The 25–27 May 2005 Mount Logan Storm. Part I: Observations and Synoptic Overview

Home , Mount Logan, Mount Logan (Quebec)

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G. W. K. MOORE Department of Physics, University of Toronto, Toronto, Ontario, Canada

GERALD HOLDSWORTH Arctic Institute of North America, University of Calgary, Calgary, Alberta, Canada

(Manuscript received 24 April 2006, in final form 28 September 2006)

ABSTRACT

In late May 2005, three climbers were immobilized at 5400 m on Mount Logan, Canada’s highest mountain, by the high-impact weather associated with an extratropical cyclone over the Gulf of Alaska. Rescue operations were hindered by the high winds, cold temperatures, and heavy snowfall associated with the storm. Ultimately, the climbers were rescued after the weather cleared. Just prior to the storm, two automated weather stations had been deployed on the mountain as part of a research program aimed at interpreting the climate signal contained in summit ice cores. These data provide a unique and hitherto unobtainable record of the high-elevation meteorological conditions associated with an intense extratropical cyclone. In this paper, data from these weather stations along with surface and sounding data from the nearby town of Yakutat, Alaska, satellite imagery, and the NCEP reanalysis are used to characterize the synoptic-scale conditions associated with this storm. Particular emphasis is placed on the water vapor transport associated with this storm. The authors show that during this event, subtropical moisture was transported northward toward the Mount Logan region. The magnitude of this transport into the Gulf of Alaska was exceeded only 1% of the time during the months of May and June over the period 1948–2005. As a result, the magnitude of the precipitable water field in the Gulf of Alaska region attained values usually found in the Tropics. An atmospheric moisture budget analysis indicates that most of the moisture advected into the Mount Logan region was preexisting water vapor already in the subtropical atmosphere and was not water vapor evapo- rated from the surface during the evolution of the storm. Implications of this moisture source for understanding of the water isotopic climate signal in the Mount Logan ice cores will be discussed.

1. Introduction Western Cordillera leaving only upper-level moisture available for transport into northwestern North Mount Logan, Canada’s highest mountain, is located America (Asuma et al. 1998; Lackmann et al. 1998; in the heavily glaciated Saint Elias Mountains of the Smirnov and Moore 1999). The extreme height of Yukon Territory just to the east of the Gulf of Alaska Mount Logan [5959 m above sea level (ASL)] is such (Figs. 1 and 2). The mountain is situated in a region of that it intercepts much of this upper-level moisture significant climatological importance. For example, it is (Moore et al. 2003). located at the end of the major North Pacific storm An ice core containing a 300-yr record of annual track (Blackmon 1976; Hoskins and Hodges 2002) snow accumulation extracted from the summit region along the main atmospheric pathway by which water of Mount Logan contains a number of climate signals vapor enters the Gulf of Alaska (Newell and Zhu 1994; associated with El Niño–Southern Oscillation, the Pa- Lackmann et al. 1998; Smirnov and Moore 2001). Much cific decadal oscillation, the Pacific–North America of this moisture rains out on the upwind side of the teleconnection, as well as the Walker and Hadley circulations (Moore et al. 2001, 2002a,b, 2003, 2004). As discussed in Holdsworth et al. (1992) and Moore et al. Corresponding author address: Dr. G. W. K. Moore, Depart- ment of Physics, University of Toronto, Toronto, ON M6G 2B6, (2003), many of these signals are not present in the Canada. precipitation record at Yakutat, Alaska (Fig. 1), located E-mail: [email protected] at sea level only 125 km from Mount Logan. It has been

