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Oliver M. Sun, Steven R. Jayne, Kurt L. Polzin, Bryan A. Rahter, and Louis C. St. Laurent

Abstract

Data from three midlatitude, month-long surveys are examined for evidence of enhanced vertical mixing associated with the transition layer (TL), here defined as the strongly stratified layer that exists between the well mixed layer and the thermocline below. In each survey, microstructure estimates of turbulent dissipation were collected concurrently with fine-structure stratification and shear. Survey-wide averages are formed in a “TL coordinate” z TL, which is referenced around the depth of maximum stratification for each profile. Averaged profiles show characteristic TL structures such as peaks in stratification N 2 and shear variance S 2, which fall off steeply above z TL = 0 and more gradually below. Turbulent dissipation rates ɛ are 5–10 times larger than those found in the upper thermocline (TC). The gradient Richardson number Ri = N 2/S 2 becomes unstable (Ri < 0.25) within ~10 m of the TL upper boundary, suggesting that shear instability is active in the TL for z TL > 0. Ri is stable for z TL ≤ 0. Turbulent dissipation is found to scale exponentially with depth for z TL ≤ 0, but the decay scales are different for the TL and upper TC: ɛ scales well with either N 2 or S 2. Owing to the strong correlation between S 2 and N 2, existing TC scalings of the form ɛ ~ |S|p|N|q overpredict variations in ɛ. The scale dependence of shear variance is not found to significantly affect the scalings of ɛ versus N 2 and S 2 for z TL ≤ 0. However, the onset of unstable Ri at the top of the TL is sensitively dependent to the resolution of the shears.

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Bomin Sun, C. Bruce Baker, Thomas R. Karl, and Malcolm D. Gifford

Abstract

Temperature measurements from the U.S. Climate Reference Network (USCRN) instrument system were compared to the Automated Surface Observing System (ASOS) ambient air temperature measurements and were examined under different regimes of wind speed and solar radiation. Influences due to observing practice differences and the effects of siting differences were discussed.

This analysis indicated that the average difference between the ASOS and USCRN temperatures is on the order of 0.1°C. However, problems were noticed that were possibly related to the ASOS shield effectiveness, including a solar radiation warm effect under calm conditions and the dependence of ASOS minus USCRN temperature on wind speed.

The ASOS and USCRN time of observation difference was on the order of ∼0.05°C, with a warmer ASOS daily T max and a cooler ASOS daily T min. The local effect complicates the bias analysis because it depends not only on local heating/cooling, but it can be strongly modified by cloudiness, wind, and solar radiation.

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Woosuk Choi, Chang-Hoi Ho, Doo-Sun R. Park, Jinwon Kim, and Johnny C. L. Chan

Abstract

Prediction of tropical cyclone (TC) activity is essential to better prepare for and mitigate TC-induced disasters. Although many studies have attempted to predict TC activity on various time scales, very few have focused on near-future predictions. Here a decrease in seasonal TC activity over the North Atlantic (NA) for 2016–30 is shown using a track-pattern-based TC prediction model. The TC model is forced by long-term coupled simulations initialized using reanalysis data. Unfavorable conditions for TC development including strengthened vertical wind shear, enhanced low-level anticyclonic flow, and cooled sea surface temperature (SST) over the tropical NA are found in the simulations. Most of the environmental changes are attributable to cooling of the NA basinwide SST (NASST) and more frequent El Niño episodes in the near future. The consistent NASST warming trend in the projections from phase 5 of the Coupled Model Intercomparison Project (CMIP5) suggests that natural variability is more dominant than anthropogenic forcing over the NA in the near-future period.

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Dasol Kim, Chang-Hoi Ho, Doo-Sun R. Park, Johnny C. L. Chan, and Youngsun Jung

Abstract

In this study, the variation of tropical cyclone (TC) rainfall area over the subtropical oceans is investigated using the Tropical Rainfall Measuring Mission precipitation data collected from 1998 to 2014, with a focus on its relationship with environmental conditions. In the subtropics, higher moving speed and larger vertical wind shear significantly contribute to an increase in TC rainfall area by making horizontal rainfall distribution more asymmetric, while sea surface temperature rarely affects the fluctuation of TC rainfall area. This relationship between TC rainfall area and environmental conditions in the subtropics is almost opposite to that in the tropics. It is suggested that, in the subtropics, unlike the tropics, dynamic environmental conditions are likely more crucial to varying TC rainfall area than thermodynamic environmental ones.

