Background:

A common feature of the spring and early summer along the east coast of England and Scotland is the dreaded North Sea haar (or sea fret, as it is also sometimes called). Often plaguing the eastern side of the country through this time of year, it can persist for days, bringing stubbornly cool, dull, overcast and misty conditions with fog at the coast and over any modest elevation inland. At most risk are those localities exposed to an onshore east to north-easterly flow and it can appear suddenly and unexpectedly, though its development and persistence is dependent on a number of critical factors, these will be explored on this blog for a case study that occurred in April 2018.

The synoptic chart (see Figure 1; right) for 06 Z on 12 April 2018 shows a well-established anticyclone across NE Europe and W Russia extending towards the UK, with a moistening E to NE flow being pulled towards East Anglia.

A cold sea:

During the late winter and early spring, a succession of cold waves swept westwards across northern Europe bringing severe cold to the UK. A legacy of this sustained onslaught of icy easterly winds was a colder than normal North Sea (see Figure 2, below), with much of the sea area more than 1 degree Celsius below normal. In this area of the sea north of around 52 deg N, SSTs were analysed between PS 05 and PS 06 deg C. This colder than normal sea provided the first ingredient required to bring about a persistent haar.

Synoptic situation:

High pressure had become centred over Scandinavia over the previous few days with a warm plume of air drawn north and then westwards in a developing east to north-easterly flow (see Figure 1, above). With time, pressure fell across France, Belgium and the Netherlands allowing the flow to back north-easterly over the UK and southern North Sea, this then allowed the relatively warm and moist flow with dew points of between +7 and +9 degC to be pulled south-westward over the anomalously cold sea.

This initially warm and relatively moist air then quickly cooled, the lowest layers of which soon became saturated with fog starting to develop as it was cooled below its dew point. Over time this cool and saturated air gradually deepens and matures through a combination of turbulent mixing and sustained cooling from below, to eventually develop a relatively deep saturated “foggy” layer which can become colder than the SST and lead to development of an 'unstable fog' with the development of an SALR (Saturated Adiabatic Lapse Rate) profile through depth to the fog top.

Weak convection initiated from the relative warmth over the sea then aids the further deepening of this "foggy" layer and along with turbulent mixing of warm and dry air at the fog top from entrainment, balances any radiative loss from the fog top. This process is dependent on a number of factors, more critically the difference between sea surface temperature (SST) and the initial air mass temperature and dew point; once the dew point surpasses the SST then the process starts to accelerate. It is then possible that these processes reach an equilibrium, which allowing for no atmospheric changes, could then persist indefinitely.

The factors were sufficient in this case to readily develop extensive sea fog, a process replicated well (often too efficiently) by the Met Office high resolution output from the UKV. The length of sea track would also play a critical role and may explain (for example), as well as the slightly higher SSTs in the area, why little or no fog was recorded further southwest than the Dover Strait. This heavily modified airmass then presented its own forecasting challenges across the east of the UK, as it arrived at the coast transformed from its original relatively warm and less saturated starting point over continental Europe.

As can be seen from the actual and forecast tephigram ascents (see Figure 3, left) there was a well-marked modification to the warm continental airmass over the UK downwind of the North Sea. On the Watnall 11 Z ascent on the right there is a saturated profile up to around 2000 FT AGL (approx. 2300 FT AMSL). This is replicated on the 09 Z ascent for Larkhill (in blue on the right) and the AMDAR (in black on the right) reported near to Stansted airport at 0944 Z. It is notable that all ascents are moister, colder and deeper than the GM T+3 ascents for 09 Z near to Stansted airport, and this may go some way to explain discrepancies that developed between the model output and reality, as will be discussed in detail later.

Model performance:

Initially the extent and height of low cloud forecast by the UKV (see Figure 4, right) verifies well at 09 Z with the actual observations. Across East Anglia, widely the cloud base is at 200 FT or below with hill and coastal fog, and the slightly higher cloud base across London is picked out well. However, perhaps because of the issues noted above, by 12 Z (see Figure 5, below, left) the verification goes quite badly awry over southeast England in particular with a forecast improvement in cloud base not realised. Despite performing relatively well across Lincolnshire and around the Wash, the performance is poor over East Anglia with the cloud base observed typically between 100 and 300 FT still, but forecast by the UKV across Essex to be >700 FT. The biggest errors here area near to Stansted airport.

