Why rainfall may reduce when the ocean warms

Transcription

1 Why rainfall may reduce when the ocean warms Tim Hewson*, Ian Boutle** and Benedetta Dini** *ECMWF, Reading, UK **Met Office, Exeter, UK Work mainly undertaken at the Met Office Slide 1

2 Contents Introduction and hypothesis Components to test the hypothesis: 1. Ensemble runs 2. Idealised runs 3. Observations Summary and implications

3 Introduction and hypothesis

4 January mean SST and 10m winds from ERA Interim

5 July mean SST and 10m winds from ERA Interim

6 Motivation Airmasses that arrive in NW Europe have usually spent a long time over the ocean prevailing winds are west to southwesterly So, to what extent are characteristics of those airmasses ie weather - determined by the ocean temperature? Air temperature in the eastern north Atlantic is closely correlated with sea surface temperature (SST) it is well documented that the average temperature difference here between Sea and Air is small, and that this varies little But what about SST influences on humidity, and the direct impact that may have on cloudiness and rainfall (and the additional indirect impact, then, on air temperature over land)?

7 Relative Humidity Some classical ideas cite Clausius-Clapeyron (specific humidity at saturation increases with T) as evidence that warmer oceans may lead to higher rainfall But Simmons et al (JGR 2010) found that rainfall in Europe correlates more closely with relative humidity (RH) than with specific humidity (q) (both measured at screen level) So if relative humidity decreases as the ocean warms we could see a reduction in rainfall, even if specific humidity increases

8 Hypothesis cloud and rain are favoured downwind of anomalously cold SSTs We propose that the gradient in SST (anomaly) along the incoming airmass trajectory can influence boundary layer relative humidity, and thereby cloud base, cloudiness and rainfall Illustrative profiles: Structure of middle and upper troposphere assumed to depend primarily on synoptic scale systems Atmospheric profile at start of NE ward bound trajectory towards UK (T & Td) 600mb Lower tropospheric temperature profiles at end of trajectory across ocean (Td unchanged) in: Scenario 1 ( Neutral SST anomalies ) Scenario 2 ( Cold east Atlantic ) Scenario 3 ( Warm east Atlantic ) 1000mb ~6 C

9 Possible tests of the hypothesis 1) Operational ensemble runs with different SSTs 2) Idealised numerical model simulations 3) Observational studies 4) Examine re-forecast climatology as a fn. of SST pattern 5) Single column model simulations not yet But we want to capture the myriad of possible atmospheric configurations for the incoming airmasses, as they pass over the ocean makes (5) difficult, and potentially risky (1) to (4) are much better in this regard

10 1. Ensemble runs with different SSTs

11 ECMWF forecasts for week 2 (day 12-18) ensemble means (50 members). DT 2 Jul 09. OPER SST PPN MSLP cloud and rain are favoured downwind of anomalously cold SSTs, and vice-versa ~0.5 C / 1000km reduction C mm mb 30% increase in total ppn RERUN WITH OBSERVED SST OBS OBS

12 2. Idealised numerical model simulations

13 Experiment Design Use a different model the Met Office Unified model - in an idealised channel, mid-latitude, over-ocean configuration, with periodic W-E boundary conditions (following Boutle, BL Met, 2010) for cyclone/front simulation ~40km resolution, full model physics SST = fn (latitude), prescribed, and evolving in a simple way, to simulate passage W to E of a typical N Atlantic cyclone and its fronts Initialisation: Upper level (westerly) jet core, zero surface wind, surface air temperature = SST, thermal wind balance RH=fn(z), tapers down from 80% at surface (based on re-analysis climatologies) Temperature perturbation applied at all levels helps trigger cyclone development ~3 days to spin up, analyse later times SST evolution is key

14 SST configuration Real SST example from Nov 2012 LAT 70N Simulation Later in simulation analagous to E - weaker gradients - warmer ~4 C 30N SST Start of simulation analagous to W - strong gradients