DOI: 10.1175/JHM586.1

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FIG. 2. Mount Logan from the south-southwest. Mount Logan FIG. 1. Topography of the Mount Logan region (m) from the 30 rises over 4000 m above the Seward Glacier that can be seen in the arc-second resolution Global Topographic dataset (GTOPO30). foreground. proposed that this discrepancy is the result of the fact by the storm on the Arctic Institute of North America that the surface and upper-level moisture transports (AINA) pass (5400 m ASL) was the focus of attention in the region are only weakly coupled (Moore et al. throughout North America (Bjarnason and Jardine 2001, 2003). The concept of two distinct moisture 2005; D’Oro 2005; Mickleburgh 2005). Observations by streams is consistent with surface snow pit samples Gerald Holdsworth, who had just established the AWS from various elevations on Mount Logan (Holdsworth at the King Col site at 4130 m ASL, indicate that the et al. 1991). These data show a discontinuity in the weather changed around noon (local time) on the 25 oxygen isotope ratio with elevation that suggests a May when snow started to fall and whiteout conditions different source region for the precipitation that falls commenced. The temperature was approximately at low elevations as compared to that at higher eleva- Ϫ12°C at 1700 local standard time. The wind speed tions. increased late on 25 May and remained high through- Quantitative information on the mechanism or out the night with peak wind speeds estimated to be on mechanisms through which these tropical climate sig- the order of 15 m sϪ1. Temperatures also increased to nals occur in the ice core is at present still unclear. approximately Ϫ5°C during this period. The winds re- Extratropical cyclones are known to be associated with mained high and gusty on 26 May and temperatures moisture transport in the region (Asuma et al. 1998; dropped to Ϫ15°C during the night of 26 May. At 5400 Lackmann et al. 1998; Smirnov and Moore 1999, 2001) m ASL, the winds on 26 May were also gusty, with and so it is reasonable to assume that the climate signals speeds estimated to be in excess of 30 m sϪ1 (L. Billy arise as a result of the modulation in the track, inten- 2005, personal communication). Conditions improved sity, and moisture source region of these systems by on 27 May allowing for the trapped climbers to be res- larger-scale atmospheric circulations, such as ENSO cued late in the day. During the storm, approximately (Shapiro et al. 2001). To improve our understanding of 48 cm of low-density snow accumulated at the King Col these mechanisms, as well as details of the vertical site. Using a standard density of 0.1 g cmϪ3, this corre- structure of the moisture transport associated with Gulf sponds to approximately 48 mm of water. Similar of Alaska cyclones, a network of automatic weather amounts of low-density snow also fell at 5400 m ASL. stations (AWS) was established on Mount Logan in At Yakutat, 41 mm of rain fell on 26 May with an May of 2005. These new systems complement existing additional 12 mm on 27 May. AWS established in 2000–01 by a consortium coordi- In this paper, we will use data from two AWS along nated by the Geological Survey of Canada to provide with surface and sounding data from Yakutat, Alaska, high-frequency meteorological data (wind speed and satellite imagery, and the National Centers for Envi- direction, humidity, pressure, and temperature) at vari- ronmental Prediction (NCEP) reanalysis (Kalnay et al. ous elevations on the mountain. 1996; Kistler et al. 2001) to characterize the synoptic- During the installation of the network, an approach- scale conditions associated with this storm. Particular ing extratropical cyclone resulted in a period of particu- emphasis is placed on the water vapor transport asso- larly severe weather on Mount Logan on 25–26 May ciated with this storm and the source region for the 2005. The plight of three descending climbers trapped precipitation that fell near the summit.

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FIG. 3. MODIS true-color composite satellite imagery from approximately midday (local time) or approximately 2000 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. Mount Logan is indicated by the “ϩ.” The location of the surface low, as determined from the National Oceanic and Atmospheric Administration (NOAA) surface analysis, is indicated by the “L.”

2. Synoptic overview Figure 4 shows the evolution of the sea level pressure, 10-m wind, and surface latent heat flux through- The evolution of the storm is depicted in Fig. 3, which out the period of interest from the NCEP reanalysis. shows Moderate Resolution Imaging Spectroradiom- The convention used is that positive latent heat fluxes eter (MODIS) true-color composite satellite imagery are associated with evaporation from the surface from 23 to 26 May 2005. In particular, the images show (Moore and Renfrew 2002). At 1200 UTC 23 May, one the evolution of the system from a weak region of low can see the low pressure center and the concomitant pressure centered at 38°N, 152°W on 23 May (Fig. 3a) cyclonic circulation situated near 38°N, 150°W. The sys- to a mature extratropical cyclone over the Gulf of tem did not undergo an appreciable deepening as it Alaska centered at 54°N, 144°W on 26 May, with a moved northward. However, the presence of a region well-developed comma-shaped cloud field extending of high pressure to the east did result in large pressure southward from the low’s center (Fig. 3d). The south- gradients producing surface wind speeds on the order erly flow ahead of the cold front was associated with the of 15 m sϪ1. On 25 and 26 May (Figs. 4c,d), one can see warm conveyor belt (WCB) that transports heat and evidence of a deeper low pressure system approaching momentum poleward (Harrold 1973; Semple 2003). the Gulf of Alaska from the west. However, this system

