Thermal Shockwaves in an Upland River

Thermal shockwaves in an upland river

1 however, mean annual T varies between Conversely, Langan et al. (2001) assert Robert L. Wilby , w 1 4 and 27°C in polar and equatorial regions, that the thermal impact of storm events Matthew F. Johnson respectively (Punzet et al., 2012). is seasonally dependent and can yield Tw and Julia A. Toone2 We deployed a high resolution ther- increases when heat is advected from ripar- mistor array to investigate rapid T vari- ian wetlands. Subehi et al. (2010) show the 1 Department of Geography, w ations linked to weather conditions. We importance of hillslope gradient and water Loughborough University favour these instruments because of table depth in determining the hydrological 2 Environment Agency, Trentside Office, their low cost and accuracy, which ena- pathway (i.e., relative mix of surface/sub- Nottingham ble detailed sampling of Tw in space and surface runoff) and hence Tw response to

Weather – March 2015, Vol. 70, No. 3 70, No. Vol. – March 2015, Weather time. Alternatives such as airborne remote rainfall in small forested catchments. Brown Introduction sensing can yield high quality information and Hannah (2007) found that short-dura-

River temperature (Tw) is a fundamental on spatial patterns of surface Tw but only tion low-intensity storms falling onto an determinant of water quality, chemical for snapshots in time and where the river alpine catchment can occasionally produce

rates, biological processes, and organism is not obscured (Torgersen et al., 2001). Tw increases of up to 2.3 degC. behaviour (Webb et al., 2008). As such, there Conversely, site-specific monitoring of From the above synopsis it is evident that

is concern that rising Tw and changing pat- heat budget components provides detail meteorological controls of river tempera-

terns of river flow under anthropogenic on temporal variations in Tw but is less feature responses are mediated by antecedent climate change could adversely impact sible for large numbers of locations (Webb river flow and catchment conditions (such freshwaters (Caissie, 2006; Orr et al., 2014). et al., 2008). Fibre optic methods offer high as soil temperature and moisture content), Hence, there are calls for active manage- spatial and temporal resolution but only type of land cover, topography, contribu- ment of surface/groundwater flows and over distances of up to several kilometres tions from groundwater, attributes of the riparian vegetation to safeguard cool refu- (Krause et al., 2013). generating event, and so forth. The purpose gia and vulnerable species (Hansen et al., Here, we focus on acute as opposed to of this study is to use a high-resolution ther-

2003). At the same time it is recognised that chronic Tw changes because of their poten- mistor array to: 1) establish a typology of

such interventions should be founded on tial for sudden onset and significant eco- rapid, sub-daily Tw changes, and 2) present

process-understanding obtained through system impacts (McHenry and Long, 1995; empirical evidence of the magnitude of Tw field observation and modelling (Wilby Beaulieu et al., 2013; Bruno et al., 2013). change, rate of downstream propagation

et al., 2010). Rapid Tw increases (thermopeaks) have and dispersion of these short-lived phe- Field studies of the energy fluxes control- been reported under a range of artificial nomena. The ultimate aim of the research

ling Tw show that approximately 70% of heat conditions including thermal discharges is to improve understanding of the drivers input originates from solar radiation, with from power stations (Langford and Aston, of thermal ‘shockwaves’ for temperate river the remainder provided mainly by friction 1972); warm water releases in winter ecosystems and to raise awareness of their of water with the river bed and banks, and (Webb, 1995; Dickson et al., 2012) and cool potential significance. sensible heat transfer from the atmosphere water in late spring/summer (Zolezzi et al.,

(Webb et al., 2008). Air temperature (Ta) is 2011) from hydropower reservoirs; as well widely used as a surrogate for solar radia- as warm runoff from summer storms over Study area, data and methods

tion in empirical models for predicting Tw paved surfaces (Herb et al., 2008; Sabouri The study catchment and field experiments variations over timescales of days to months et al., 2013). are described by Wilby et al. (2012), Johnson