15 SST evolution Day 0 : Western set up, atmosphere in balance with SST Day 0 3 : Linear evolution of SST towards Eastern set up (atmosphere evolves above cyclone spins up due to baroclinic instability) Day 3+ : Eastern SST pattern persisted, atmosphere continues to evolve 3 cases run Neutral, Warm East Atlantic, Cold East Atlantic 0 3

16 SST (day 3+) and mslp (day 3) Neutral case

17 SST (day 3+) and mslp (day 3) Cold E Atlantic Strong gradient In SST along incoming airmass trajectory

18 SST (day 3+) and mslp (day 3) Warm E Atlantic Weak gradient In SST along incoming airmass trajectory

19 Temporal evolution MSLP and total ppn rate -Neutral SST case Average large scale ppn rate all runs Cold E Atlantic Neutral Warm E Atlantic mm/hr 4 7

20 Day 4 Warm E Atlantic MSLP and large scale ppn rate MSLP and average RH in lowest 1km

21 Day 4 Cold E Atlantic MSLP and large scale ppn rate MSLP and average RH in lowest 1km

22 Day 7 Warm E Atlantic MSLP and large scale ppn rate MSLP and average RH in lowest 1km

23 Day 7 Cold E Atlantic MSLP and large scale ppn rate MSLP and average RH in lowest 1km

24 46 N Cross-Sections (W-E) Day 4 Theta-w and RH Warm E Atlantic Theta-w and Ppn rate

25 46 N Cross-Sections (W-E) Day 4 Theta-w and RH Cold E Atlantic Theta-w and Ppn rate

26 46 N Cross-Sections (W-E) Day 7 Theta-w and RH Warm E Atlantic Theta-w and Ppn rate

27 46 N Cross-Sections (W-E) Day 7 Theta-w and RH Cold E Atlantic Theta-w and Ppn rate

28 Fluxes - profiles just on warm side of cold front - day 5, 47N SST Fluxes derived from turbulence in BL scheme Moistening in q Cooling propogates upwards Condensation on ocean surface

29 Resumé of simulations Initially surface fluxes are strong, due to developing advection related to cyclogenesis, and due to evolving SSTs; later on fluxes reduce generally as the atmosphere re-harmonizes with the ocean In C, due to a greater warm sector sensible heat flux into the ocean, the lower troposphere is cooler than in W, but this also means that RH in C stays higher in the warm sector, with more low cloud and lower cloud bases in C as a result In W there is more advection of dry near surface air (RH) into the frontal zone, by the frictionally-turned low level wind, which would elevate the level of latent heat release in forced frontal ascent Due to high RH in C ppn intensity tends to increase downwards, via accretion; in W it remains relatively constant or reduces downwards via evaporation Greater sensible heat fluxes upward into the cold post-front air in W, because of higher SSTs, help to reduce the frontal thermal gradient markedly compared to C, and so inhibit any tendency for frontogenetically-forced ascent in W Ppn intensity initially is similar in C and W, but later on, as a result of all of the above, ppn activity dies more rapidly in the W case Pressure rises occur as the fronts tend to weaken in both cases W is the Warm East Atlantic case C is the Cold East Atlantic case

30 Key Points In the zone well south of the cyclone track (which is the zone that typically would affect much of NW Europe): Throughout the simulation lower tropospheric RH (and so low cloud) is greater when the ocean is colder Earlier in the simulation, when the cyclone is still developing, rainfall rates are similar in both SST scenarios Later in the simulation, when the parent cyclone has matured, and MSLP is rising, in the warm ocean case the trailing cold front dies more rapidly and precipitation rates are lower So can we see any of the above in real observations?...

