Dry Early May Weather

Early May Weather
Observations from the Bermuda Weather Service. Temperature and dew point on top, wind direction on bottom. To the right of the yellow line, the relative humidity (indicated by how close together the temperature and dew point are) increases as winds settle out of a south-southeasterly (135-180°) direction.

April ended by clearing out a very warm and humid airmass that was accompanied by patches of fog that advected across the island. This left the start of May quite dry with northerly winds and a ridge of high pressure to the north. As the ridge shifted to the east of Bermuda, northerly flow gradually veered southerly bringing in more humid air. High pressure has remained centered generally to the east of Bermuda since the fifth of May maintaining humid south-southeasterly flow.

This setup is fairly common for this time of year: ridges of high pressure settling in near Bermuda bringing persistent settled and mild weather across the western Atlantic. The Bermuda-Azores high typically starts to build in during late-May/early-June to its July peak extent and intensity, prevailing winds come from the south and southwest with a warm and humid the norm.

This high pressure setup is generally effective at keeping rain-making frontal systems at bay. However, it appears the ridge extending across Bermuda might weaken enough to allow a weakening front to bring some showers to the island over Friday. High pressure then re-establishes itself with a return to more settled weather over the weekend. The weak front is unlikely to be able to bring any real change in the airmass and the warm humid weather should continue without much break.

May is typically one of the driest months in Bermuda. Long periods without much rainfall are characteristic of April through June. Water availability can become an issue when several such dry spells occur in one year – particularly so as water demand increases in the warm season.

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Ship Tracks Around Bermuda

2018-02-23 060220 UTC
GOES-East Geocolor imagery centred on Bermuda at 06:02 UTC, 23 February 2018. This multi-spectral product highlights low clouds (e.g. stratocumulus) as light blue shades. The brighter light blues indicate thicker low cloud.

[Watch animation]

High pressure over the western Atlantic over the last few days has been associated with persistent stratocumulus cloud cover. This typically thin layer of low-altitude cloud is a fairly common feature over the eastern half of ocean basins, and its occurrence around Bermuda is not uncommon – particularly in the cooler months.

Strong high pressure comes with subsidence. Air is compressed and warms as it descends. The descending air, with origins in the mid/upper-atmosphere is also quite dry.  This warm, dry air cannot make it all the way to the surface because it doesn’t have enough momentum to push through the cooler layer near the surface (the boundary layer). This leads to a stable layering of air that manifests as warmer drier air over cooler more humid air.

The boundary layer is often well mixed with an even distribution of heat and moisture (and can become increasingly humid over the ocean). Meanwhile, the warm air above acts like a lid over the cooler boundary layer preventing mixing between the two layers. Moisture trapped in the well mixed layer can form a sheet of stratocumulus cloud given the right conditions.

Stratocumulus_Annotated
Potential temperature (the temperature that dry air would theoretically have if it was brought to the surface without external heating), Specific humidity (the mass of water vapour for every kilogram of air), and Relative Humidity (the ratio between the actual water vapour content and the water vapour content needed for saturation). Profiles measured by weather balloon at 00:00 UTC 23 February 2018 at Bermuda.

Exhaust from ships contains aerosols that act as cloud condensation nuclei. These are airborne particles that water can condense onto to form cloud droplets. The aerosols from the ships tend to form more, and smaller, cloud droplets making the clouds contaminated with ship exhaust appear brighter and thicker to satellite instruments.

The ship tracks in the animation above appear to advance from east to west or west to east, following the path of the ships. Meanwhile, another cloud enhancement appears to advance southeastward, starting from Bermuda. This track follows the wind direction in the boundary layer (unlike the ship tracks). It could be the result of aerosols from BELCO or the Incinerator acting to enhance the cloud brightness through the same mechanism as ship tracks.