(Johnson et al., 2014). The most significant Natural causes of rapid Tw reductions have et al. (2014) and online at http://www. heat outputs are radiation, evaporation, also been documented. For example, Smith luten.org.uk. The Loughborough University

bed conduction and sensible heat transfer and Lavis (1975) describe decreases in Tw Temperature Network (LUTEN) comprises 36

across the water-air interface. Local factors associated with individual rainfall and snow- sites with synchronised Tw and Ta measure- such as landscape shading, river orientation melt events. Maybeck (2004) also recounts ments in the rivers Dove and Manifold of and vegetation cover affect receipts of solar how a summer thunderstorm released suf- the English Peak District (Figure 1). Pasture radiation; groundwater fed streams may ficient cool water into the Pemigewasset is the main land use in both catchments,

have increased longwave, latent and sen- River, New Hampshire for Tw to fall by 7 degC and elevations range from 150 to 450m. sible heat losses (in winter); channel mor- within 6h. Similarly, Gammons et al. (2005) This study employs a subset of the data

phology, river flow and artificial structures note a modest Tw reduction associated with collected in the Dove catchment (sites D1 influence heat advection from upstream. a hail storm in Butte, Montana, USA, and to D23) which is fed largely by groundwa-

Global mean annual Tw is approximately Lange and Haensler (2012) observed signifi- ter seepage plus flows from non-thermal

14°C, but there are large regional variations. cant drops in Tw when post-drought storm and semi-thermal springs along the mar-

Maximum Tw can reach 35°C in equatorial, runoff was dominated by event water in the gin of the limestone outcrop (Edmunds, 92 arid and warm temperate climate zones; Black Forest, Germany. 1971). These sources discharge at relatively constant temperatures year-round, thereby

lowering ambient Tw in summer but increas- Thermal shockwaves in an upland river ing Tw in winter (Johnson et al., 2014). Parts of the headwaters and lowest reaches of the Dove are in deep landscape shade, whereas middle sections are most exposed to direct solar radiation. Two Gemini Tinytag Aquatic 2 thermistors recorded maximum, mean and minimum temperatures every 15min at each site:

one on the river bed measuring Tw, and the other at about 2m above the water

surface measuring Ta. Tinytag sensors have a quoted accuracy of 0.2 degC, which we checked under field and laboratory condi- Weather – March 2015, Vol. 70, No. 3 tions (Johnson and Wilby, 2013). To avoid heating by direct solar radiation sensors were fixed in deep shade. Data reported here are for 3 years, from March 2011 to February 2014. 15min river flow data were acquired from two gauges kept by the Environment Agency (EA). The Hollinsclough (HC) gauge is in the upper Dove and has a drainage area of 8km2; the Izaak Walton (IW) gauge is downstream and represents a catchment of 83km2 (Figure 1). According to the National River Flow Archive, both river flow records are defined as natural to within 10% at the 95th percentile (although for HC river flow, estimates >0.8m3s−1 are based on rating tables and should be treated with caution). Note also that there are more than 100 shal- low weirs and occasional sub-surface field drains in the lower reaches of the Dove (between D17 and D24). Daily precipitation totals were acquired from UK Met Office stations at Buxton (53°15′N, 1°55′W) and Ashbourne (53°1′N, 1°44′W). These lie ~10km to the north of the source and south of Dovedale (D24) respectively. Hourly precipitation data were obtained from stations at Coton-in-the-Elms Figure 1. LUTEN monitoring sites (red circles), EA gauging stations (black circles), limestone outcrop (52°44′N, 1°38′W), Keele (53°0′N, 2°16′W), (grey shading), sandstone and mudstones areas (unshaded). D10 is the reference site used to Leek (53°8′N, 1°59′W) and Hollinsclough detect rapid changes in Tw across the network as a whole. (Source: Wilby et al. (2014).) (53°12′N, 1°54′W) within the headwaters of the Dove. We objectively identified thermopeaks

using the rate of change (ROC) of Tw over 1h at site D10 (Figure 1). We did not dis- criminate between rising or falling limbs but only took note of the most extreme value (whether positive or negative) each day. We then ranked all days and extracted the 12 largest absolute ROC values along with antecedent precipitation total, air temperature (at D10) and concurrent river flow in each case. Site D10 was chosen because of its central location within the catchment (above the limestone gorge) and because the record is 97% complete. Moreover, the