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Ϫ Ϫ FIG. 4. Sea level pressure (contours, mb), 10-m winds (vectors, m s 1), and latent heat flux (W m 2, shading) from the NCEP reanalysis at 1200 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. became stalled just north of the Aleutian Islands. With ward the Gulf of Alaska. In the Mount Logan region, regard to the latent heat flux field, one can see that the temperatures on the 500-mb surface rose by approxi- largest fluxes occur along the southern flanks of the mately 10°C between 23 and 26 May as warmer air from regions of high pressure in the subtropics. Presumably the south was advected into the region. in these regions, the advection of colder and drier air The 6.7-mm brightness temperature from the Geo- from the high latitudes allows for a large transfer of stationary Operational Environmental Satellite (GOES) moisture from the ocean’s surface. At higher latitudes, imager provides information on the water vapor in the the latent heat flux was significantly smaller and on upper troposphere between 500 and 200 mb (Soden and occasion was negative indicating that condensation was Bretherton 1993). In addition, given the large gradient occurring as warm and humid subtropical air came into in water vapor across the tropopause, this channel is contact with colder sea surface temperatures. also sensitive to the presence of tropopause folds The upper-level flow associated with this storm is (Wimmers et al. 2003; Wimmers and Moody 2004). Fig- shown in Fig. 5. On 23 May, there was a weak upper- ure 6 shows this channel from the GOES West satellite level short-wave trough present with an axis along throughout the period of interest. The moisture stream 165°W (Fig. 5a). Over the next 3 days, the trough associated with the developing system on 23 May can moved eastward amplified, extending into the subtrop- be seen in Fig. 6a, as well as the approaching upper- ics (Figs. 5b–d). During this time, its orientation along level trough. On subsequent days, the moisture’s move- with the positioning of the surface low was favorable ment is apparent as the system tracks northward and for the long-range transport of subtropical moisture to- matures (Figs. 6b,c). Finally on 26 May (Fig. 6d), one

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Ϫ FIG. 5. Geopotential height (contours, km), horizontal winds (vectors, m s 1), and temperature (shading, K) on the 500-mb pressure surface from the NCEP reanalysis at 1200 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.” has a well-developed intrusion of upper-tropospheric went large vertical displacements. For example, the air water vapor extending from the subtropics to the Gulf parcel that ended up at 5000 m on Mount Logan orig- of Alaska, as well as an adjacent region of low humidity inated in the subtropical North Pacific at 2000 m near that is indicative of the presence of the dry intrusion or 30°N, 140°W. The 3000-m trajectory had a similar path tropopause fold. to that at 5000 m, while the air parcel that ended up at We conclude this section with Fig. 7, which shows 2000 m underwent a descent from approximately 2500 back trajectories for parcels that were situated in the m and had a source region over the extratropical North Mount Logan region (60°N, 140°W) at 1200 UTC 26 Pacific near 45°N, 150°W. Also shown in Fig. 8 is the May. Trajectories are 96 h in length and are shown for relative humidity of the air parcels along the various parcels that ended up at 500, 2000, 3000, and 5000 m trajectories. There is a distinct difference in the evolu- ASL. The trajectories were computed using the Hybrid tion of the humidity along the 500- and 2000-m trajec- Single-Particle Lagrangian Integrated Trajectory tories as compared to that along the 3000- and 5000-m (HYSPLIT) model (Draxier and Hess 1998). The ones. In particular, the air parcels along the two lower- source region for the air parcel that ended up at 500 m level trajectories underwent a dramatic moistening over on Mount Logan is over the Gulf of Alaska. This parcel the last 12–24 h of their journey. This is quite different did not experience any significant vertical movement. than what happened along the two upper-level trajec- In contrast, the air parcels that ended up at 2000, 3000, tories, where there was a moistening from 24 May on- and 5000 m have more complex trajectories and under- ward.

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FIG. 6. GOES West water vapor image at 1200 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.”

