Daily Archives: February 26, 2014

Storm Day! Part 2

Greetings from rain-soaked Bodega Head!

We have now picked up over an inch and a half of rain since this morning at 7 am. We have had periods of light rain, heavy rain, big ole fat rain, and rain that even seemed to come up from underneath. The short range, high-res MM5 forecast suggested that we would get 1.5 inches for all of wednesday:

DRI-MM5-24hrAccum_1200UTC0226-1200UTC0227

24 hr accumulated precipitation forecast by 4 km MM5 valid at 4 am UTC on Feb 26, 2014. Credit: DRI-CEFA.

We reached Atmospheric River conditions here at the BML HMT station just after midnight and have been under an AR ever since. The surface met time-series over the last 48 hours or so looks like:

HMT-BBYsfc_0100UTC-022714

Sfc met variables collected at the 10m tower at BML for the 48 hours ending at 5pm PST on Feb. 26, 2014. Remeber that the time ordinate increases to the right! Credit: NOAA-HMT.

My favorite panels are the lowermost 2. Winds have been southerly or south-southeasterly all day. This has been the case throughout deep layer overhead:

HMT-BBY449UFlx_0100UTC-022714

Time-vertical profile of low-layer winds, freezing level, and upslope water vapor flux. Credit: NOAA-HMT.

Since the 449 MHz wind profiler is alive and kicking again, I couldnt resist posting a plot of the near-surface winds. For the most part the wind profiles themselves have been uniform and southerly in the lowest 5 km. The approaching storm is a very deep system, and since we are on the east flank, we will continue to get southerly winds until the first cold front passes us.

The radar senses virtual temperature (via density) using RASS. The black dots in the later stages of the picture correspond to the freezing level above BBY, which is currently about 1.9 km high. Notice that after 2 pm, the winds in the lowest 2 km have veered. As a result, the freezing level has raised slightly. Veering winds correspond to warm-air advection (WAA), so this low-level warming should be expected.

We did not have a prolonged period of westerly winds as this storm approached, and therefore did not see a coastal jet. Interestingly, even though we did not, and we have not had terribly stable stratification, Cazadero has recieved less rain this event than BBY. Upslope water vapor flux corresponds to the low-layer integrated component of the wind which points from BBY to CZC multiplied by the water vapor mixing ratio. In the lower panel of the plot above, this value has been increasing along with wind speed. It is expected that as upslope wv flux increases, orographic enhancement will cause more rain to fall at CZC.

CZC has thus-far recieved near 1.4 inches. Often, this rain has formed in a deep layer extending above the freezing level:

HMT-CZCSNR_0100UTC-022714

Timeseries of S-Band radar signal-to-noise ratio above CZC. Credit: NOAA-HMT.

The freezing level can be diagnosed from this plot by the location of the “bright band” of echos near 1.9 km.

Even though we have struck out on it so far, we may yet get a short lived coastal jet. The first front is expected to come ashore at BML tonight near 10 pm. This is depicted in the MM5 forecast below:

DRI-MM5-925hPaUT_1200UTC0226-0600UTC0227

925 hPa winds (barbs and isotachs) from 4 km MM5 valid at 0600 UTC Feb 26, 2014. Credit: DRI-CEFA.

After the front, we expect a short (until 1 pm, PST Thursday) period of westerlies, during which we may have the right conditions for a jet.

Winds will turn southerly again Thursday into Friday, until another front reaches us in advance of Friday’s storm system.

At this point, to call today’s and Friday’s system separate is probably improper. The upper level potential vorticity maximum associated with Fridays storm is so rapidly deepening, that it will engulf the PV max associated with today’s storm soon:

See that here Credit: U Hawaii

At the lower levels, this rapid upper level deepening is going to intensify a small, but intense extratropical cyclone which will hit California early Saturday morning:

See the fun cyclogenesis Credit: U Hawaii

Just for fun, I thought I would post what the high-resolution forecast of this event looks like at 72 hours simulation time.

WCoast_TC

10 m wind and SLP from 4 km MM5 valid at 12 UTC on Mar 1, 2014. Credit: DRI-CEFA.

Is that a west coast TC?