Tw series at D10 is most highly correlated (average r = 0.98) with all other sites in the Dove, so is regarded to be representative. Finally, we estimated thermal and flood (or hydrodynamic) wave velocities with the Figure 2. Example ROC series for site D10 during March 2012 to February 2013. expectation that they are asynchronous. This 93 is because the thermal peak behaves as a passive tracer and is advected downstream b at the velocity of water molecules depending on river flow depth and channel rough- +1.3(D6) ness. This advective heat transport is further

influenced by local heat exchange with the b 13 7 May 2013 stream bed, hyporheic exchange, or storage within in-stream dead-zones (Evans et al.,

1998; Neilson et al., 2010; Westhoff et al., +1.2 (D6) 2010; 2011). Meanwhile, the flood wave travels more quickly than water molecules, b with the velocity depending on free surface Thermal shockwaves in an upland river Thermal shockwaves width, volume, and depth of flow relative to

pre-event conditions (Toffolon et al., 2010). +1.2 (D6)

If the thermal wave was triggered by a b storm, event initiation was assumed to be

the time of peak rainfall intensity meas- 0 −0.002 −0.003 −0.001 ured at Hollinsclough (the closest gauge). 0.079 0.076 0.072 0.069 Downstream variations in thermal wave (maximum rate of change of water temperature per hour); temperature of change water (maximum rate w

velocity were then determined using dis- T a tance between sites and time of arrival of a

peak Tw. Velocities for hydrodynamic waves were estimated using the known distance +2.63

Weather – March 2015, Vol. 70, No. 3 70, No. Vol. – March 2015, Weather between HC and IW (27.9km) and the dif- ference in time of arrival of the hydrograph a peak at each gauge (to the nearest 15min). +5.09 Results Our 3 year monitoring period included a several notable hydrological events. For example, spring 2011 had the most severe and widespread April snow storm since range (diel air temperature range); Max Δ range (diel air temperature 1981. Violent storms in late June and early a T July 2012 brought flash flooding across numerous locations in England and Wales (Almond, 2013; Clark and Webb, 2013). These episodes also contributed to the wettest April to July in nearly 250 years and brought an abrupt end to one of the most significant prolonged droughts in a century (Kendon et al., 2013; Parry et al., 2013). The first week of May 2013 was notable for the amount of bright sunshine, which reached 14h on some days. Winter 2013/2014 was confirmed as the wettest since 1766 in the England and Wales precipitation series, with more than 130% average rainfall locally in the Peak District (Met Office, 2014). Figure 2 shows the ROC trace for site D10 in 2012/2013. This series is strongly correlated (r = 0.9) with ROC data upstream and downstream (sites D6 and D16 respectively). ROC values seldom exceed ±1 degC h−1, so

events with rapid Tw change clearly stand out (as in the case of 4 April 2012, 28 June 2012, 5 July 2012 and 27 January 2013). Across the

3 years of data, the 12 largest ROC scores (pre-event pre-event (change between ΔQ discharge); and peak discharge); 0 all exceeded ±0.8 degC h−1 at D10 (Table 1). Overall, the most extreme ROC (15min) for any site was +3 degC, recorded at site D1 D23 +0.8 +0.8 −0.5 +0.8 +2.0 +1.2 +0.9 −0.8 +0.7 +0.7 +0.7 +0.7 between 2030 and 2045 UTC on 5 July 2012. ) D10) (various) +1.0 +1.9 (D20) +2.0 (D20) −1.1 (D6) +1.0 +1.0 (D6) +3.2 (D11) +2.4 (D10) −1.0 +1.6 (D1) −1.3 (D10) +0.9 +1.5 (D6) +2.8 +2.4 +1.2 −1.3 +1.1 +1.1 +1.1 +1.2