3. In situ observations of the storm at this location are available as 3-hourly values, while sounding data are available daily at 0000 and 1200 During this event, two AWS on Mount Logan were UTC. Figure 1 gives information on the relative loca- operational and as a result they provide a unique tions of Mount Logan and Yakutat. The AWS are ap- dataset with which to document the meteorological proximately 120 km from Yakutat. conditions on the mountain associated with this The time series of the various meteorological fields weather system. One of the stations was situated on a from the three locations is shown in Fig. 9. With regard rocky outcrop close to the Ogilvie Glacier (60°37ЈN, to the pressure field, the anomaly time series (Fig. 9a) 140°47ЈW; elevation 2929 m). This system measured show that the amplitude of the pressure fall associated wind speed and direction, pressure, specific humidity, with the system was highest at sea level and diminished and temperature. The other system was situated at the with height. The timing of the maxima in pressure was King Col site (60°35ЈN, 140°36ЈW; elevation 4130 m). coincident at all three locations, while that of the This system measured pressure and specific humidity minima associated with the storm was lagged at higher and temperature. Both systems were supplied by elevations by approximately 12 h. The temperature Campbell Scientific Canada. Figure 8 shows photo- time series (Fig. 9b) indicates that the fluctuations are graphs of the locales for these two AWS. For this paper, typically in phase at the three sites with the amplitude we will use hourly averaged values of the various me- tending to decrease with height. All three stations ex- teorological fields. To provide a context for the AWS perienced a warming between 25 and 26 May that was measurements, we will also make use of observations associated with the approaching low pressure system. from the National Weather Service Office at Yakutat Temperatures continued to rise on Mount Logan until (59°39ЈN, 139°37ЈW, elevation 12 m). The surface data 27 May. This may have been associated with the second

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FIG. 8. Automatic weather station locations on Mount Logan: (a) Ogilvie Glacier (60°37ЈN, 140°47ЈW; elevation 2929 m); and (b) King Col (60°35ЈN, 140°36ЈW; elevation 4179 m). Looking upward from the Ogilvie Glacier site, one sees the King Trench with the King Col in the upper left-hand corner.

storm on 26 May (Fig. 9c). The magnitude of this change was largest at Yakutat. On 27 May, all sites experienced a pronounced reduction in the specific humidity. This was most likely associated with the arrival of the dry slot (Fig. 6d). In addition, the King Col site showed another dramatic moistening near 1800 UTC 27 May that did not occur at the Ogilvie Glacier site. Fi- nally, the wind speed time series (Fig. 9d) clearly shows that there is little correlation between the Ogilvie Gla- cier and Yakutat sites. In addition, wind speeds at the Ogilvie Glacier site are considerably higher than those at Yakutat. Indeed, maxima in wind speeds at Ogilvie FIG. 7. (a), (b) Back trajectories for air parcels at various el- Glacier near 1200 UTC 24, 1200 UTC 25, 6000 UTC 26, evations arriving on Mount Logan at 1200 UTC 26 May 2005. The and 1200 UTC 27 May are associated with minima in locations of each air parcel at 12-h intervals are indicated by “*.” Each trajectory is 96 h in length. The location of Mount Logan is wind speeds at Yakutat. indicated by the “ϩ.” (c) The relative humidity (%) of the air Yakutat sounding data from the height of the storm, parcels along the various trajectories. 1200 UTC 26 May, are shown in Fig. 10. Wind speeds at the 3000- to 5000-m levels were on the order of 20 to 25 low pressure system that was entering the region (Figs. msϪ1 and were considerably higher than the hourly 4 and 5). With regard to specific humidity, the most averaged values observed on Mount Logan. In contrast, significant feature at all three sites was the pronounced as described in the introduction, climbers on the moun- moistening that was associated with the arrival of the tain at these levels observed peak wind speeds to be on

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Ϫ FIG. 9. Meteorological measurements of (a) pressure (mb); (b) temperature (°C); (c) specific humidity (g kg 1), and (d) wind speed (m sϪ1) during the 25–27 May 2005 storm as observed from Yakutat (thick solid lines, 12 m ASL), Ogilvie Glacier (thick dashed lines, 2929 m ASL), and King Col (thin solid lines, 4130 m ASL). Data from Yakutat are 3-hourly values, while those from Ogilvie Glacier and King Col are hourly averages. The pressure time series is shown as an anomaly from the corresponding average values over the period 24–28 May 2005.