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Ryan C. Scott, Timothy A. Myers, Joel R. Norris, Mark D. Zelinka, Stephen A. Klein, Moguo Sun, and David R. Doelling

Abstract

Understanding how marine low clouds and their radiative effects respond to changing meteorological conditions is crucial to constrain low-cloud feedbacks to greenhouse warming and internal climate variability. In this study, we use observations to quantify the low-cloud radiative response to meteorological perturbations over the global oceans to shed light on physical processes governing low-cloud and planetary radiation budget variability in different climate regimes. We assess the independent effect of perturbations in sea surface temperature, estimated inversion strength, horizontal surface temperature advection, 700-hPa relative humidity, 700-hPa vertical velocity, and near-surface wind speed. Stronger inversions and stronger cold advection greatly enhance low-level cloudiness and planetary albedo in eastern ocean stratocumulus and midlatitude regimes. Warming of the sea surface drives pronounced reductions of eastern ocean stratocumulus cloud amount and optical depth, and hence reflectivity, but has a weaker and more variable impact on low clouds in the tropics and middle latitudes. By reducing entrainment drying, higher free-tropospheric relative humidity enhances low-level cloudiness. At low latitudes, where cold advection destabilizes the boundary layer, stronger winds enhance low-level cloudiness; by contrast, wind speed variations have weak influence at midlatitudes where warm advection frequently stabilizes the marine boundary layer, thus inhibiting vertical mixing. These observational constraints provide a framework for understanding and evaluating marine low-cloud feedbacks and their simulation by models.

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C.B. Leovy, C-R. Sun, M.H. Hitchman, E.E. Remsberg, J.M. Russell, III, L.L. Gordley, J.C. Gille, and L.V. Lyjak

Abstract

Data from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) for the period 25 October 1978–28 May 1979 are used in a descriptive study of ozone variations in the middle stratosphere. It is shown that the ozone distribution is strongly influenced by irreversible deformation associated with large amplitude planetary-scale waves. This process, which has been described by McIntyre and Palmer as planetary wave breaking, takes place throughout the 3–30 mb layer, and poleward transport of ozone within this layer occurs in narrow tongues drawn out of the tropics and subtropics in association with major and minor warming events. Thew events complement the zonal mean diabatic circulation in producing significant changes in the total column amount of ozone.

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S. P. Burns, J. Sun, A. C. Delany, S. R. Semmer, S. P. Oncley, and T. W. Horst

Abstract

Techniques for improving the relative accuracy of longwave radiation measurements by a set of pyrgeometers [the Eppley Laboratory Precision Infrared Radiometer (Model PIR)] are presented using 10 PIRs from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). The least squares–based optimization technique uses a field intercomparison (i.e., a time period during which all the PIRs were upward looking and set up side by side) to determine a set of optimization coefficients for each PIR. For the 10 CASES-99 PIRs, the optimization technique improved the standard deviation of the difference of downwelling irradiance between the PIRs from ±0.75 to ±0.4 W m−2 (for nighttime data). In addition to presenting the optimization method, various PIR data quality checks are outlined and applied to the PIR data. Based on these quality checks, the measured case and dome temperatures of the CASES-99 PIRs were all reasonable. Using the 10 CASES-99 PIRs, simple estimates of the average nighttime net radiative flux divergence within the layer between 2 and 48 m were determined and resulted in cooling rates over a range from 0 to −1.3°C h−1, depending on the assumptions made for the upwelling irradiance at 2 m. The effect of the coefficient optimization on the calculated net radiative flux divergence is explored.

