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Richard H. Johnson and Jeffery R. Zimmerman

Abstract

Aircraft dropwindsonde and conventional sounding data have been used to document the boundary layer structure over the South China Sea during the period of a 3-day moderate cold surge that occurred during the December 1918 Winter Monsoon Experiment (Winter MONEX).

Throughout the cold surge event, equatorward deepening of the cumulus layer is observed over the northern South China Sea (to the south of a coastal cloud-free region) as cold air streaming of the coast encounter substantial surface, sensible and latent heat fluxes. Following the onset of the cold surge, the cumulus-layer depth increases over the central South China Sea. However, this depth decreases over the northern South China Sea at a distance of 200–300 km from the China coast. The depth and strength of the cumulus (or subsidence) inversion are greatest new the China coast (∼600–700 m, 4°–6°C) and weaken toward the central South China Sea (∼200 m, 1°C), whew they eventually become undetectable. Possible explanations for the observed variations are suggested.

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Kingtse C. Mo, J. R. Zimmerman, E. Kalnay, and M. Kanamitsu

Abstract

June 1988 has been classified as one of the hottest and driest months on record in the United States. This study used the NMC Medium-Range Forecast(MRF) T40 model to simulate circulation features of June 1988 and to investigate the relationship between sea surface temperature anomalies (SSTA) and circulation patterns in the Northern Hemisphere. Three control experiments have been performed using three different initial conditions, separated by one day (21, 22, and 23 May 1988) and using SSTA fixed at the starting date. The three forecasts, and their average, are remarkably skillful in the Northern Hemisphere. The observed anomaly of June 1988, a wave train with a persistent ridge in the north-central United States and a northward shifting of the jet stream in the Pacific–North America area, is very well simulated in each of the integrations. All three experiments were repeated using the same initial conditions, but with climatological SST. The wave train generated is similar to that in the control experiments, but it is not as robust. The simulated jet streams are also similar to those in the control experiments. Two experiments with the 1988 SSTA, but with initial conditions of 22 May 1987 and 22 May 1989 were also run. The circulation patterns generated by these runs are very different from those of 1988, indicating that the persistence of the anomalous ridge in the north-central United States after late May 1998 was not due to the SSTA of the May 1988 alone.

A barotropic analysis was done to obtain the normal modes associated with the 300-mb streamfuncton of the June climatology. The analysis indicates the existence of a slowly growing mode with structure similar to the anomalies of 1988. This result, as well as the numerical experiments, suggests that the persistence of the June 1988 wave train may be associated with initial conditions, which were in a rather stable regime. The SSTA may have helped to strengthen the pattern, but the wave train associated with the 1988 drought could not have been generated by SSTA alone.

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John E. Zimmerman, Phillip J. Smith, and David R. Smith

Abstract

A study of the sensitivity of a weak winter extratropical cyclone to latent heat release (LHR) is presented using 48-h simulations of the cyclone's evolution derived from three versions of the LFM model: a MOIST simulation in which full model physics was employed, a DRY simulation in which all latent heating was removed, and a DOUBLE MOIST simulation in which the effect of latent heating on the temperature field was doubled. Results indicate that a deepening cyclone occurs in the DOUBLE MOIST simulation, a near steady-state cyclone in the MOIST simulation, and a filling cyclone in the DRY simulation. Thus, for this case the presence and intensity of LHR is of critical importance to this cyclone's intensification. In addition, using height tendency diagnoses, it is concluded that for this case in the lower troposphere the dominant LHR influence is direct, through the explicit diabatic heating forcing in the height tendency equation. In contrast, in the middle and upper troposphere this direct LHR role is no longer dominant, but rather shares its importance with the indirect effect, represented by the influence of LHR on the dynamical forcing mechanisms.

