Southern England. This struck me as no surprise, I’ve needed to water the garden practically every day for weeks, but as was pointed out to me, this could well bring back a British media summer staple phrase, missing since 2006, “hosepipe ban“. Soil moisture of course is going to be largely based on the recent rain… Only 15.4mm at the atmospheric observatory in May up to the 27th (largely attributable to the Shonk Effect).

So will we be back to the summers of old where the words hose pipe ban are mentioned by the press at every hint of rain? A trip to the CEH website allows us a look at the groundwater will show signs of the soil dryness seen, but a look at that map shows the water levels in the aquifers are normal or above normal, with our closest site, “Stonor Park” showing a little above normal.  The 2010 rainfall accumulation until the end of April has shows very near average rainfall, this will be the reason the groundwater levels are near normal.

It would only be fair for me to mention that Thames water have a good record when it comes to hosepipe bans. The ban in 2006 to early 2007 was their first for 15 years and came after several years of drought, so this region is not prone to hosepipe bans.

Looks like we shouldn’t be too worried yet, but if the recent dry period continues, perhaps the media will again be filled with hosepipe bans. This is the point to turn to the Met Office’s summer forecast…  So failing that, I suggest if water levels get low in Reading, we simply give Jon Shonk further roles in WCD.

Science communication is a hot topic in the media. The accuracy of weather forecasts, in particular long-range projections, is often found to come under fire. In forecasting there is no single future eventuality we can be certain will occur. There are always uncertainties associated with the observations and models used to make predictions. Nowadays ensemble based methods are widely used to help explore the effects of these uncertainties on the forecast. An ensemble of many forecasts is run which all start from slightly different initial conditions. The starting points are carefully chosen so that the evolutions of the forecasts represent the range of possible outcomes. All the forecasts are then used to calculate probabilities of certain weather events occurring.

Verifying a probabilistic forecast is no easy task. Each forecasted event has a yes or no outcome, so can we say an 85% chance was a skilful prediction if the event occurs? To come up with a meaningful measure we look at the performance of the model over many forecasts. If the event occurs on average 85% of the time that the forecasted chance was 85% we can deem the forecast to be reliable. This will of course mean there are at times ‘false alarms’, when the forecast predicted a high chance of occurrence but the event does not happen in reality.

A well known example of communicating uncertainty in scientific understanding is the IPCC’s ‘likelihood’ scale, with each likelihood category based on a percentage threshold. Ideas of this kind go back many years, a Monthly Weather Review article by W. E. Cooke in 1906 proposed a 5 point scale for describing forecast uncertainty. Of course a system of this kind is far from perfect and individual interpretations of a given phrase will be wide ranging. An interesting review on the history of probability forecasts can be found here.

The World Meteorological Organisation (WMO) says that ‘communicating the uncertainty of the forecast is vital to users. It allows them to make better decisions that are attuned to the reliability of the forecast. It also helps to manage the expectations of users for accurate forecasts.’

Aside from communication aspects, the WMO also make clear that verification is an integral part of the forecast process. If we have any hope of providing clear and useful information to the public we need to understand the models we use and how reliable they are at making the predictions we seek. Increasing media coverage and public interest has highlighted the need for us all to give lucid explanations, whether it be describing the coming weekend’s weather to your parents or discussing future climate change with policy makers. Why don’t you take Andrew’s survey on the communication of seasonal forecasts and have your say?

‘If a man will begin with certainties he shall end in doubts; but if he will be content to begin with doubts he shall end in certainties.’ Francis Bacon