Gravity wave events and polar stratospheric clouds over the Antarctic Peninsula from spaceborne lidar observations

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Sheila French
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1 Gravity wave events and polar stratospheric clouds over the Antarctic Peninsula from spaceborne lidar observations V. Noel, A. Hertzog, H. Chepfer Laboratoire de Météorologie Dynamique, IPSL/CNRS 1

2 1. Polar Stratospheric Clouds [PSCs] form during polar winter nighttime (May - Sep) play an active part in the formation of the seasonal ozone hole sun-activated reactions on PSC particles transform passive species (e.g. HCl) into active chlorine and bromine that cause ozone loss Sedimentation of PSC particle scavenge nitric acid, slowing down the reconversion of active chlorine into passive species 30km 10km PSC as seen by CALIOP: altitude, composition 2 CALIPSO orbit track

3 PSC particles H2O NAT "type Ia" NATrock Ice "type II" STS "type Ib" H2SO4 HNO3 Particle nucleation is driven by mixing ratios, temperature, pressure In reality, PSCs are often mixtures of all particle types This talk will focus on Ice and NAT PSCs 3

PSC formation and GW - ice For 5 ppmv water vapour, the ice frost point T ice

standards Ice PSCs are relatively rare and occur primarily near the Peninsula

Example of GW-caused PSC- June 27 2006 GW Ice PSC observed above the

4 PSC formation and GW - ice For 5 ppmv water vapour, the ice frost point T ice goes from ~ 189K (60 hpa) to 183K (20 hpa) pretty cold even by stratospheric standards Ice PSCs are relatively rare and occur primarily near the Peninsula or the Transantarctic mountains - GW are a primary influence T < Tice Ice PSC Example of GW-caused PSC- June GW Ice PSC observed above the Peninsula by CALIOP Peninsula 173K 183K 193K 203K 213K 223K Modelled WRF temperatures 4

5 PSC formation and GW - NAT T NAT is ~6K warmer than T ice : 189K to 195K - very frequent Homogeneous NAT nucleation is inefficient (2-4 days), NAT PSC formation is slow 5

PSC formation and GW - NAT T NAT is ~6K warmer than T ice : 189K to 195K - very frequent Homogeneous NAT nucleation is inefficient (2-4 days), NAT PSC formation is slow NAT crystals can nucleate

6 PSC formation and GW - NAT T NAT is ~6K warmer than T ice : 189K to 195K - very frequent Homogeneous NAT nucleation is inefficient (2-4 days), NAT PSC formation is slow NAT crystals can nucleate heterogeneously on crystals from Ice PSCs How important are GW for the formation of the PSC population? June Upstream: clear-sky Downstream : NAT PSC Ice PSC NAT PSC "Mountain-wave seeding" upstream downstream 6

7 Methodology Model WRF PSC CAL Observations Volume T<T ice T<T NAT Volume PSC ice/nat Compare, taking into account GW events 7

2. Model domain 2000 km 100 cells WRF model (3.2.1) 120 vertical levels, p_top = 5hPa 38 levels in 120-10 hpa range (57 10 6 km 3 ) Initialized with ERA-Interim reanalyses 0.

8 2. Model domain 2000 km 100 cells WRF model (3.2.1) 120 vertical levels, p_top = 5hPa 38 levels in hpa range ( km 3 ) Initialized with ERA-Interim reanalyses 0.75 resolution stratospheric wind and temperature JJA hours timestep 8

9 Example of GW event - July 19th 2006 T<T ice ΔWmax GW identified by the frequency of profiles affected by strong vertical winds, i.e. > 1m/s 9

1% 1.2% of the domain (10-120 hpa) affected by GW

10 GW activity 2006 Frequency of GW-affected profiles, 2006 June: 2.1% July: 1.8% August: 0.1% 1.2% of the domain ( hpa) affected by GW in High GW activity in June, July Severe drop in August

events (<1day) do not seem to affect temperatures PSC case study GW event criteria: existence of winds > 1 m/s for

11 wind speed and temperatures wind speed m/s GW w > 1 m/s GW GW 29 days / 78 (37%) affected by GW GW events are linked to colder temperatures, but no clear relationship between intensity of winds and cold temperatures reached Short GW events (<1day) do not seem to affect temperatures PSC case study GW event criteria: existence of winds > 1 m/s for at least 24 h temperature [K] To evaluate V Tice and V TNAT, fluctuations of T ice and T NAT need to be accounted for 11

12 Ice and NAT frost points ~5 ppmv Daily H 2 O [ppmv] ~4 ppmv derived from MLS profiles of H 2 O and HNO 3 mixing ratios above the Peninsula WRF: T(t,x,y,P) MLS: T ice (t,p) and T NAT (t,p) V Tice (t,x,y,p) V TNAT (t,x,y,p) 12

