Round 2 is on its way

Greetings from Bodega Head!

After a good soaking yesterday, the sun has come out for a brief hello.

 

Recap of Wednesday’s rain (round 1)

We received around 2 and a quarter inches of rain here yesterday, largely between 7 am Wednesday morning and 8 pm Wednesday night. A frontal boundary passed us around 7 pm Wednesday. It wasn’t a classical cold front, which can be seen from the 449 MHz wind profiler and RASS timeseries:

HMT-BBY_UTvFlux_1900UTC0227

Time-vertical cross-section of wind and virtual temperature from 449 MHz radar + RASS at BBY for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

As can be seen in the wind profiles, winds turned from SSE to SW after the boundary, and the lower atmosphere actually warmed. As we will see, freezing levels did not change much after the boundary either.

The profiling radars at Cazadero (CZC) and Santa Rosa (STR) capture this beautifully:

HMT-CZCSNR_19UTC0227

Time-Vertical cross section of signal-to-noise ratio from the S-Band Radar at CZC for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

HMT-STRSNR_19UTC0227

Time-vertical cross section of signal-to-noice ratio from the S-Band Radar at STR for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

For each case, the bright band does not ascend or descend after the time of frontal passage. It is apparent that CZC received a period of very heavy rain between 6 pm and 8 pm, near the frontal boundary. In fact, cloud echo-top heights reached to nearly 9 km AMSL. The wind profiler here at BBY shows that this period coincided with a dramatic increase in upsolpe water vapor flux:

HMT-BBY_UiwvFlux_1900UTC0227

Time-vertical cross section of wind and timeseries of upslope IWV at BBY for the period ending at 11 am PST on Feb 27, 2014. Credit NOAA-HMT.

In this case, the upslope wind direction is defined along the vector which points from BBY to CZC. as Southerly winds really sped up near 6 pm yesterday, upslope IWV also shot up, and for the period where the value exceeded 20 knot-inches, precipitation was enhanced at CZC compared to BBY.

Potential for dust in precip. samples:

Yesterday in this post, I mentioned that we observed dust at the surface for most of the morning, while the storm was coming ashore. We also expected high altitude dust in the storm’s central circulation. There are a few ways that elevated dust could reach our site at the ground. 1) Dust aerosol could be scavenged by falling raindrops, which then make it into our precipitation sampler. 2) Dust aerosol could act as cloud condensation nuclei, which grow into or become incorporated in a rain-drop or precipitating ice and make it into our precipitation sampler. 3) Dust aerosol could act as ice nuclei, which grow into ice-phase precipitation particles, melt on their path to the surface and make it into our precipitation sampler.

We are most interested in the last possibility, which means that we would really like to collect dust during periods when the precipitation profiling radar is telling us that there is cloud ice over our heads (i.e. a brightband is present) in a deep layer.

Even at STR, whose radar likely mimicked the cloud structure that existed over us at BBY, there were long periods of high cloud-top echoes over brightband rain yesterday. Given that we expected a deep tropospheric layer of Asian dust, particularly in the early phases of this storm, that is good news for our precipitation residue analysis!

Looking ahead to Round 2:

Since late yesterday evening we have entered an interlude between storms. Winds were southwesterly for a short time, but will soon become southerly in advance of the next system. We are going to experience a rare event here on the Northern California coast. We are going to see a small but powerful extratropical cyclone whose circulation center passes to our south. We expect the low pressure center to come onshore early Saturday morning near Monterrey Bay. Before this time, we will encounter another frontal boundary here at BBY. According to model forecasts, a mesoscale band of strong SE winds will cross the coast near 4 am PST tomorrow morning:

MM5-10mU_12UTC0227-12UTC0228

10m wind speed and wind barbs forecast by 4 km MM5 valid at 4 am PST on Feb 28, 2014. Credit: DRI-CEFA.

After this time, winds will shift to SE, then rotate to S, then E, then eventually NE and Northerly as the low pressure center comes fully onshore. We expect high wind speeds at several points tomorrow, as the particular series of events mimics a rainband, then near-miss of a tropical storm. Pretty unique for the west coast!

The bottom line is that it should start raining again tonight at around 4 am and we could have periods of rain through 1 pm Saturday. We may pick up 1-2 additional inches of rain here.

-Andy

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