Later, at 15 Z (see Figure 6, below, right) the performance is even worse, and whilst there have been some improvements (note: 300 FT at Wattisham) the general picture is little different to 6 hours earlier and yet the UKV is forecasting widely GRN cloud bases (700–1499 FT) inland and even some areas of >1500 FT or WHT near to Bury St. Edmunds and across London, well in excess of 1000 FT in error. As discussed earlier the GM did not have the boundary layer modelled as cold, or saturated through as much depth of the lowest 2500 FT as was measured from AMDARs and sondes taken that morning. It is therefore likely that greater insolation was forecast to break through and gradually lift the cloud through the day, this itself allowing further insolation and a faster clearance, and so on. The errors were even more notable the day before (11 April) with (see Figure 7, bottom right) even larger discrepancies between the model output and reality during the day. Note that here the cloud was forecast to break entirely south of London and into southern Essex, when in reality the cloud did lift but remained BKN or OVC at 700–1000 FT across all but the far south coast. Forecast temperatures were also, as you would expect, significantly in error (see Figure 8, bottom left). Large parts of East Anglia were forecast to reach up to 14 deg C with an absolute maxima of 15.4 deg C near to Santon Downham (BestData suggesting 13.9 degC at Wattisham) – however as you can see (the actual maxes are listed over the top) generally the highest temperatures were barely above 10 deg C with Charsfield the highest at 11.1 deg C.

The forecast:

Figure 9, bottom, shows the visible satellite imagery taken from 1615 Z on 11 April 2018 and at 0645 Z on 12 April 2018. It shows the extent of sea fog and low stratus in the southern part of the North Sea (somewhat complicated by altocumulus above), and how variable and unpredictable its extent can be. Overall, the high resolution output from the UKV did a good job on predicting and capturing its extent, though the GM (as would be expected with its lower resolution) was less good, and both were unable to represent its persistence inland across East Anglia and southeast England through both 11 April and 12 April.

Figure 10 shows the same view as the right of the previous figure, but a few hours later at 0900 Z on 12 April 2018 with surface METAR and SYNOP observations overlaid. The low cloud and sea fog is extensive, however note the small gap that lies just north of the coast of Belgium and the Netherlands that was rapidly filling in, showing how inhomogeneous the air mass can be, and therefore making errors in the forecast that much more likely. The biggest uncertainty in these types of events is the extent of low cloud and stratus upwind, and once this is known it makes forecasting its evolution that much easier – as such this is much more of a nowcasting event and makes TAF writing beyond 18 hours very difficult to do!

This event was complicated by a number of different factors which would have slowed any inland improvements, namely:

• Areas of Ac and Ac cast developed over the top of the stratus, limiting somewhat insolation during the day.
• A strengthening north-easterly flow, enough to continually replenish the saturated low-level air inland continually through the day.
• A legacy of saturated ground from an extensive anomalously wet spell at the end of March and start of April, diverting what insolation there was into evaporating the lying water and surface moisture before any sensible heating could take place.

The forecaster at Wattisham noted the extensive low cloud and sea fog upwind and adapted their forecast for the day (see Figure 11, below left) to allow only a small improvement in the cloud base and visibility with a clearance not expected.

This was a difficult forecast to issue and went against the more optimistic forecast from the model (UKV, E4 and GM) which expected the cloud base to improve to GRN (>700 FT) during the day and would have provided pilots at Wattisham airfield the opportunity to carry out some limited flying in the local area. In reality, the observations (see Figure 12, below right) show that the low stratus and hill fog persisted through until the early afternoon, and even then only briefly improved to a cloud base of 300 FT AGL, with a visibility of 2–3 KM, therefore preventing all operations at the airfield through the day for a second consecutive day.

Conclusion:

This case study shows how persistent and stubborn haar and sea fog can be in eastern coastal districts. It also underlines the importance of nowcasting in these situations, and emphasises how important careful study of the observations is, along with frequent verification against the model. The UKV can do a good job at representing and developing sea fog when conditions are favourable, though the depth to which the fog matures was not in this case represented well over East Anglia and southeast England. Often significant modification away from the model is required to give the correct story to pilots and it is worth being prepared to make significant changes even if it can appear to be quite pessimistic at first!