31 3. Observational study

32 Results briefly summarised: Used 38 years of radiosonde, rainfall and SST data (SST = Hadley Centre monthly means) Strong support for the findings from the numerical model study Boundary layer RH distribution depends strongly on SST upstream Small SST anomalies (<1C) are important Precipitation also relates to SST upstream 26% more wet days (>10mm) for cold E Atlantic scenario, despite pressure not being any lower Signals are stronger for categories of higher MSLP (analogous to later in the model runs) Statistical tests were applied to the distributions (Kolmogorov-Smirnov): distributions were significantly different (at > 99.95% confidence level) for the higher pressure categories Average RH, lowest km, Valentia (higher pressure category) sondes DATA B warmer than A B colder than A in SST anomaly terms

33 SST anomaly difference vs MSLP

34 Example histograms COLD E Valentia RH lowest 1km (all four mslp categories) COLD E NEUTRAL WARM E moister WARM E Midlands Rainfall (two highest mslp categories) wetter

35 Example Warm E Atlantic

36 Example Cold E Atlantic

37 Summary

38 Summary hypothesis accepted! Results from studies of observational data, of ensemble runs with changed boundary conditions, and of idealised cyclone simulations all suggest the following influence of SST anomalies on weather in NW Europe, when winds blow from their prevailing west to southwesterly direction : More (low) cloud Lower cloud bases More wet days Less sunshine Colder days Less (low) cloud Higher cloud bases Fewer wet days More sunshine Warmer days

39 Mechanisms and discussion The primary mechanism seems to be the extra cooling of the boundary layer in the C case which simultaneously increases relative humidity Also, from a dynamical standpoint, collapse of trailing cold fronts appears to be promoted more in the W case Focus has mainly been on large scale rainfall, not convective. One might expect convective to be greater in the W case, and to be relevant for coastal regions with onshore flow (as in Persson et al, MWR, 2005, California, & Lenderink et al, Clim Dyn, 2009, Netherlands). However, simulations suggested that convective rainfall was not always greater in the W case, and in the observational study it was implicitly included, yet a signal still emerged. Geography such as coastal orientation, is probably very important. There may also be positive (and negative?) feedbacks between atmosphere and SST (and soil moisture, etc) not considered in this talk This work also highlights the importance of correct SST representation for long range predictions, and so may, in due course, help unlock some long range predictability in rainfall Sub-surface ocean temperatures also play a key role, as they modulate SST response to atmospheric forcing. These need also to be measured and correctly initialised in models Two-way atmosphere-ocean coupling from day 0 may also be beneficial for shorter range weather forecasts small SST changes (<1C) make a difference. Coming very soon to the ECMWF ENS (but not yet to HRES) W is the Warm East Atlantic case C is the Cold East Atlantic case

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41 Specific Humidity Some classical ideas cite Clausius-Clapeyron (specific humidity at saturation increases with T) as evidence that warmer oceans may lead to higher rainfall RELATED EXAMPLE from Lenderink et al (Clim Dyn, 2009): -Aug 2006 SST (left) SST -2C -Cyclonic, convective NW ly regime -Ppn over Netherlands -Short range forecast simulations, with different SSTs SST anomaly -24h totals summed (T+12 to T+36)

42 Temporal evolution MSLP and total ppn rate -Neutral SST case Closer to low track -analogous to S Norway Average large scale ppn rate all runs Cold E Atlantic Neutral Warm E Atlantic mm/hr

43 Evolving SST in the ECMWF Ensemble Stippling shows where SST differences compared to uncoupled runs are greater than 1C (day 10)..

44 Vertical Ocean temperature profiles in IFS spring / early summer example ~5C Ocean surface Reality Anomalous heat storage =ocean model level Coupling is better but SST change may be too slow in certain situations ~10m Current EPS Response to cooling from above temperature

45 Some Tips? Watch out for differences between Control and calval=3 uncoupled control Precipitation / RH / cloud cover / cloud base may all vary at times, even for the same synoptic pattern Biggest influence of coupling likely in spring when large vertical sea temperature gradients can develop Watch out also for SST reduction being too slow in spring time after a warm spell (due to 10m upper level)