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K. E. Garman, K. A. Hill, P. Wyss, M. Carlsen, J. R. Zimmerman, B. H. Stirm, T. Q. Carney, R. Santini, and P. B. Shepson

Abstract

Although the ability to measure vertical eddy fluxes of gases from aircraft platforms represents an important capability to obtain spatially resolved data, accurate and reliable determination of the turbulent vertical velocity presents a great challenge. A nine-hole hemispherical probe known as the “Best Air Turbulence Probe” (often abbreviated as the “BAT Probe”) is frequently used in aircraft-based flux studies to sense the airflow angles and velocity relative to the aircraft. Instruments such as inertial navigation and global positioning systems allow the measured airflow to be converted into the three-dimensional wind velocity relative to the earth’s surface by taking into account the aircraft’s velocity and orientation. Calibration of the aircraft system has previously been performed primarily through in-flight experiments, where calibration coefficients were determined by performing various flight maneuvers. However, a rigorous test of the BAT Probe in a wind tunnel has not been previously undertaken.

The authors summarize the results of a complement of low-speed wind tunnel tests and in-flight calibrations for the aircraft–BAT Probe combination. Two key factors are addressed in this paper: The first is the correction of systematic error arising from airflow measurements with a noncalibrated BAT Probe. The second is the instrumental precision in measuring the vertical component of wind from the integrated aircraft-based wind measurement system. The wind tunnel calibration allows one to ascertain the extent to which the BAT Probe airflow measurements depart from a commonly used theoretical potential flow model and to correct for systematic errors that would be present if only the potential flow model were used. The precision in the determined vertical winds was estimated by propagating the precision of the BAT Probe data (determined from the wind tunnel study) and the inertial measurement precision (determined from in-flight tests). The precision of the vertical wind measurement for spatial scales larger than approximately 2 m is independent of aircraft flight speed over the range of airspeeds studied, and the 1σ precision is approximately 0.03 m s−1.

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R. H. Moss, S. Avery, K. Baja, M. Burkett, A. M. Chischilly, J. Dell, P. A. Fleming, K. Geil, K. Jacobs, A. Jones, K. Knowlton, J. Koh, M. C. Lemos, J. Melillo, R. Pandya, T. C. Richmond, L. Scarlett, J. Snyder, M. Stults, A. M. Waple, J. Whitehead, D. Zarrilli, B. M. Ayyub, J. Fox, A. Ganguly, L. Joppa, S. Julius, P. Kirshen, R. Kreutter, A. McGovern, R. Meyer, J. Neumann, W. Solecki, J. Smith, P. Tissot, G. Yohe, and R. Zimmerman

Abstract

As states, cities, tribes, and private interests cope with climate damages and seek to increase preparedness and resilience, they will need to navigate myriad choices and options available to them. Making these choices in ways that identify pathways for climate action that support their development objectives will require constructive public dialogue, community participation, and flexible and ongoing access to science- and experience-based knowledge. In 2016, a Federal Advisory Committee (FAC) was convened to recommend how to conduct a sustained National Climate Assessment (NCA) to increase the relevance and usability of assessments for informing action. The FAC was disbanded in 2017, but members and additional experts reconvened to complete the report that is presented here. A key recommendation is establishing a new nonfederal “climate assessment consortium” to increase the role of state/local/tribal government and civil society in assessments. The expanded process would 1) focus on applied problems faced by practitioners, 2) organize sustained partnerships for collaborative learning across similar projects and case studies to identify effective tested practices, and 3) assess and improve knowledge-based methods for project implementation. Specific recommendations include evaluating climate models and data using user-defined metrics; improving benefit–cost assessment and supporting decision-making under uncertainty; and accelerating application of tools and methods such as citizen science, artificial intelligence, indicators, and geospatial analysis. The recommendations are the result of broad consultation and present an ambitious agenda for federal agencies, state/local/tribal jurisdictions, universities and the research sector, professional associations, nongovernmental and community-based organizations, and private-sector firms.

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R. H. Moss, S. Avery, K. Baja, M. Burkett, A. M. Chischilly, J. Dell, P. A. Fleming, K. Geil, K. Jacobs, A. Jones, K. Knowlton, J. Koh, M. C. Lemos, J. Melillo, R. Pandya, T. C. Richmond, L. Scarlett, J. Snyder, M. Stults, A. Waple, J. Whitehead, D. Zarrilli, J. Fox, A. Ganguly, L. Joppa, S. Julius, P. Kirshen, R. Kreutter, A. McGovern, R. Meyer, J. Neumann, W. Solecki, J. Smith, P. Tissot, G. Yohe, and R. Zimmerman
Open access