Three types of event were distinguished −1 −1 by inspection of the full data set (Table 1). )) HC IW −0.001 −0.007 −0.001 +0.028 0 0 +0.421 +0.205 0 +11.35 +1.980 +7.12 +3.23 +9.44 −0.010 −0.020 −0.030 −0.020 (°Ch (°Ch

)) HC IW 0.071 0.767 0.070 0.752 0.093 0.979 0.178 2.040 0.278 2.780 0.351 3.080 0.330 3.280 0.958 2.260 1.150 1.140 1.080 1.060 −1 −1

Overall, the most rapid positive T changes −1 −1 s s

w Q total); (precipitation 3 3 s s 3 3 were preceded by intense rainfall (IR); the tot (mm) (mm) (mm) Buxton HC Ashbourne 0 0 0 0 0 0 8.4 11.4 18.8 0 0 0 4.7 16.8 17.2 11.7 18.2 30.6 17.3 5.4 13.6 0 0.6 6.6 0 0 0 0 0 0 0 0 0 0 0 0 (m (m

two most extreme negative ROCs were 0 0 tot tot tot D6 was the most upstream site after flood damage in summer/winter 2012/2013. after flood damage in summer/winter site D6 was the most upstream Values outside the range used to calibrate the gauge. the gauge. calibrate outside the range used to Values P P Q P rainfall). IR (intense or snowfall); of change); SH (solar heating); SM (snowmelt ROC (rate Table 1 Table 2014. 2011 and February at site D10 between March changes in Tw Properties with most rapid of 12 events Variable Sites 1 May 2011 4 May 2011 4 Apr 2012 6 May 2012 28 Jun 2012 5/6 Jul 2012 10 Jul 2012 27 Jan 2013 1 May 2013 2 May 2013 6 May 20 Key: P a b ROC rankMax ΔTw D10 10 9 11 12 1 2 4 3 7 6 8 5 Event typeEvent All SH SH SM SH IR IR IR SM SH SH SH SH Q ΔQ (m ΔQ (m 94 linked to heavy snowfall and/or melt (SM). range (°C)Ta Max ΔTw D10 11.3 18.3 2.4 17.6 11.1 14.0 4.5 5.3 17.5 19.7 22.2 20.2 Thermal shockwaves in an upland river Weather – March 2015, Vol. 70, No. 3

Figure 3. Example thermal waves produced by (a) solar heating (4 May 2011), (b) snow fall (4 April 2012), (c) intense rainfall (28/29 June 2012) and

(d) intense rainfall (5/6 July 2012). A 12 degC range is used for Tw , and 20 degC range for Ta . Accompanying hydrographs and air temperatures are also shown. The river flow gauges are Hollinsclough (HC) and Izaak Walton (IW).

The remaining events occurred on days solar heating (SH) under clear sky conditions. Figure 3 shows examples of the down- with zero precipitation, stable or falling The most extreme ROC (per hour) at D10 stream propagation of thermal waves river flow, and large diel Ta ranges (typically for each category of event was +2.8 degC generated by these three mechanisms.

>17 degC). These events were all preceded for IR, −1.3 degC for SM, and +1.2 degC for Under intense SH events, Tw increases by nocturnal cooling and then by intense SH (Table 1). most rapidly between mid-morning and 95 mid-afternoon at upstream sites. The ther- (a) mal wave is modified by heat exchanges between atmosphere and river as it advects downstream, typically arriving at sites above the spring line (D16) between 1700 and 2000 UTC (Figure 3(a)). The magnitude of

the diel Tw range depends on the volume of flow (i.e., thermal inertia of the water body), strength of solar shortwave radiation, net longwave radiation, convection, and evaporation heat fluxes (Evans et al., 1998). The

damped response of Tw between D16 and Thermal shockwaves in an upland river Thermal shockwaves D23 shows that groundwater inputs reduce the diel range even when there is strong radiant forcing upstream. Under SM events (such as 4 April 2012) relatively cold water enters the drainage network either directly as precipitation or (b) indirectly as melt-water from ablating snow cover. The thermal sag drifts downstream

such that absolute Tw minima experienced at downstream sites occur hours after detec- tion in the headwaters (Figure 3(b)). Again,

