https://www.nickdavis.space/home daily 1.0 2025-03-30 https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/8372d547-c699-47de-a0cb-899c368a3060/geopotential_map_long_lead.png Home The stratospheric circulation leading up to the 2021 cold air outbreak in observations and experimental forecasts. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/1674666510538-DBH7J3W8NLV3P0J9XCVJ/grabity.jpg Home Refining the global grid over North America can resolve gravity wave dynamics, including horizontal propagation that isn't captured by parameterizations. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/c17dd91a-3645-4eb5-8cfc-c780fee416d7/composites_simple.png Home Forecasts that are unsuccessful and successful at predicting a sudden stratospheric warming, and the verification (MERRA2). Here, "unsuccessful" means a false negative - a forecast should have predicted a sudden warming, but didn't. All composites relative to the sudden warming date. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/32df92e5-f0f1-4d35-bef1-21ef534389b5/wave_flux_meridional_mean.png Home Pathways of planetary-scale atmospheric waves across the Pacific indicate that less than 3% of the wave forcing for the event reflected off the polar vortex. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/788660c5-dbff-48e2-b0f7-754d61bc0a91/mixedspace.png Home Spectrally decomposing the circulation in space and time provides deep insights into vertical transport. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/ad5ee9e9-ebee-4cdf-aa8f-5f0c4fd7ca3d/fig5.png Home The zonal wind responses, as for temperature. Equal and opposite responses highlight which processes drive - and which diffuse - the global response. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/86d0f22e-c45b-4e46-968e-0126fc4332e8/fig4.png Home Regression of wave drag, residual mean angular momentum flux convergence, and net momentum tendency on stratospheric jet deceleration during a sudden stratospheric warming. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/1637012124784-8RGNR7V2TQGSZR6ZKY1T/na_forecasts.jpg Home In forecasts, surface temperatures over North America are frigid - even when the stratosphere is artificially engineered to shut down the wave reflection mechanism. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/2fc1cddf-bf46-4d0c-b40c-b188e078b1b5/gravity_wave.png Home A gravity wave produced by flow over the Rocky Mountains reaches well into the stratosphere, and is associated with a localized injection of stratospheric ozone toward the surface. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/51eb832f-be6a-4d48-890a-6844f13dadcd/fig4.png Home The Hadley and Ferrel cell responses, as for temperature. By restricting the circulation responses, we can build a more direct and predictive understanding of global change. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/1a727118-5f72-417e-9029-9eec5639bf99/fig9.png Home Modeling all of these complicated changes in the atmospheric circulation with a simple mathematical hypothesis. A special model experiment confirms the hypothesis (red). https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/c71ab93a-96d1-44f9-b061-9c401d2a2cfc/mom_balance_progression.png Home Composites of jet-level momentum budget terms with the (black) correct zonal mean and (red) the zonal mean from the other composite. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/025c927f-9821-4b03-827b-e7611b46cb73/fig11.png Home Idealized modeling experiments are ubiquitous - but the forcing they impose is fundamentally at odds with the real effects of greenhouse gases. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/9b534183-f932-417e-b45a-9fea13f86b9a/fig3.png Home The temperature response to a CO2-like forcing in an idealized model with varying degrees of the circulation held to its control state. The minor differences between these experiments is key to unlocking the global response to forcings. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/d2e9bfed-1b62-4c02-b5ff-23264eb2a163/fig11.png Home Tropical upper-tropospheric heating is often applied to idealized models as an analogue to CO2 forcing, but this is opposite to the actual CO2 forcing. This study uses a model with simplified but interactive radiation to avoid the pitfalls of choosing your own forcing. https://www.nickdavis.space/about daily 0.75 2025-03-30 https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/53cf8995-0d4f-42c9-9c88-b5255f3b881e/AW_BF21_Before-190-ZF-5384-22182-1-001-001.jpg About https://www.nickdavis.space/the-february-2021-cold-air-outbreak daily 0.75 2023-01-25 https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/e44d6a04-eef2-45e5-8f23-8301b27076ae/verification.jpg The February 2021 cold air outbreak - It’s the extremes that matter. We almost always experience average weather. We expect it to be colder in winter and warmer in summer, and we expect it to rain and snow, but not too much. We build our lives and infrastructure around these expectations. It’s the extreme weather, the once-in-a-lifetime events that are difficult to forecast and prepare for, that impact our lives and cost us billions of dollars in damages. In February of 2021, the United States suffered a severe cold air outbreak that killed hundreds, broke the power grid, and disrupted the economy for weeks. Scientists and forecasters scrambled for an explanation. https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/f063ce89-4a67-47d9-9cfc-b608d39c4d50/polar_vortex.jpg The February 2021 cold air outbreak - Was it the polar vortex? Researchers have hypothesized that cold air outbreaks in the United States may be caused by a “stretching” of the stratospheric polar vortex. While the physics are complicated, the idea is simple. Large-scale waves constantly travel up to the stratosphere. When the vortex becomes stretched, it reflects these waves back down toward the surface, where they can invigorate weather systems and produce extreme cold. While it’s true that the vortex was stretched in the weeks before the February 2021 cold air outbreak, how would you actually test whether the stretching and wave reflection caused it? https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/f219fcf9-cac8-4230-9368-4900f9317de6/na_forecasts_simple.png The February 2021 cold air outbreak - Use a forecast model - cleverly. The atmosphere in a forecast model is initialized as close to observations as possible so that the forecast most represents reality. But what if you wanted to test which part of the atmosphere was most important for the future evolution of the weather? You could choose to not initialize, or to “scramble”, different parts of the atmospheric initial conditions, and see how the forecast changes. When the stratospheric initial conditions are scrambled, the forecast still predicts extreme cold. On the other hand, when the initial conditions in the troposphere, or the weather layer of the atmosphere, are scrambled, the forecast predicts extreme warmth. How is it that the stratosphere had no impact on the extreme cold, while the lowest layer of the atmosphere had full control? https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/2db12902-b19f-49cd-9190-042ec863a4a6/geopotential_map_long_lead.png The February 2021 cold air outbreak - Check the mechanism. In both observations and the standard forecast, wave activity travels upward into the stratosphere over Eurasia and is reflected off of a stretched vortex and back down over North America. This process is completely absent in the forecast with scrambled stratospheric initial conditions. How is it possible that the vortex stretching and wave reflection had no impact on the extreme cold? https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/0b5a4289-38e2-4aba-8218-084c44ab88a9/wave_flux_meridional_mean.png The February 2021 cold air outbreak - Examine the physics with great care. It is true that the wave activity that originated over Eurasia was reflected downward across the Pacific - but a careful analysis of the flux streamlines shows that these waves never reached the troposphere over North America. All of the wave forcing in the troposphere over North America originated from North America. In other words, the reason that the forecast with scrambled stratospheric initial conditions - and no vortex stretching or wave reflection - still predicted the extreme cold over North America, is because the waves never reached the troposphere, anyway. https://www.nickdavis.space/ssw-positive-feedback daily 0.75 2023-05-25 https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/efce0fbd-0f74-4624-bb1d-a16b073143dc/composites_simple.png https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/ae5c5b4e-e1b6-4c73-ba33-091bdf7dabe6/fig4.png https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/6fd36e3d-68bb-4798-a4da-bca53b9193a8/mom_balance_progression.png https://images.squarespace-cdn.com/content/v1/6192c9943757b171b429086e/d3236e58-de89-424e-a7c8-2681eb774240/Screen+Shot+2023-02-06+at+11.44.13+AM.png