Human’s Role is Less Than for Most Parameters Forecasting Pressure Human’s Role is Less Than for Most Parameters

Three Types of Pressure Surface or Station {Pressure Sea Level Pressure (SLP) Altimeter Setting (ALT)

Pressure Observations ASOS is the best…the gold standard Ships generally the worst. Less near active weather today. Smartphones now provide pressure observations

ASOS Pressure Sensor

Model Pressure Reduction Lots of issues to worry about

Hypsometric Reduction Used by Most Models No time averaging of the surface temperature is employed.  In order to calculate the mean temperature, the model "surface" temperature, and the standard atmosphere lapse rate (6.5 C / 1000m) are used.

At Night During Warm Season: phony troughs under terrain during night

Why? Models usually use the U.S. Standard Atmosphere for pressure reduction (Shuell Method). During night the atmosphere can become more stable than U.S. Standard Atmosphere at low levels. Actual air temp warms less rapidly towards the surface in real atmosphere than in model.

Why? Thus, the model atmosphere under terrain is warmer than in the real world producing unrealistic low pressure. Away from terrain there are no reduction issues.

During the day, phony trough inland at low elevations Fig. 5. Composites of sea level pressure (solid lines, hPa) and 1000-hPa temperature (color shading, °C) using the (left) Shuell and (right) Mesinger methods for JJA at 0000 UTC.

Why? During day, the real atmosphere over the central valley is near dry adiabatic (9.8 C per km), while over the mountains we assume U.S. Standard atmosphere valley (6.5C per km). Becomes cooler at low levels inside the mountains…thus higher pressure.

High-Resolution Can Greatly Improve Pressure Forecasts Near Terrain

He S 12-km

Effects of Terrain

Terrain Effects Pressure ridge on windward side Pressure trough on leeward side Dependent on Froude number Fr = U/hN Larger pressure perturbations with larger Froude numbers

For Pressure You Are Dependent on the Models

2016 Update Some smartphones are measuring pressure information Positive data assimilation experiments using this data source.

Some of the smartphones/pads with pressure sensors Iphone 6 Samsung Galaxy III, IV, V, VI Nexus 4 and 10 Some Sony, Nokia, and Chinese phones

By the end of 2016: 1 billion smartphones with pressure sensors

Initial test: 114K Pressures in One Hour over US

UW 36-4 km Testbed DART EnKF System WRF Model Run in real-time operationally and for test periods. 4-km 36-km

Convergence Zone Case A “poorly-forecast” convergence zone forms around 1400 UTC (6AM PDT) and moves south across north Seattle during the morning commute

Convergence Zone Case 3-hour forecasts from fully cycled EnKF of simulated composite reflectivity valid at 1500 UTC, Oct. 24, 2011

Pressure Verification