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Thomas Spengler, Joseph Egger, and Stephen T. Garner

Abstract

The process of hydrostatic adjustment in a vertical column is discussed in the context of rain formation and sedimentation. The authors assume an event of instantaneous condensation in a midatmospheric layer that removes mass from the gas phase and produces latent heating. It is shown that the rain formation leads to a change of the surface pressure after a short period of acoustic wave activity. There is, however, no hydrostatic surface effect once the particles reach terminal velocity. It is not until the rain reaches the ground that the surface pressure decreases consistently with the mass removed by the phase change.

Only the mass removal introduces perturbations below the layer of rain formation, where it acts to stretch the lower levels, reducing pressure and temperature. Above the layer of rain formation, the effects of latent heating dominate over the effects of mass removal by an order of magnitude.

The hydrostatic adjustment time is found to be approximately equal to e 2 Na −1 (340 s, where Na is the acoustic cutoff frequency and e is the Euler constant) and is proportional to the temperature of the isothermal basic state. The energy distribution is found to be dominated by the latent heating. However, the mass removal significantly alters the amount of energy lost due to work done by the pressure perturbations. The implications for numerical modeling are discussed.

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J. E. Kristjánsson, I. Barstad, T. Aspelien, I. Føre, Ø. Godøy, Ø. Hov, E. Irvine, T. Iversen, E. Kolstad, T. E. Nordeng, H. McInnes, R. Randriamampianina, J. Reuder, Ø. Saetra, M. Shapiro, T. Spengler, and H. Ólafsson

From a weather forecasting perspective, the Arctic poses particular challenges for mainly two reasons: 1) The observational data are sparse and 2) the weather phenomena responsible for severe weather, such as polar lows, Arctic fronts, and orographic influences on airflow, are poorly resolved and described by the operational numerical weather prediction (NWP) models. The Norwegian International Polar Year (IPY)– The Observing System Research and Predictability Experiment (THORPEX) project (2007–10) sought to significantly improve weather forecasts of these phenomena through a combined modeling and observational effort. The crux of the observational effort was a 3-week international field campaign out of northern Norway in early 2008, combining airborne and surface-based observations. The main platform of the field campaign was the Deutsches Zentrum für Luft- und Raumfahrt (DLR) research aircraft Falcon, equipped with lidar systems for profiling of aerosols, humidity, and wind, in addition to in situ measurements and dropsondes. A total of 12 missions were flown, yielding detailed observations of polar lows, Arctic fronts, and orographic low-level jets near Spitsbergen, the coast of northern Norway, and the east coast of Greenland. The lidar systems enabled exceptionally detailed measurements of orographic jets caused by the orography of Spitsbergen. Two major polar low developments over the Norwegian Sea were captured during the campaign. In the first polar low case, three f lights were carried out, providing a first-ever probing of the full life cycle of a polar low. Targeting observations by the aircraft in sensitive areas led to improvements in predicted track and intensity of the polar low. Here highlights from the field campaign, as well as from ongoing follow-up investigations, are presented. Highlights from the development of a new limitedarea model ensemble prediction system for the Arctic, as well as an exploitation of new satellite data [Infrared Atmospheric Sounding Interferometer (IASI) data], are also included.

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EXECUTIVE COMMITTEE, David D. Houghton, Paul D. Try, Warren M. Washington, Robert T. Ryan, Margaret A. LeMone, Richard S. Greenfield, Richard E. Hallgren, and Kenneth C. Spengler
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EXECUTIVE COMMITTEE, E. G. Droessler, E. W. Bierly, R. E. Hallgren, R. G. Fleagle, D. S. Johnson, T. H. Vonder Haar, K. C. Spengler, and D. F. Landrigan
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EXECUTIVE COMMITTEE, Donald R. Johnson, Robert T. Ryan, William D. Bonner, James R. Mahoney, Kristina B. Katsaros, Ronald D. McPherson, Richard E. Hallgren, and Kenneth C. Spengler
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EXECUTIVE COMMITTEE, Robert T. Ryan, Warren M. Washington, Donald R. Johnson, William D. Bonner, Margaret A. LeMone, Ronald D. McPherson, Richard E. Hallgren, and Kenneth C. Spengler
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EXECUTIVE COMMITTEE, E. W. Bierly, C. J. Murino, E. G. Droessler, R. E. Hallgren, H. D. Orville, T. H. Vonder Haar, K. C. Spengler, and D. F. Landrigan
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EXECUTIVE COMMITTEE, Warren M. Washington, David D. Houghton, Robert T. Ryan, Donald R. Johnson, Margaret A. LeMone, Alexander E. MacDonald, Richard E. Hallgren, and Kenneth C. Spengler
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I. A. Renfrew, R. S. Pickart, K. Våge, G. W. K. Moore, T. J. Bracegirdle, A. D. Elvidge, E. Jeansson, T. Lachlan-Cope, L. T. McRaven, L. Papritz, J. Reuder, H. Sodemann, A. Terpstra, S. Waterman, H. Valdimarsson, A. Weiss, M. Almansi, F. Bahr, A. Brakstad, C. Barrell, J. K. Brooke, B. J. Brooks, I. M. Brooks, M. E. Brooks, E. M. Bruvik, C. Duscha, I. Fer, H. M. Golid, M. Hallerstig, I. Hessevik, J. Huang, L. Houghton, S. Jónsson, M. Jonassen, K. Jackson, K. Kvalsund, E. W. Kolstad, K. Konstali, J. Kristiansen, R. Ladkin, P. Lin, A. Macrander, A. Mitchell, H. Olafsson, A. Pacini, C. Payne, B. Palmason, M. D. Pérez-Hernández, A. K. Peterson, G. N. Petersen, M. N. Pisareva, J. O. Pope, A. Seidl, S. Semper, D. Sergeev, S. Skjelsvik, H. Søiland, D. Smith, M. A. Spall, T. Spengler, A. Touzeau, G. Tupper, Y. Weng, K. D. Williams, X. Yang, and S. Zhou

Abstract

The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway.

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