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J. Jin, X. Gao, Z.-L. Yang, R. C. Bales, S. Sorooshian, R. E. Dickinson, S. F. Sun, and G. X. Wu

Abstract

A comparative study of three snow models with different complexities was carried out to assess how a physically detailed snow model can improve snow modeling within general circulation models. The three models were (a) the U.S. Army Cold Regions Research and Engineering Laboratory Model (SNTHERM), which uses the mixture theory to simulate multiphase water and energy transfer processes in snow layers; (b) a simplified three-layer model, Snow–Atmosphere–Soil Transfer (SAST), which includes only the ice and liquid-water phases;and (c) the snow submodel of the Biosphere–Atmosphere Transfer Scheme (BATS), which calculates snowmelt from the energy budget and snow temperature by the force–restore method. Given the same initial conditions and forcing of atmosphere and radiation, these three models simulated time series of snow water equivalent, surface temperature, and fluxes very well, with SNTHERM giving the best match with observations and SAST simulation being close. BATS captured the major processes in the upper portion of a snowpack where solar radiation provides the main energy source and gave satisfying results for seasonal periods. Some biases occurred in BATS surface temperature and energy exchange due to its neglecting of liquid water and underestimating snow density. Ice heat conduction, meltwater heat transport, and the melt–freeze process of snow exhibit strong diurnal variations and large gradients at the uppermost layers of snowpacks. Using two layers in the upper 20 cm and one deeper layer at the bottom to simulate the multiphase snowmelt processes, SAST closely approximated the performance of SNTHERM with computational requirements comparable to those of BATS.

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J. D. Doyle, D. R. Durran, C. Chen, B. A. Colle, M. Georgelin, V. Grubisic, W. R. Hsu, C. Y. Huang, D. Landau, Y. L. Lin, G. S. Poulos, W. Y. Sun, D. B. Weber, M. G. Wurtele, and M. Xue

Abstract

Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave breaking was impacted by the numerical dissipation, numerical representation of the horizontal advection, and lateral boundary conditions. Small vertical wavelength variations in the shear and stability above 10 km contributed to significant changes in the structures associated with wave breaking. Simulation of this case is ideal for testing and evaluation of mesoscale numerical models and numerical algorithms because of the complex wave-breaking response.

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J. Sun, S. P. Burns, D. Vandemark, M. A. Donelan, L. Mahrt, Timothy L. Crawford, T. H. C. Herbers, G. H. Crescenti, and J. R. French

Abstract

A remote sensing method to measure directional oceanic surface waves by three laser altimeters on the NOAA LongEZ aircraft is investigated. To examine feasibility and sensitivity of the wavelet analysis method to various waves, aircraft motions, and aircraft flight directions relative to wave propagation directions, idealized surface waves are simulated from various idealized aircraft flights. In addition, the wavelet analysis method is also applied to two cases from field measurements, and the results are compared with traditional wave spectra from buoys. Since the wavelet analysis method relies on the “wave slopes” measured through phase differences between the time series of the laser distances between the aircraft and sea surface at spatially separated locations, the resolved directional wavenumber and wave propagation direction are not affected by aircraft motions if the resolved frequencies of the aircraft motion and the wave are not the same. However, the encounter wave frequency, which is directly resolved using the laser measurement from the moving aircraft, is affected by the Doppler shift due to aircraft motion relative to wave propagations. The wavelet analysis method could fail if the aircraft flies in the direction such that the aircraft speed along the wave propagation direction is the same as the wave phase speed (i.e., the aircraft flies along wave crests or troughs) or if two waves with different wavelengths and phase speed have the same encountered wavelength from the aircraft. In addition, the data noise due to laser measurement uncertainty or natural isotropic surface elevation perturbations can also affect the relative phase difference between the laser distance measurements, which in turn affects the accuracy of the resolved wavenumber and wave propagation direction. The smallest waves measured by the lasers depend on laser sampling rate and horizontal distances between the lasers (for the LongEZ this is 2 m). The resolved wave direction and wavenumber at the peak wave from the two field experiments compared well with on-site buoy observations. Overall, the study demonstrates that three spatially separated laser altimeters on moving platforms can be utilized to resolve two-dimensional wave spectra.

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