Weather – March 2015, Vol. 70, No. 3 70, No. Vol. – March 2015, Weather the markedly different Tw behaviour at D23 compared with upstream reflects the input of relatively warm groundwater. The thermal constancy of spring sources is evidenced by the similarity of the signal for D23 under SH and SM forcing (Figure 3(a) and (b)). The most dramatic thermal response occurs after IR events (Figure 3(c) and (d)). The exact shape of the thermopeak depends on the attributes of the generating storm (duration, maximum intensity, and direction of travel relative to the main (c) channel orientation). The thermal wave on 28 June 2012 (Figure 3(c)) was initiated by an intense storm with maximum precipitation rate >13mmh−1 at Hollinsclough (Figure 4(a)). The hydrograph response at the upstream gauge was subdued compared with downstream, reflecting the SW to NE passage of the frontal system and zone of maximum rainfall intensity, which was located to the south. The peak rainfall intensity on 5/6 July 2012 was lower (~9mmh−1) than on 28 June 2012 (Figure 4(a) and (b)), but a greater total fell onto ground that was already saturated (as indicated by the higher initial river flows in Table 1). Storm onset was marked by a

rapid drop in Ta and near immediate rise in river flow at the HC gauge (Figure 3(d)). Figure 4. Hourly rainfall records associated with IR events on (a) 28 June 2012, (b) 5/6 July 2012, Coincident thermal peaks at D1 and D16 and (c) 10 July 2012. See Table 1 for full details. may indicate earlier arrival of a local rain- storm at the downstream site. An alternative wave velocities, as indicated by shorter travel wave between HC and IW varied between explanation is that runoff from paved sur- times to fixed sites (Figure 5). Non-linear 270 and 435min (equivalent to 1.1–1.7ms−1) faces in the hamlet of Mill Dale added warm time-distance relationships show that the compared with 619–951min (or 0.5–0.8ms−1) water to the river between D16 and D20. thermal wave is decelerating. The most for the thermal wave. The spring site (D23) exhibited a damped rapid hydrodynamic wave followed high The travel time analysis indicates that fol- but earlier than expected thermal response intensity rainfall over the whole catchment lowing IR events stream biota are exposed to both IR events. This was attributed to a (28 June 2012), whereas the most rapid first to rising river flow then second to a sub-surface field drain that discharges from thermal wave trailed lower intensity rainfall thermal shock. The separation time between the west-facing escarpment into the river but relatively high totals in the headwaters the two waves increases with distance from between sites D22 and D23. (5/6 July 2012). Hence, depending on the source. Under the most extreme case (28 Consistent with theory, hydrodynamic spatial-temporal dynamics of the storm, the June 2012) the estimated time interval 96 wave velocities were higher than thermal estimated travel time of the hydrodynamic between arrival of the hydrodynamic and (a) three most extreme IR events all occurred during a two week period of remarkable transition from drought to flooding (Parry Thermal shockwaves in an upland river et al., 2013). However, additional thermal waves emerge when the ROC threshold is relaxed to a threshold of ±0.5 degC h−1. For example, following >60mm rainfall at Buxton on 27 July 2013, a ROC of +0.6 degC h−1 was recorded. Another event on 18 July 2011 was triggered by >65mm rainfall at Leek which led to an abrupt increase in river flow and was marked by the arrival of warmer water from upstream of D23. This diluted the signature of cooler spring water Weather – March 2015, Vol. 70, No. 3 (b) and caused Tw to change +1.6 degC between 0400 and 0415 UTC at D23 (Figure 6). Primary and secondary IR events in the Dove are generally associated with daily rainfall totals >20mm at Buxton. Analysis of records since year 1961 suggests that rainfall totals of this magnitude or more occur between zero to four times per summer (June to August). However, variations in the features of individual thermal waves are explained by several other factors operating in the Dove. First, abrupt heating or cooling is superim-