High pressure and the settled weather it brings has been a theme for much of February 2018. Few mid-latitude cyclones have impacted the island so far this month. As a result, the cold air has stayed away and temperatures have remained far above normal. The lack of cyclones has also meant that precipitation totals are below normal and no days this month have experienced gale force winds. Sea surface temperatures are also far above normal as a result.

The month is expected to end with changeable weather as high pressure gives way. A cold front then takes the opportunity to push southeastward across Bermuda with some rain and showers followed by cooler air.

Further Reading:
Coakley, J. A., R. L. Bernstein, and P. A. Durkee, 1987: Effect of Ship-stack Effluents on Cloud Reflectivity. Science, 237, 1020-1022.
Hudson, J. G., 2000: Cloud Condensation Nuclei and Ship Tracks. J. Atmos. Sci., 57, 2696-2706.
Wood, R., 2012: Stratocumulus Clouds. Mon. Wea. Rev., 140, 2373-2423.

Weather Recap: Bermuda 2017

Air Temperatures

BermudaClimate2017_temps
Daily temperature data for Bermuda. The blue bars represent the observed daily temperature range from the minimum temperature at the bottom of the bar, to the maximum temperature at the top of the bar. The blue shaded region represents temperatures below the normal daily minimum to the daily record low, the brown shaded region represents the normal daily range, and the red shaded region represents temperature above the normal daily maximum to the daily record high. Data are sourced from NCDC and BWS and should be considered unofficial.

The year started out very variable with large swings in temperature between near record highs and record lows. Near the end of February a string of unusually mild and quiet weather persisted for several days with temperatures setting a new daily record high of 73.9°F (23.3°C) on the 26th, beating the previous record for the day of 73.0°F (22.8°C) set in 1975.

February’s warm and quiet spell was rudely interrupted in early March. An strong cold front brought very cold air to Bermuda, including the year’s only daily record low. This was part of a cold air outbreak that lasted several days. Gales and blustery showers with isolated hail accompanied a daily record low on the 5th March when the low reached 47.7°F (8.7°C) breaking the previous record of 52°F (11.1°C) set in 1968.

April through September was much quieter with smaller swings between extreme temperatures. Mild dry periods and the occasional cool rainy days with slightly higher than normal temperatures dominated this period.

As Autumn progressed, temperatures resumed their typical variability, but mainly swinging between normal and higher than normal temperatures. The year’s second daily record high temperature was set on the 7th of December when temperatures reached 77.4°F (25.2°C), beating the previous record high of 77°F (25.0°C) set in 1978.

Precipitation

BermudaClimate2017_precip
Daily year-to-date rainfall. The green areas are the observed rainfall for 2017. The yellow line represents the normal year-to-date rainfall. The dark green shaded region represents the surplus in year-to-date rainfall, while the brown shaded region represents the deficit in year-to-date rainfall. Data are sourced from the NCDC and BWS and should be considered unofficial.

The year started with a significant rain event on the 5th January on which a month’s worth of rain fell in a single day leading to some flooding. This 5.34″ (135.6 mm) of rain was the heaviest rain for a meteorological day at any time during the winter months (Dec-Jan-Feb), and the fifth wettest day on record.

Slightly below normal precipitation fell through the remains of winter and by mid-spring, extended dry periods were taking hold. April 2017 was the second driest April on record with only 0.73″ (18.5 mm) of rain falling in the entire month. This dry pattern continued through May and early June.

2.09″ (53.1 mm) of rain fell on the 11th June setting a daily record (beating the previous record of 0.53″ (13.5 mm) set in 1988) and signaling an end to the dry spell. From mid-June through the end of the year, near normal precipitation totals were observed. This maintained a year-to-date deficit of roughly 5.00″ (127.0 mm). Other daily records were 1.21″ (30.7 mm) on the 18th July (1.14″/29.0 mm, 1993), and 1.77″ (45.0 mm) on the 19th October (1.50″/38.1 mm, 1963).