3. Comparison with CALIOP observations 2006 For each CALIPSO orbit crossing the domain Identify closest WRF run (3h) Extract vertical profiles of WRF temperatures

profiles on CALIOP vertical grid Quantify volume of T<T ice, T<T NAT 13 Detect PSCs based on backscatter thresholding J J A S Identify PSC types following

13 3. Comparison with CALIOP observations 2006 For each CALIPSO orbit crossing the domain Identify closest WRF run (3h) Extract vertical profiles of WRF temperatures in stratosphere ( hpa) along the CALIPSO orbit track WRF required to take into account "imperfect" satellite sampling ~4% of the domain per day Reproject profiles on CALIOP vertical grid Quantify volume of T<T ice, T<T NAT 13 Detect PSCs based on backscatter thresholding J J A S Identify PSC types following classification scheme from Pitts et al Quantify volume of ice and NAT PSC Orbit-level comparison of V Tice, V PSCi and V TNAT, V PSCn aggregated over 1-day periods CAL

14 V Tice vs. V PSCi 2006 sampled PSC volume T<Tice volume CAL WRF GW Ice PSC 50 days (72%) in 2006 all occur on days with T<T ice, on 74% of those days closely follow fluctuations in T ice (except end August) GW 29 days (42%), all sampled 25 show ice PSCs (85%) no obvious correlation with increase in T<T ice or PSC volume (sampling?) 14

15 V TNAT vs. V PSCn 2006 GW sampled PSC volume T<TNAT volume CAL WRF NAT PSC missing CALIPSO data on 100% of sampled days in 2006, all days T<T NAT follows fluctuations in T ice, although the volume is relatively much smaller (due to nucleation inefficiency) relationship with GW even less clear 15

16 Ice PSCs and GW 170 days GW (38%) days T<T ice (85%) 260 days Ice PSC (62%) sampled days Occurrence of T<T ice 85% of days, GW or no GW Occurrence of Ice PSC 86% of days T < T ice GW 65% of days T < T ice no GW volume with T < T ice GW days: less small volumes of T<T ice smaller average volume of T<T ice higher volume of Ice PSC +70% on average More efficient PSC formation, thanks to colder temperatures? 16 volume of ice PSC

17 NAT PSCs and GW According to the mountain-wave hypothesis, the formation of NAT PSC should be enhanced during gravity wave events (heterogeneous vs. homogeneous nucleation) V PSCn / V TNAT should increase during GW Not really the case on average V PSCn / V TNAT 17

V PSCn / V TNAT mapped Mapping the spatial evolution of V PSCn /V TNAT reveals new details V PSCn / V TNAT, GW V PSCn / V TNAT, no GW 0.20 0.15 0.10 0.

18 V PSCn / V TNAT mapped Mapping the spatial evolution of V PSCn /V TNAT reveals new details V PSCn / V TNAT, GW V PSCn / V TNAT, no GW V PSCn / V TNAT GW events are linked to a downstream increase in nucleation efficiency (East of the Peninsula), coupled to a decrease upstream (West) The ~33% increase can be attributed to heterogeneous nucleation on ice PSCs The cause of the ~33% decrease is unclear 18

19 Summary Fluctuations in observed volumes of ice and NAT PSCs follow modelled volumes of T<T ice and T<T NAT Differences in nucleation efficiency Volume Ice PSC = 0.6 * volume T < T ice Volume NAT PSC = 0.15 * volume T < T NAT Over the Peninsula, T < T ice are not more frequent during GW events (85% of the time in any case) If GW, more ice PSCs are observed, PSC volume higher by ~70% on average More ice crystals for heterogeneous nucleation of NAT during GW Mountain-wave seeding effect evidenced by ~33% downstream increase in NAT nucleation efficiency compensated by a unexplained symmetric upstream decrease 19

20 20

21 backup slides 21

22 Outlook Use actual PSC identification from Pitts et al. 20x20 km might not be enough to simulate the intensity and extent of GW Extrapolate total volume of NAT PSC created from GW events to the total domain, from sampled results 22

23 GW activity and temperatures WRF vol. GW vol. with T < T ice vol. with T < T NAT All years 1.2% 4.1% 41.6% 0.6% 4.2% 40.2% 0.8% 5.9% 38.4% 0.4% 5.1% 35.1% 1.1% 6.2% 38.6% 0.8% 5.3% 38.7% J J A J J A J J A 0.7% 1.2% 0.5% 2.2% 5.1% 8.5% 32 % 39 % 43 % The intensity of GW activity does not seem to affect significantly the volume T < T ice or T < T NAT 23

24 CALIOP detection 24

25 notes - why not using the WRF mixing ratios? 25

Simulation of Polar Ozone Depletion: An Update

Simulation of Polar Ozone Depletion: An Update Image taken from www.zmescience.com D. Kinnison (NCAR), S. Solomon (MIT), and J. Bandoro (MIT) February 17, 2015 WACCM Working Group Meeting, Boulder Co.