posed on diel and annual Tw regimes, which are governed by seasonal variations in solar (c) heating and river flow (Webb and Walling, 1985). Second, thermal wave propagation and longitudinal dispersion are affected by the extent of pre-heating and ponding behind the weirs between D16 and D23. Third, absence of tributaries limits scope for mixing of thermally contrasting water in the upper Dove, but this is not the case where field drains and springs feed into the lower reaches. Evidence of distinct thermopeaks in the main stem of the dendritic network of the river Manifold (not shown) implies that the influence of surface runoff from tributaries may not be significant in these Figure 5. Times to thermopeak (red dots) and hydropeak (blue dots) following intense rainfall (IR) small catchments. events on (a) 28 June 2012, (b) 5/6 July 2012, and (c) 10 July 2012. Note that there are less data We venture that IR events may be gener- for July 2012 because sites D15 and D17 were washed out by high flows in the previous month. ating thermal waves in other natural headwaters in the UK. For the Dove, the thermal thermal waves at IW was >11h. Furthermore, by direct snowfall and/or melt-water runoff; wave amplitude was greatest ~10km from at sites above the spring zone, the duration or heating by runoff produced by intense source, dissipating thereafter even before of the event (as indicated by the time for summer storms. Heating by solar radia- encountering cooler groundwater. Hence,

Tw to return to pre-event values) increases tion typically yields maximum Tw at down- we speculate that catchments with areas downstream due to turbulent diffusion and stream sites just after sunset (Figure 3(a)). <100km2 and flashy hydrological regimes shear flow dispersion. For example, the ther- However, a noteworthy feature of IR events might be particularly vulnerable (but scope mal wave on 28 June 2012 increased Tw for is that abrupt changes and maximum Tw for testing this hypothesis elsewhere is con- 645min at site D1 and 855min at D10. can occur during the night, at times when strained by data availability). Other factors

The following section considers the wider Tw would otherwise be dominated by net such as channel gradient, roughness, pool- implications of these findings, including the radiant losses (Evans et al., 1998). Moreover, riffle sequencing, connectivity to floodplain, potential significance of thermal waves for IR events generate the most rapid sub-daily and artificial structures influence the local freshwater taxa. Tw changes: up to +3 degC in 15min for velocity of the thermal wave and thereby the most extreme case (D1 on 5 July 2012). space-time variations in heat budget. These observations arise from a small Land cover and soil moisture conditions Discussion sample of events that were identified objec- determine the fraction of solar energy par- Based on data from LUTEN we have tively via ROC values. In general, IR events titioned into sensible heating of the surface detected three mechanisms that can gen- are expected to be most severe when Ta > prior to storm onset. For instance, streams erate rapid responses in river temperatures. Tw, there has been strong solar heating of draining urbanised catchments are known These are: heating by solar radiation under the land surface, the storm is intense, and to exhibit thermopeaking after strong solar clear skies and low flow conditions; cooling initial river flow is low (Herb et al., 2008). The heating and heavy rainfall (Herb et al., 2008; 97 (a) Thermal shockwaves in an upland river Thermal shockwaves

(b) Weather – March 2015, Vol. 70, No. 3 70, No. Vol. – March 2015, Weather

(c)