Sea Temperatures

Sea surface temperatures around Bermuda were far above normal for much of 2017. Light winds and prolonged periods of settled weather in spring and early summer limited the amount of mixing between the warm surface waters and the cooler water just below – priming the summer for above normal temperatures. This year no direct impacts from tropical cyclones was a welcome change to the pace set over the last decade, but also meant they were unable to provide the mixing that would cool the sea surface temperatures in late summer/early autumn.

The Bermuda Weather Service noted in monthly climate summaries that the monthly mean sea surface temperatures were among the five highest recorded since 1950 for September and October, and among the three highest for November.

Another part of this story is the presence of a nearby warm ocean eddy. This eddy lingered in the area for most of autumn. The eddy not only contributed to seas surface temperature anomalies, but also brought abnormally high sea surface heights. This came on top of elevated sea heights due to the astronomical Spring Tide phenomenon, and led to periods of generally minor coastal flooding throughout Autumn.

Abnormal Tides

Bermuda Esso Pier Water Levels.png
19 Sep to 12 Oct 2017 Water levels from Bermuda Esso Pier, St. George’s Island. Verified water level observations in green, preliminary water level observations in red. Forecast water level due to the predicted astronomical tide in blue. Water level is with respect to mean higher high water, where positive values indicate normally dry land is inundated. Alternative reference water level marks are on the right.

Over the weekend, the combination of the spring tide and a high amplitude ocean eddy resulted in localized coastal flooding around low-lying areas of Bermuda. Tides were running around 1.5 ft above expected levels which were already higher than normal thanks to a spring tide.

The role of the Astronomical Tides:

The astronomical tides are driven primarily by the gravitational effects of the Moon on the ocean. When the Moon is directly overhead, the water rises in response to the Moon’s gravitational pull. When the Moon is directly underfoot, the water rises again to balance the pull of the Moon on the opposite side of the Earth.

During “Spring” tides, the gravitational pull of the Sun on the oceans acts in the same direction as that from the Moon. This results in higher than normal tides and tidal ranges. Conversely, during “Neap” tides, the gravitational pull of the Sun is acting perpendicular to that of the Moon and lower than normal tides and tidal ranges can be expected. Looking at the blue line in the above figure, higher tides associated with the Spring tide can be seen around the 20th September and again last weekend, while lower tides associated with Neap tide can be seen around the 28th September.

Additionally, the Sun and Moon have to be aligned in space for their gravitational pull to act in the same direction. This manifests as a New Moon when the Moon is between the Sun and the Earth, and a Full Moon when the Earth is in between the Sun and the Moon. Both New and Full moon are associated with Spring tides. The Lunar cycle (including one Full and one New Moon) repeats roughly every 29 days and so you can expect a Spring tide a little more than every fortnight.

[More on Spring and Neap Tides]

Finally, the Moon follows an elliptical orbit around the Earth and so is closer or further away twice per orbit. Every ~7.5 Spring tides, the moon reaches its closest distance to Earth during a New or Full Moon. When the Moon is closer to Earth (perigee), the tides are slightly higher than normal. The opposite is true for when the Moon is furthest from Earth (apogee). Tides during last weekend’s Spring tide were higher than the 20th September’s Spring tide because the Moon was near/at perigee last weekend, and not during the 20th September.

The role of Ocean Eddies:

2017280atsha
Sea surface height anomalies showing a positive anomaly greater than 30 cm near Bermuda (circled in black). This was associated with an anticyclonic eddy that intensified as it tracked southwestward toward Bermuda over the last two months.

Ever present in the ocean, eddies can manifest as regions of higher (positive) or lower (negative) sea surface height anomalies. The flow around these sea surface height anomalies is often close to balanced and so they can persist for a long time as they track across the ocean surface. These anomalies are typically small, less than 30 cm.

Typical flow around a positive sea surface height anomaly is clockwise (anticyclonic), and counter-clockwise (cyclonic) for a negative sea surface height anomaly in the northern hemisphere.