the order of 15 to 30 m sϪ1. This may be the result of the boundary of the column, diffusion, and the liquid and observed gusty nature of the wind that may represent solid phase transports of water, then conservation of the complex and highly turbulent nature of the inter- water vapor requires that action between the large-scale wind field and topogra- ѨW ͒ ͑ ϭ Ϫ ١ phy (Wood 2000). Also shown in Fig. 10 is the magni- ϩ Ѩ · Q E P, 2 tude of the water vapor transport, defined as t q ϭ qV, ͑1͒ where P is the precipitation rate, E is the evaporation rate (both at the surface), W is the precipitable water where q is the specific humidity and V is the horizontal ␳ 1 s wind. The profile shows the presence of two maxima W ϭ ͵ qdp, ͑3͒ in the transport, one near 1.5 km and a larger one near g 0 3.5 km. and Q is the vertically integrated water vapor transport

␳ 1 s 4. The hydrometeorology of the storm Q ϭ ͵ q dp . ͑4͒ g We conclude our description of the characteristics of 0

this storm with a discussion of the water vapor trans- In the above integrals, p is the pressure with ps being port associated with it. We will emphasize the synoptic- the surface pressure and g is the acceleration due to scale characteristics of this transport as depicted in the gravity. NCEP reanalysis. Our starting point will be the conser- Problems have been identified in the NCEP reanaly- vation of water vapor in an atmospheric column sis with all terms in the water vapor conservation Eq. (Peixoto and Oort 1992; Smirnov and Moore 2001). If (2). For example, the reanalysis does not assimilate sat- one ignores the flux of water vapor across the upper ellite-based retrievals of water vapor (Trenberth and

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cific humidity) and short-term forecast fields (precipitation and evaporation) in the NCEP reanalysis. Notwithstanding these issues, the hydrometeorological fields from the NCEP reanalysis have been used in a number of studies. For example, Mo and Higgins (1996) compared the atmospheric moisture fields in the NCEP reanalysis to those in the National Aeronautics and Space Administration (NASA) Data Assimilation Office (DAO) reanalysis. They found overall agreement between the two reanalyses, although there were some regional differences. Trenberth (1999) used the hydrometeorological data from the NCEP reanalysis to examine the role of advection and evaporation in the recycling of atmospheric moisture and found that for a typical marine extratropical cyclone approximately 70% of the storm’s precipitation came from water vapor already in the atmosphere at the time of its formation. Cohen et al. (2000) investigated the interannual variability in the meridional component of the vertically integrated transport of the atmospheric water vapor in the NCEP reanalysis. They found that the zonal mean of this field was similar to that derived from the radiosonde-based estimate of Peixoto and Oort (1992) but that the magnitudes were larger. Allan et al. (2002) compared the NCEP reanalysis precipitable water field over the Tropics to that from the NASA Water Vapor Project (NVAP) water vapor dataset (Randel et al. 1996) that is a blend of radiosonde and satellite data. Apart from a systematic bias between the two, Allan et al. (2002) concluded that precipitable water vapor field from the NCEP reanalysis was in “excellent agreement with observations.” There is clearly some uncertainty with regard to the quality of the hydrometeorological FIG. 10. Sounding data from Yakutat at 1200 UTC 26 May 2005: (a) wind speed (m sϪ1) and (b) magnitude of water vapor trans- fields in the NCEP reanalysis. Nevertheless, it repre- port (g kgϪ1 msϪ1). sents the only readily available dataset that allows one to compute all the terms in the water conservation Eq. (1) for this event. Guillemot 1998; Trenberth et al. 2005). As a result, With these caveats in mind, we show in Fig. 11 the there is an underestimation in the precipitable water precipitable water Eq. (3) from the NCEP reanalysis field over the tropical ocean Eq. (3) in the reanalysis as during the period of interest. On 23 May (Fig. 11a), one compared to retrievals of the same field from the Spe- can see a clear meridional gradient in the field with cial Sensor Microwave Imager (SSM/I) instrument larger values in the Tropics. The maxima, near 35°N, (Wentz 1997; Trenberth and Guillemot 1998). Errors 150°W, on this day was associated with the nascent cy- have also been identified in the latent heat flux, that is, clone identified in Figs. 3a and 4a. It is interesting to evaporation, field in the reanalysis, which lead to an note there is a pronounced minimum in the latent heat overestimate in the magnitude of this field in conditions flux field in this region at this time (Fig. 4a) suggesting characterized by high wind speeds and large air–sea that this moisture anomaly was present prior to the temperature differences (Moore and Renfrew 2002; formation of the disturbance. On subsequent days, the Renfrew et al. 2002). The reanalysis also underesti- northward movement and intensification of the low mates the magnitude of the precipitation over the oce- pressure system resulted in an elongation and north- anic tropical interconvergence zones (Trenberth and ward extension of this anomaly leading to values of Guillemot 1998). In addition, the terms in Eq. (2) are a precipitable water in the Gulf of Alaska region as high mixture of analyzed fields (pressure, wind speed, spe- as 25 mm on 26 May (Fig. 11d). The southward trans-