Other curiosities from today:

In this post, I mentioned that we encountered low level dust this morning, and that this system was relatively very dust laden. We also found Petaluma Gap flow for most of the morning. This can be seen in the surface station map from near 10 am PST:

MWest-Obs-USLP_1800UTC0226

Surface station temperature and sea-level pressure from stations in N. California. Credit: Meso-West, U Utah.

High pressure over the CV and East Bay, compared to relative low pressure in the North Bay and coastal Sonoma, are causing flow through the Petaluma Gap to our site in this picture.

As a result, we measured very elevated particle counts, NO, NOx and Ozone for much of today. It’s a strange result, but sometimes when it is raining the air is more polluted at Bodega Bay than when it is sunny and high pressure is in control.

This is especially true in light of the data we have recently worked up for periods of quiet conditions from early in the trip. We now have an idea of just how clean the air can be here when flow is onshore. A post reviewing that information will appear here tomorrow.

-Andy.

which is going to accelerate and intensify Friday’s system, so that it will come right on the heels of today’s rain.

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From Mongolia, with Love

Greetings from a rain-soaked Bodega Head!

We have picked up nearly an inch of rain since 7 am PST.

I missed a post last night due to an internet outage, so I’m going to leave two posts here this afternoon. Ill get to the weather, our rain outlook for the rest of the day, Petaluma Gap flow and maybe a Coastal Jet in post two. For this post, I want to revisit the cross-Pacific transported dust we have been anticipating.

Yesterday evening’s GEOS-5 run was still bullish on our chances of seeing dust pass overhead this morning near 10 am:

GEOS-5_NAmer_dustAOT_00UTC0226-18UTC0226

Dust AOT over North America valid at 1800UTC on Feb 26, 2014. Credit: NASA-GMAO.

Lo and behold, we started picking up many dust-like spectra from our ATOFMS, Laverne, around mid-morning. We also saw ice-nuclei activity in the CFDC increase. If you animate the following loop:

GEOS-5 dust global loop Credit: NASA-GMAO.

You can watch the nature of this dust as it enters the eastern Pacific. Dust travels around the south flank of the storm as it heads East, and in doing so, eventually intercepts the Pacific coast. Dust traveling this pathway certainly would have had to cross the baroclinic zone associated with a trailing cold front from West to East, meaning the air parcels would have experienced forced descent along isentropic surfaces. Thus, if the forecast was reality, any dust traveling around the South flank of the storm would have descended toward the surface.

Our instruments here at the surface are seeing dust, so score one for the model.

We are also expecting that dust may still be present at high levels after the cold front with this storm. That feature is not well captured in the model forecast, so why are we anticipating it?

The storm became occluded on Monday, Feb. 24 while it was still in the Central Pacific. Here is a surface analysis for 1800 UTC on 02/24:

OPC-NPacSfc_1800UTC0224

Surface analysis for Gameboy Color (R) valid at 1800 UTC on Feb 24, 2014. Credit: NOAA-OPC.

During this time, dust from a large plume leaving NE Asia was making it’s way across the Pacific Ocean too. Before we try to find it in relation to the storm, lets look at the MODIS Level 2 cloud top temperature for this approximate time:

Terra_CTT_NCPac_0224

Cloud top temperature over the Central Pacific on Feb 24. Credit: NASA-GSFC.

The MODIS (Terra) swath over the storm is from approximately 20 UTC. The deep purple colors indicating very cold cloud tops form a comma shape, which is classically associated with an occluded extratropical cyclone. The left inside of the comma is near the low pressure center and is an area where dry air is intruding into the storm circulation. It is relatively free of clouds.

MODIS-Terra observed aerosol optical depth in this relatively cloud free region (the aerosol products rely on visible wavelengths and cannot function over cloudy regions.):

Terra_AOT550_NCPac_0224

Aerosol Optical Thickness at 550 nm over the North Central Pacific on Feb 24, 2014. Credit: NASA-GSFC.

The cloud free region shows elevated aerosol optical thickness, suggesting that the dust has made it into this  part of the storm circulation, which is behind the cold front, along with the dry air.

For us, this means we are expecting more dust to be included in the rain samples and ambient air near the time the cold front passes. Observing the shift in insoluble residue chemistry from the rain samples may tell us something interesting about the chemical history of the dust as it was processed during its journey.

-Andy