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Xiaodong Hong, Martin J. Leach, and Sethu Raman

Abstract

Variable vegetation cover is a possible trigger for convection, especially in semiarid areas due to differential surface forcing. A two-dimensional numerical model with explicit cloud physics and a detailed vegetation parameterization scheme is used to investigate the role of vegetation differences in triggering convective cloud formation. The ground surface in all simulations includes two irrigated vegetation areas with a dry steppe in the center of the domain. The effects of atmospheric stability, ambient moisture profile, and horizontal heating scale are investigated.

Atmospheric stability controls the growth of convective circulations. Thermal circulations form at the interfaces between the vegetated areas and the dry steppe. In the more stable environment, two distinct convective cells persist; they merge into one cell in the less stable cases. The existence of low-level moisture controls the timing and persistence of clouds that form. An interesting result is the earlier dissipation of clouds in less stable cases, as greater mixing with drier air from aloft leads to the dilution of the cloud water. Since the largest thermal forcing exists at the interfaces, length of the steppe interacts with the stability to control the merger of the cells. The two cells merge quickly into one for narrow horizontal heating. For the widest heating scale studied, no merger occurs.

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Carolyn A. Reynolds, James D. Doyle, and Xiaodong Hong

Abstract

The initial-state sensitivity and interactions between a tropical cyclone and atmospheric equatorial Kelvin waves associated with the Madden–Julian oscillation (MJO) during the DYNAMO field campaign are explored using adjoint-based tools from the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). The development of Tropical Cyclone 5 (TC05) coincided with the passage of an equatorial Kelvin wave (KW) and westerly wind burst associated with an MJO that developed in the Indian Ocean in late November 2011. COAMPS 18-h adjoint sensitivities of low-level kinetic energy to changes in initial state winds, temperature, and water vapor are analyzed for both TC05 and the KW to document when the evolution of each system is sensitive to the other. Time series of sensitivity patterns confirm that TC05 and the KW low-level westerlies are sensitive to each other when the KW is to the southwest and south of TC05. While TC05 is not sensitive to the KW after this, the KW low-level westerlies remain sensitive to TC05 until it enters the far eastern Indian Ocean. Vertical profiles of both TC05 and KW sensitivity indicate lower-tropospheric maxima in temperature, wind, and moisture, with KW sensitivity typically 20% smaller than TC05 sensitivity. The magnitude of the sensitivity for both systems is greatest just prior to, and during, their closest proximity. A case study examination reveals that adjoint-based optimal perturbations grow and expand quickly through a dynamic response to decreased static stability. The evolution of moist-only and dry-only initial perturbations illustrates that the moist component is primarily responsible for the initial rapid growth, but that subsequent growth rates are similar.

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Xiaodong Hong, Craig H. Bishop, Teddy Holt, and Larry O’Neill

Abstract

This paper examines the sensitivity of short-term forecasts of the western North Pacific subtropical high (WNPSH) and rainfall to sea surface temperature (SST) uncertainty using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). A comparison of analyzed SSTs with satellite observations of SST indicates that SST analysis errors are particularly pronounced on horizontal scales from 100 to 200 km, similar to the mesoscale eddy scales in the Kuroshio region. Since significant oceanic variations occur on these scales, it is of interest to examine the effects of representing this small-scale uncertainty with random, scale-dependent perturbations. An SST ensemble perturbation generation technique is used here that enables temporal and spatial correlations to be controlled and produces initial SST fields comparable to satellite observations. The atmospheric model develops large uncertainty in the Korea and Japan area due to the fluctuation in the horizontal pressure gradient caused by the location of the WNPSH. This, in turn, increases the variance of the low-level jet (LLJ) over southeast China, resulting in large differences in the moist transport flux from the tropical ocean and subsequent rainfall. Validation using bin-mean statistics shows that the ensemble forecast with the perturbed SST better distinguishes large forecast error variance from small forecast error variance. The results suggest that using the SST perturbation as a proxy for the ocean ensemble in a coupled atmosphere and ocean ensemble system is feasible and computationally efficient.

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Xiaodong Hong, Simon W. Chang, Sethu Raman, Lynn K. Shay, and Richard Hodur

Abstract

Hurricane Opal (1995) experienced a rapid, unexpected intensification in the Gulf of Mexico that coincided with its encounter with a warm core ring (WCR). The relative positions of Opal and the WCR and the timing of the intensification indicate strong air–sea interactions between the tropical cyclone and the ocean. To study the mutual response of Opal and the Gulf of Mexico, a coupled model is used consisting of a nonhydrostatic atmospheric component of the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), and the hydrostatic Geophysical Fluid Dynamics Laboratory’s Modular Ocean Model version 2 (MOM 2).

The coupling between the ocean and the atmosphere components of the model are accomplished by conservation of heat, salt, momentum, as well as the sensible and latent heat fluxes at the air–sea interface. The atmospheric model has two nests with spatial resolutions of 0.6° and 0.2°. The ocean model has a uniform resolution of 0.2°. The oceanic model domain covers the Gulf of Mexico basin and coincides with a fine-mesh atmospheric domain of the COAMPS. The initial condition for the atmospheric component of COAMPS is the archived Navy Operational Global Atmospheric Prediction System operational global analysis, enhanced with observations. The initial ocean condition for the oceanic component is obtained from a 2-yr MOM 2 simulation with climatological forcing and fixed mass inflow into the Gulf. The initial state in the Gulf of Mexico consists of a realistic Loop Current and a shed WCR.