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FIG. 11. Precipitable water (contours and shading, mm) from the NCEP reanalysis at 1200 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. The location of Mount Logan is indicated by the “ϩ.” The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.” port of dry air behind the low resulted in the develop- period of interest. The figure shows the northward ad- ment of minima in the precipitable water field of 10 mm vection of a pulse of moisture from the subtropics on 23 as far south as 30°N on 25 May (Fig. 11c). An exami- May (Fig. 13a) into the Gulf of Alaska region on 26 nation of the statistics of the 6-hourly precipitable wa- May (Fig. 13d). This advection was associated with the ter field from the NCEP reanalysis during the months track of the low pressure system identified in Figs. 3 and of May and June from 1948 to 2005 indicate that 4 and resulted in the extension of the filamentary pre- anomalies of this magnitude occur less than 1% of the cipitable water feature into the Gulf of Alaska region time. (Figs. 11 and 12). An examination of the statistics of the As a check of the precipitable water from the NCEP 6-hourly vertically integrated water vapor transport reanalysis, we show in Fig. 12 the same field retrieved from the NCEP reanalysis during the months of May from the SSM/I dataset (Wentz 1997) during the period and June from 1948 to 2005 indicates that the magni- of interest. A comparison of the two figures indicates, tude of the transport into the Gulf of Alaska region apart from the finescale structure present in the SSM/I associated with this storm occurs less than 1% of the precipitable water field that is absent from the coarser time. resolution NCEP reanalysis field, there is good agree- Figure 14 shows the precipitation rate and sea level ment. pressure from the NCEP reanalysis during the period of Figure 13 shows the vertically integrated water vapor interest. As the low pressure system under investigation transport Eq. (3) from the NCEP reanalysis during the intensifies from 23 to 26 May, its precipitation field also

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FIG. 12. Precipitable water (shading, mm) from the remote sensing science SSM/I dataset for the afternoon passes of the F13 satellite on (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. The location of Mount Logan is indicated by the “ϩ.” The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.” becomes larger in magnitude with the highest values to time (Fig. 15b) indicates that in the vicinity of the posi- the north and east of the low. On 26 May, the precipi- tive pole, there was a net flux of water out of the at- tation rate in the Gulf of Alaska region attained values mosphere with the opposite occurring in the vicinity of in excess of 2 mm hϪ1 (Fig. 14d). the negative pole. The divergence of the vertically in- Given the characteristics of the NCEP reanalysis and tegrated water vapor transport has a tripolar structure the systematic errors that exist in all the hydrometeo- with a region of convergence near 40°N, 140°W and rological fields, one cannot expect that water vapor is regions of divergence near 38°N, 170°W and 28°N, conserved in the dataset (Trenberth and Guillemot 140°W. This suggests that at this time in the storm’s 1998). It is nevertheless instructive to attempt to diag- evolution, the developing anomalies in the precipitable nose the magnitude of the various terms in Eq. (1). water field, the filament of high values of Q extending Figures 15 and 16 show the rate of change of precipi- northward near 40°N, 140°W and the minima in Q near table water, precipitation–evaporation, and the diver- 35°N, 160°W (Figs. 11a,b), are the result of the convergence of the vertically integrated water vapor transport gence/divergence in the integrated water vapor trans- at 1200 UTC 23 and 26 May 2005. On 23 May (Fig. 15a), port field rather than surface fluxes. there exists a dipole in the rate of change of precipi- On 26 May, there are two dipoles in the rate of table water field with the positive pole near 40°N, change in the precipitable water field (Fig. 16a). There 140°W and the negative pole near 38°N, 170°W. Ex- is a meridional dipole along 140°W that is associated amination of the precipitation–evaporation field at this with the storm under investigation, as well as a zonal