Over the weekend, a positive sea surface height anomaly associated with an anticyclonic eddy was tracking near Bermuda with amplitude estimated to be more than 30 cm (1 ft) via satellite measurements. Coinciding with the spring tide and Lunar perigee, this resulted in abnormally high water levels and some coastal inundation.

See some media mention of the tides here: Royal Gazette; Bernews (1); Bernews (2)

With sea level rise associated with climate change, it is reasonable to expect this mostly nuisance level of inundating events to occur more frequently as water level anomalies don’t have to be as extreme for flooding to occur.

Hurricane Nicole Post Storm Report Released

nicole-terra-cpa
NASA Terra/MODIS True Color satellite imagery of Hurricane Nicole near the time of closest point of approach 13 Oct 2016.

After each season, the National Hurricane Center prepares ‘post-storm reports’ on each tropical cyclone that formed in the North Atlantic and Eastern Pacific basins. These reports include data collected in real-time that may not have been available during operational analysis, and therefore can sometimes lead to revision of track or intensity. This was the case for 2014’s Hurricane Fay.

nicole-txkf-2-min-gusts-kts
Time trace of wind gusts (red ‘x’) and wind directions (green ‘triangles’) from the airport reproduced from the NHC report. We see the sharp decrease in wind gusts and sharp wind shift as the calm eye of Nicole crossed.

Today, the National Hurricane Center released its report on 2016’s Hurricane Nicole. Their analysis on observations from Bermuda suggest that widespread category one conditions occurred on the island with isolated areas seeing category two conditions. Aircraft reconnaissance measurements near the time of closest point of approach to Bermuda indicate that Nicole was still a category three hurricane with maximum sustained winds near 105 kts (120 mph). The island received impacts from the left-front quadrant of Nicole’s eyewall, missing these strongest winds (located in the right-front quadrant) and thus this was classified as a strike.

[strike, direct-hit, indirect-hitlandfall – Impact Terminology Defined]

Hurricanes are centers of extreme low pressure. Winds spiral inwards, towards centers of low pressure and in the northern hemisphere, this manifests as a counter-clockwise circulation.  Observations from Bermuda indicated that winds backed (turned counter-clockwise with time) from an easterly direction to a northerly, and then northwesterly direction suggesting that the center of circulation remained to east of the island and therefore did not make landfall, despite Bermuda entering the calm eye of the hurricane.

nicole-bws-radar
Bermuda SRI Radar imagery showing steady and heavy rain rates persisting over Bermuda (red ‘+’) for hours before the drier eye and southwestern quadrant passed over the island. Reproduced from the NHC report.

Nicole’s impacts on Bermuda were also remarkable in that the hurricane made for one of the top-5 wettest meteorological days on record at the airport, and the Bermuda Weather Service was able to release a weather balloon in the eye that measured the highest precipitable water here since 1973 at 2.93″.

[What is Precipitable Water?]

Notable for an erratic early track and meteorological evolution (including two periods of rapid intensification), Hurricane Nicole will go down in the record books as the fourth early-October hurricane impact on Bermuda in three years. Once Nicole passed Bermuda, the cyclone underwent a complex transition into a powerful extratropical cyclone in the North Atlantic where it continued to produce storm force winds for several days.

Current Weather:

GRAPH_The%20Crescent_2017-02-16-1208-L.png
Wind measurements at the Crescent, in the northern marine area via Bermuda Weather Service. Note peak in winds just before 9am 16 Feb 2017(sustained, to/gusts, middle).

Bermuda is now well into its ‘winter’ season where changeable weather associated with gales is commonplace. Yesterday’s cold front brought a brief warm and humid spell with southwesterly gales and thunderstorms that brought heavy rain to the island.

Locally severe wind gusts, mainly confined to the marine area, were also observed. The Crescent Buoy, in the northern marine area, measured a peak thunderstorm gust of 54 kts. Winds on island appear to have remained below severe levels (i.e. < 50 kts) with a peak gust of 43 kts measured at the airport prior to any thunderstorm activity.