The 72-h simulation of the coupled system starting from 1200 UTC 2 October 1995 reproduces the observed storm intensity with a minimum sea level pressure (MSLP) of 918 hPa, occurring at 1800 UTC 4 October, a 6-h delay compared to the observation. The rapid intensification to the maximum intensity and the subsequent weakening are not as dramatic as the observed. The simulated track is located slightly to the east of the observed track, placing it directly over the simulated WCR, where the sea surface temperature (SST) cooling is approximately 0.5°C, consistent with buoy measurements acquired within the WCR. This cooling is significantly less over the WCR than over the common Gulf water due to the deeper and warmer layers in the WCR. Wind-induced currents of 150 cm s−1 are similar to those in earlier idealized simulations, and the forced current field in Opal’s wake is characterized by near-inertial oscillations superimposed on the anticyclonic circulation around the WCR.

Several numerical experiments are conducted to isolate the effects of the WCR and the ocean–atmosphere coupling. The major findings of these numerical experiments are summarized as follows.

  1. Opal intensifies an additional 17 hPa between the times when Opal’s center enters and exits the outer edge of the WCR. Without the WCR, Opal only intensifies another 7 hPa in the same period.

  2. The maximum surface sensible and latent heat flux amounts to 2842 W m−2. This occurs when Opal’s surface circulation brings northwesterly flow over the SST gradient in the northwestern quadrant of the WCR.

  3. Opal extracts 40% of the available heat capacity (temperature greater than 26°C) from the WCR.

  4. While the WCR enhances the tropical cyclone and ocean coupling as indicated by strong interfacial fluxes, it reduces the negative feedback. The negative feedback of the induced SST cooling to Hurricane Opal is 5 hPa. This small feedback is due to the relatively large heat content of the WCR, and the negative feedback is stronger in the absence of the WCR, producing a difference of 8 hPa in the MSLP of Opal.

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Yingying Sha, Zhengguo Shi, Xiaodong Liu, Zhisheng An, Xinzhou Li, and Hong Chang

Abstract

Numerical simulations were conducted to determine the impact of the Tian Shan Mountains and Pamir Plateau on arid conditions over interior Asia. These topographies are crucial for the differentiation of the precipitation seasonality among the subregions in the west, east, and north of the Tian Shan Mountains and Pamir Plateau, namely, arid central Asia, the Tarim basin, and the northern plains. Before the uplift of the Tian Shan Mountains and Pamir Plateau, the precipitation seasonality over the east arid subregion was consistent with that over the west arid subregion, with maximum rainfall in spring and winter and minimum rainfall in summer. After the uplift of the Tian Shan Mountains and Pamir Plateau, the original precipitation seasonality in the west was strengthened. As the precipitation in the east arid subregion increased in summer but decreased in winter and spring, the precipitation seasonality in the east changed to peak in summer, while the precipitation in the north arid subregion showed the opposite change. The precipitation alteration corresponded well with the change of vertical motion. With the modulation of atmospheric stationary waves, the remote East Asian monsoon was also impacted. Though enhanced southerly wind blew over East Asia, the monsoon precipitation over the east coast of China and subtropical western Pacific Ocean was significantly reduced as an anticyclonic circulation appeared. The Tian Shan Mountains and Pamir Plateau also contributed to the intensification of the East Asian winter monsoon.

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William A. Komaromi, Xiaodong Hong, Matthew A. Janiga, Carolyn A. Reynolds, James A. Ridout, and James D. Doyle

Abstract

Given the prohibitive expense of running a global coupled high-resolution model for multiweek forecasts, we explore the feasibility of running a limited-area model forced by a global model on monthly time scales. Specifically, we seek to understand the constraints of the accuracy of lateral boundary conditions (LBCs) produced by NAVGEM on the skill of limited-area COAMPS forecasts. In this study, we analyze simulations of the successive MJO events of November 2011. In the NAVGEM simulations, the effect of ocean boundary conditions are examined, including fixed sea surface temperature (SST), observed SST, and coupled SST with HYCOM. With fixed SST, the second MJO fails to develop, highlighting the importance of the ocean response in the ability to model successive MJO events. Next, we examine the dependence of the regional COAMPS skill on the global model forecast performance. It is found that even when using the inferior but computationally inexpensive uncoupled NAVGEM for LBCs, coupled COAMPS can accurately predict the successive MJO events. A well-resolved atmospheric Rossby wave that slowly propagates westward in the COAMPS domain contributes to increased predictive skill. Ocean coupling and the ability of the model to sufficiently warm the ocean during the convectively suppressed phase also appears to be critical. Last, while COAMPS exhibits a significant moist bias, the sign and magnitude of the vertical and horizontal moisture flux appear to be consistent with reanalysis, a necessary attribute of any model to be used in multiweek MJO prediction.

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