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Ϫ Ϫ FIG. 13. Vertically integrated water vapor transport (vectors, kg m 1 s 1) and its magnitude (contours and shading, kg mϪ1 sϪ1) from the NCEP reanalysis at 1200 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. The location of Mount Logan is indicated by the “ϩ.” The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.” dipole centered near 40°N, 170°W that is associated this regard, the 26 May 2005 event on Mount Logan with the cyclone approaching the Gulf of Alaska from provides an excellent case study as two weather stations the west (Figs. 4c,d). With regard to the storm under were deployed on the mountain just prior to the onset investigation, the meridional dipole indicates that there of the high-impact weather. The weather stations were is an increase in the precipitable water in the vicinity of deployed as part of an initiative to understand the cli- Mount Logan and an elongated region, collocated with mate signals contained in an ice core extracted from the the filamentary structure in Fig. 11d, extending south- summit region of Mount Logan (Holdsworth et al. ward to the Hawaiian Islands where there is a decrease 1992; Moore et al. 2002a,b, 2004). These data provide in precipitable water. At this time, the precipitation- a unique and hitherto unobtainable record of the high- evaporation field is positive in the vicinity of Mount elevation meteorological conditions associated with Logan indicating that this region is a sink for atmo- an extratropical cyclone in the Gulf of Alaska region. spheric water vapor. As was the case, on 23 May, the They were used along with surface and sounding divergence in the vertically integrated water vapor data from the nearby town of Yakutat, Alaska, satel- transport field (Fig. 16c) tends to compensate for the lite imagery, and the NCEP reanalysis to character- changes in the precipitable water field. ize the synoptic-scale conditions associated with this storm. This particular storm evolved out of a poorly orga- 5. Summary nized disturbance along a trailing cold front over the The characterization of the weather in a particular subtropical North Pacific on 22 May. Over the next 4 mountainous region of course requires in situ data. In days, the system intensified and moved northward

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Ϫ FIG. 14. Sea level pressure (contours, mb) and precipitation rate (shading, mmh 1) from the NCEP reanalysis at 1200 UTC (a) 23; (b) 24; (c) 25; and (d) 26 May 2005. reaching the Mount Logan region on 26 May as a well- These air parcels did not undergo any significant verti- defined extratropical cyclone (Figs. 3 and 4). The sys- cal motion. Such trajectories are consistent with the tem was associated with a well-defined upper-level low-level cyclonic circulation associated with an extra- trough and a tropopause fold (Figs. 5 and 6). tropical cyclone. The relative humidity of the air par- Back trajectories suggest that 2000–3000 m appear to cels along the lower-level trajectories also had a mark- be the approximate height at which there exists a sepa- edly different evolution as compared to that of the up- ration in trajectory characteristics, with those ending up per-level trajectories. In particular, the low-level air at lower heights in the Mount Logan region tending to parcels underwent a rapid moistening during the final have a source near the surface in the Gulf of Alaska 12–24 h of their journey, while the upper-level parcels region, while those at higher elevations tend to have a underwent a more gradual moistening over most of more distant source region (Fig. 7). The air parcels that their movement from the subtropics to the Mount Lo- end up at higher elevations on Mount Logan on 26 May gan region. This suggests that different mechanisms are for the most part have meridionally oriented trajecto- responsible for the moistening of the two sets of air ries that rise as the air parcels travel from the subtropics parcels. to the Gulf of Alaska region. Such trajectories are what The automatic weather station data from the Mount one would expect for parcels moving along the cold Logan sites indicate that the storm was associated with frontal zone in a typical extratropical cyclone. In con- a drop in pressure, an increase in temperature, an in- trast, the air parcels that end up at lower elevations crease in specific humidity, and an increase in wind have cyclonic trajectories with a nearby source region. speed (Fig. 10). In comparison with data from Yakutat,