Follow the Bermuda Weather Service for the latest official forecast, warnings, and observations for Bermuda.

Torrential January Rains

20170105-rain
Observations around Bermuda indicate a widespread 4-6″ of rain fell with isolated areas seeing >6″. Reports are sourced from Wunderground, WeatherLink, and Bermuda Weather Service.**Note that shaded areas on this map with fewer reports are less reliable.**

A deep long-wave upper level trough over the Eastern United States slowly edged eastwards on Thursday morning, this allowed a cold front to slowly advance towards and across Bermuda. Deep-layered flow out of the tropics allowed significant moisture transport across Bermuda. The frontal system, supported by upper level dynamics, was able to make use of that moisture in the form of an active band of heavy showers and thunderstorms that slowly progressed across the island, with a trailing region of light-moderate rains.

Rain totals for the meteorological day (0600 UTC to 0600 UTC) at the Bermuda Weather Service far exceeded several records, with 5.34″ of rain. This led to widespread flooding of low-lying and poor-drainage areas (see: Royal Gazette, Bernews). Furthermore, this rain total is roughly the amount of rain that Bermuda typically gets for the entire month of January. The 1981-2010 average January rainfall is 5.43″, or 5.30″ for 1971-2000 climate period.

Table: Records broken with yesterday’s rain, with reference to single, meteorological day records for the period 1949-present at Bermuda Weather Service.
Type of Record Previous Record
Record Wettest for the date 5 Jan 1.54″ (5 Jan 1994)
Record Wettest Jan Day 3.99″ (11 Jan 1986)
Record Wettest Winter (Dec-Jan-Feb) Day 3.99″ (11 Jan 1986)
5th Wettest Day (1.) 7.77″ (1 Jun 1996)
(2.) 6.77″ (13 Oct 2016 – Hurricane Nicole)
(3.) 6.21″ (31 Aug 1982)
(4.) 5.52″ (14 Jul 1980)
(5.) 5.34″ (5 Jan 2017)
(6.) 5.24″ (29 Oct 1967)

It is particularly impressive that this event made it into the top-5 wettest meteorological days because it occurred in winter. Rain events in winter typically produce lower totals for two main reasons, compared to summer events:

  1. these are typically associated with frontal systems that generally pass quickly
  2. there’s typically not as much atmospheric moisture available in winter

This event was associated with a frontal system, but it was progressing very slowly, and there was unusually high amount of atmospheric moisture available for rain because of the deep-layered flow from the tropics ahead of the system.

Model guidance performed well at picking up on the potential for a heavy rain event on the 5th since the end of December. This was reflected in the Bermuda Weather Service forecasts and forecaster’s discussion several days before the event.

More heavy rain is in the forecast

Yesterday’s front has progressed to the East of Bermuda, clearing its rainy weather with it. This is allowing much drier weather to settle in. However, an area of low pressure and frontal system, organizing over the lower Mississippi Valley today, will result in a similar frontal set-up to yesterday’s system over Bermuda at the end of the weekend.

Over the weekend, the low will push off the US East coast and track northeastwards around a long-wave trough over Eastern North America. As it does this, deep-layered southerly flow out of the tropics resumes ahead of a trailing cold front. This front will slowly progress eastwards toward Bermuda, passing on Sunday.

Model guidance is once again suggesting potential for a heavy rain event associated with this system. This is mentioned in the Bermuda Weather Service forecast and forecaster’s discussion. With soil freshly saturated, additional flooding in low-lying and poor-drainage areas is possible on Sunday.

A significant cool-down is then expected to start the work week. Temperatures are likely to struggle to reach 60°F on Monday as a continental polar airmass is drawn off of North America from the northwest across Bermuda.

See the Bermuda Weather Service for the latest official forecasts, warnings, and observations for Bermuda.