98 Figure 6. Properties of the IR event of 17-19 July 2011: (a) hourly rainfall, (b) Ta and river flow, (c) Tw at sites D1 to D23. Sabouri et al., 2013). Conversely, larger conditions other than strong solar radiation. Edmunds WM. 1971. Hydrogeochemistry catchments have greater scope for turbu- One implication of this finding is that of groundwaters in the Derbyshire Dome lent diffusion and shear flow dispersion increased riparian shade is not a complete with special reference to trace constitu- Thermal shockwaves in an upland river with distance downstream. There may also panacea for warming rivers because it ents. Report of the Institute of Geological be tributaries and groundwater units which would afford little protection against storm Sciences 71/7. British Geological Survey: Keyworth, UK. mix thermally contrasting water into the generated Tw changes. main channel flow. Further research is needed to determine Evans EC, McGregor G, Petts GE. 1998. River energy budgets with special ref- Abrupt changes in Tw are known to whether abrupt Tw increases are occurring in erence to river bed processes. Hydrol. trigger a range of sub-lethal biological other catchments and the extent to which Processes 12: 575–595. responses. For example, benthic inver- these phenomena are mediated by regional Fraser NHC, Metcalfe NB, Thorpe JE. tebrates exhibit species- and season- climate and land surface properties. It is also 1993. Temperature-dependent switch dependent avoidance strategies under unclear which taxa might be most sensitive between diurnal and nocturnal foraging carefully controlled laboratory conditions to thermal waves and the extent to which in Salmon. Proc. R. Soc. London, Ser. B 252: (Carolli et al., 2012). Moreover, because of any trends in timing, magnitude and fre- 135–139. synergistic effects, asynchronous hydro- quency of these events might be detect- Gammons CG, Shope CL, Duaime TE. Weather – March 2015, Vol. 70, No. 3 2005. A 24 h investigation of the hydropeaks and thermopeaks cause greater able as changes in ecosystem structure chemistry of baseflow and stormwater in density of invertebrate movement than and function. Some have begun to explore an urban area impacted by mining: Butte, hydropeaks or thermopeaks alone (Bruno such questions under laboratory conditions Montana. Hydrol. Processes 19: 2737–2753. et al., 2013). Due to increasing separation to mimic thermopeaking below dams, but Hansen LJ, Biringer JL, Hoffman JR. time between the two peaks with distance this work is largely restricted to invertebrate (eds) 2003. Buying Time: A User’s Manual downstream, a spatial signature in inver- behaviour. Whereas conventional (monthly, for Building Resistance and Resilience to tebrate response is expected. T is also daytime) spot-sampling of T may be barely Climate Change in Natural Systems. WWF w w Climate Change Program: Berlin. an important behavioural cue for the tim- sufficient for tracking environmental change Herb WR, Janke B, Mohseni O et al. ing of feeding in juvenile Atlantic salmon over decadal timescales, this is clearly insuf- 2008. Thermal pollution of streams by (Salmo salar), which occurs preferentially ficient for discerning biologically relevant runoff from paved surfaces. Hydrol. at night when Tw falls below 10°C (Fraser thermal cues at storm-event scales. Processes 22: 987–999. et al., 1993). Hence, arrival of a thermal Johnson MF, Wilby RL. 2013. Shield or wave during hours of darkness may be Acknowledgements not to shield: effects of solar radiation on particularly significant for nocturnal crea- water temperature sensor accuracy. Water 5: 1622–1637. tures (Wilby et al., 2014). The authors thank the landowners who Climate model projections show increased kindly granted access to the field sites, the Johnson MF, Wilby RL, Toone JA. 2014. Met Office for providing meteorological Inferring air-water temperature relation- likelihood of extreme rainfall linked to higher ships from river and catchment properties. atmospheric temperatures and vapour data, as well as the Wild Trout Trust and Hydrol. Processes 28: 2912–2928. content (Trenberth, 2011). On the other Trent Rivers Trust for supporting the pur- Kendon M, Marsh T, Parry S. 2013. The chase of equipment. Data from the LUTEN hand, a scenario of higher Ta implies that 2010-2012 drought in England and Wales. the frequency of snowfall in the UK could network may be obtained from: http://www. Weather 68: 88–95. diminish. Under these conditions, occur- luten.org.uk/ or by contacting the authors. Kendon EJ, Roberts NM, Fowler HJ et al. rence of IR events would be expected to 2014. Heavier summer downpours with climate change revealed by weather fore- increase, whereas SM events could become cast resolution model. Nat. Clim. Change 4: rarer. However, the future magnitude and References 570-576. doi:10.1038/NCLIMATE2258 frequency of thermal waves also depends Krause S, Taylor SL, Weatherill J on the antecedent conditions of the receiv- Almond C. 2013. Heavy rain and flooding et al. 2013. Fibre-optic distributed ing water course. Climate model outlooks in southwest England on 6–7 July 2012. temperature sensing for characterizing Weather 68: 171–175. for the UK are ambiguous for winter, but the impacts of vegetation coverage Beaulieu JJ, Arango CP, Balz DA et al. on thermal patterns in woodlands. there is greater consensus that river flows 2013. Continuous monitoring reveals mul- Ecohydrology 6: 754–764. could decrease in summer (Prudhomme tiple controls on ecosystem metabolism Langan SJ, Johnston L, Donaghy et al., 2012). A hotter, drier summer climate, in a suburban stream. Freshw. Biol. 58: MJ et al. 2001. Variation in river water with lower flow volumes, would predispose 918–937. temperatures in an upland stream over rivers to more frequent SH and IR events. Brown LE, Hannah DM. 2007. Alpine a 30- year period. Sci. Total Environ. 265: One high resolution (1.5km) climate model stream temperature response to storm 195–207. events. J. Hydrometeorol. 8: 952–967. experiment suggests that the frequency Lange J, Haensler A. 2012. Runoff gen- of very heavy rainfall episodes in summer Bruno MC, Siviglia A, Carolli M et al. eration following a prolonged dry period. 2013. Multiple drift responses of benthic J. Hydrol. 464: 157–164. could nearly quadruple by 2100 (Kendon invertebrates to interacting hydropeaking Langford TE, Aston RJ. 1972. The ecology et al., 2014). and thermopeaking waves. Ecohydrology of some British rivers in relation to warm 6: 511–522. water discharges from power stations. Caissie D. 2006. The thermal regime of riv- Proc. R. Soc. London, Ser. B 180: 407–419. 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Non-classic extratropical cyclones on Met Office sea-level pressure charts: double cold and warm fronts