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FIG. 15. Components of the conservation of water vapor equation from the NCEP reanalysis at 1200 UTC 23 May 2005: (a) rate of change in precipitable water (mm hϪ1); (b) precipitation minus evaporation (mm hϪ1); and (c) divergence of the vertically integrated water vapor transport (mm hϪ1). The location of Mount Logan is indicated by the “ϩ.” The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.” the magnitude of the pressure drop was reduced on Alaska region (Figs. 11 and 12). Based on the NCEP Mount Logan by approximately 50%. There was also a reanalysis dataset, values of the precipitable water field slight phase lag in the timing of the pressure minima on in the Gulf of Alaska region as high as those found on Mount Logan that is consistent with a system that tilts 26 May occur less than 1% of the time in May and June. westward with height. Other observations of the storm The intrusion was associated with a pulse of water va- from the Mount Logan AWS include a dramatic drop in por that was transported from the subtropics to the specific humidity after its passage, which may be the Gulf of Alaska region (Fig. 13). An analysis of the result of the intrusion of dry stratospheric air associated sources and sinks of precipitable water suggests that with a tropopause fold (Figs. 6 and 7). Wind speeds this filamentary intrusion was primarily the result of the during the storm were also higher on Mount Logan convergence of preexisting water vapor in the atmo- than in Yakutat (Fig. 9). Sounding data from Yakutat sphere associated with a pulse of moisture that was indicated the presence of two distinct maxima in the transported northward by the cyclone (Figs. 15 and 16). water vapor transport field (Fig. 10). The surface flux of water, precipitation minus evapora- The movement and evolution of the cyclone resulted tion, was for the most part positive indicating that the in a filamentary intrusion, characterized by tropical val- cyclone was losing moisture to the surface (Fig. 14). As ues of precipitable water, that extended into the Gulf of a result, the evaporation of water from the surface

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FIG. 16. Components of the conservation of water vapor equation from the NCEP reanalysis at 1200 UTC 26 May 2005: (a) rate of change in precipitable water (mm hϪ1); (b) precipitation minus evaporation (mm hϪ1); and (c) divergence of the vertically integrated water vapor transport (mm hϪ1). The location of Mount Logan is indicated by the “ϩ.” The location of the surface low, as determined from the NOAA surface analysis, is indicated by the “L.” makes only a minor contribution to this filament. This mosphere (Newell et al. 1992; Newell and Zhu 1994; is consistent with Trenberth (1999) who argued that Zhu and Newell 1994). This study along with others surface evaporation only contributes approximately (Smirnov and Moore 1999, 2001) emphasizes that on a 30% of the moisture that falls as precipitation in a typi- day-to-day basis, these moisture streams are highly local marine extratropical cyclone. calized in space and on these time scales are better characterized as “pulses.” This paper has also shown that there are filamentary 6. Conclusions structures in the precipitable water field that may pro- In addition to the specifics of this case, the analysis vide another way to diagnose the presence and struc- presented in this paper has several implications for our ture of organized moisture streams in the atmosphere. knowledge of atmospheric water vapor transport, as With regard to the interpretation of the climate signal well as for our understanding of climate expressions in the Mount Logan ice core, this work supports early contained in ice cores. Numerous studies have high- studies that have suggested that the Tropics and sub- lighted the role that the vertically integrated water va- tropics are the source of the moisture that falls as snow por transport field has in diagnosing the presence of high up on the mountain (Moore et al. 2001; Holds- large-scale streams of moisture, or “rivers,” in the at- worth and Krouse 2002; Moore et al. 2002b, 2003).

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Back trajectories furthermore suggest that there exists Campbell Scientific Canada and Rick Smith from the a separation in the source region for air parcels that end University of Calgary helped G. H. in preparing the up in the Mount Logan region with parcels above ap- AWS for installation. proximately 3000 m having a distant source, while those that end up in lower elevations have a more local REFERENCES source. The sounding data from Yakutat also indicated Allan, R. P., A. Slingo, and V. Ramaswamy, 2002: Analysis of the presence of two local maxima in the water vapor moisture variability in the European Centre for Medium- transport field, one near the surface and the other near Range Weather Forecasts 15-year reanalysis over the trop- 3.5 km. This separation is consistent with the observed ical oceans. J. Geophys. Res., 107, 4230, doi:10.1029/ discontinuity in the isotopic ratio of oxygen with eleva- 2001JD001132. tion that is observed on Mount Logan (Holdsworth et Asuma, Y., S. 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