Tudor Hill Observatory

tudor_hill_landscape
Tudor Hill Marine Atmospheric Observatory (Credit: BIOS)

In a project commissioned by the National Oceanic and Atmospheric Administration’s (NOAA) Earth Systems Research Laboratory (ESRL) the Bermuda Institute of Ocean Sciences (BIOS) has been the cooperating agency responsible for measuring the concentration of key atmospheric atmospheric gas species at Tudor Hill in Bermuda. This is part of NOAA/ESRL’s Global Monitoring Division’s aim to track changes in these key gas species, particularly focusing on their sources, sinks, global trends, and distributions.

[About: Global Monitoring Division]

One of the more well known gases measured at these types of observatories, including Tudor Hill, is Carbon Dioxide. An infamous greenhouse gas emitted largely through the burning of organic matter (ie. fossil fuels), the increase in Carbon Dioxide can be seen even in the middle of the Atlantic at Tudor Hill, Bermuda.

Another interesting feature is the seasonal change in Carbon Dioxide concentration controlled mainly by the biosphere. Because there is more land in the northern hemisphere, the biosphere’s influence is disproportionally weighted to what is happening in the northern hemisphere. During northern hemisphere summer, there are more photosynthetically active plants taking in more Carbon Dioxide globally than during northern hemisphere winter.

TudorHillCO2_10Nov2016.png
Measurements of concentration of Carbon Dioxide gas at Tudor Hill Bermuda since records began in 1989, including preliminary 2016 data in orange. Plotted with NOAA/ESRL’s interactive data viewer.

Attributing the trend in Carbon Dioxide to human activity is a little more complicated than just observing the trend. It comes through analysis of Carbon isotopes bonded in Carbon Dioxide. Carbon comes in two naturally occurring stable isotopes: Carbon-13 (13C) and Carbon-12 (12C). 13C has an additional neutron and therefore has slightly more mass and slightly different chemical properties.

It has been found that the biosphere (ie. plants) preferentially uptake the lighter 12C containing molecules during photosynthesis. This leaves behind less 12C in the atmosphere and so the ratio 13C/12C increases. The higher proportion of 12C in organisms is maintained even if they should become fossil fuels. When we burn organic matter (ie. fossil fuels) we release Carbon Dioxide with higher proportions of 12C into the atmosphere and the ratio 13C/12C measured in the air decreases as a result.

One way to measure the relative amount of the two stable Carbon isotopes is called “delta 13 Carbon” (δ13C). Here, the measured ratio 13C/12C is standardized by a reference ratio determined from reference research into the average properties of Carbon. That standardization is very close to one, and all the variations occur in the thousandths decimal. In practice, one is subtracted from it and then the result is multiplied by 1000.

A trend of decreasing δ13C is observed at Tudor Hill, Bermuda. This trend lends support to the idea that not only are atmospheric Carbon Dioxide concentrations increasing, but the increase might be due to human activity.

tudorhill_dc_10nov2016
All available delta thirteen C data for Tudor Hill, Bermuda. Records of this parameter are kept since 1991, including preliminary 2016 data points. Plotted with NOAA/ESRL’s interactive data viewer.

These trends and patterns are repeated across the world. It is clear that the upswing in Carbon Dioxide is not a local phenomenon, but a symptom of a global problem.

Many more projects, providing invaluable scientific insight rely on data like this collected at Tudor Hill in Bermuda and around the world. Bermuda has proved to offer a unique location to get continuous long time-series of the largely undisturbed samples of the low-level ambient marine atmosphere. As such, I expect research to continue or expand in Bermuda, particularly as issues such as increasing Carbon Dioxide concentrations become more pressing.

References:
Carbon Isotopes in Photosynthesis (Marion H. O’Leary, 1988)
NOAA/ESRL’s Global Monitoring Division
->Map of Observation Sites
->Tudor Hill’s Measurements via Interactive Data Viewer