Katy C. Mulqueen1 front, developing single cold and warm however. In the 1980s, field programmes 2 fronts, then occluding (Figure 1(a)). First targeting rapidly developing marine extra- and David M. Schultz described by Bjerknes (1919) and Bjerknes tropical cyclones and mesoscale numerical 1School of Earth, Atmospheric and and Solberg (1922), the Norwegian cyclone model simulations showed a slightly differ- Environmental Sciences, University of model provided a context for scientists ent evolution (Shapiro and Keyser, 1990). Manchester and forecasters to describe the evolution Instead of a narrowing warm sector dur- 2Centre for Atmospheric Science, of cyclones and led to the modernisa- ing occlusion, as in the Norwegian cyclone School of Earth, Atmospheric and tion of meteorology in the first half of the model, the Shapiro–Keyser cyclone model Environmental Sciences, University of twentieth century (e.g. Namias, 1980; 1983; exhibited a warm front nearly perpendic- Newton and Rodebush Newton, 1994). The ular to the cold front in a frontal T-bone, Manchester model has withstood the test of time, with a bent-back front and an eventual warm relatively few modifications. One modifica- seclusion (Figure 1(b)). Schultz et al. (1998) tion is that of the occlusion process as the and Schultz and Zhang (2007) showed that Introduction wrap-up of the air masses involved rather cyclones in large-scale diffluence tended The evolution of extratropical cyclones has than the catch-up of the warm front by the to form Norwegian cyclones and cyclones traditionally been described in the context cold front (Schultz and Vaughan, 2011). in large-scale confluence tended to form of the Norwegian cyclone model, where a Not all cyclones undergo an evolution Shapiro–Keyser cyclones: the lower friction 100 developing cyclone grows on a stationary similar to the Norwegian cyclone model, over water also facilitates the development