Advances in Upper Atmosphere Forecasting: NOGAPS-ALPHA

Introduction: The Navy Operational Global Atmospheric Prediction System (NOGAPS) provides detailed atmospheric analyses and forecasts for numerous Department of Defense (DOD) applications. The heart of NOGAPS is an atmospheric general-circulation model (GCM) with an operational capability currently extending from the surface to 30 km altitude. NRL scientists from the Space Science, Remote Sensing, and Marine Meteorology Divisions have been working jointly on a multi-year project to develop a new high-altitude version of NOGAPS extending up to 85 km and beyond (Fig. 8). There is a growing recognition in the atmospheric science community that weather disturbances at higher altitudes can penetrate to lower altitudes and influence development of surface weather features. The primary goal of this project is to support all aspects of Naval operations through better prediction of surface conditions as well as tropospheric and upper atmospheric winds, temperature, moisture, and clouds. In particular, the upward extension of the existing model now makes it possible to better understand and predict upper atmospheric disturbances that can affect satellite remote sensing applications, vehicle reentry, high-altitude unmanned aerial vehicle (UAV) performance, and high-altitude dispersal of explosively injected material. The product of this effort is the NOGAPS-ALPHA (Advanced Level Physics-High Altitude) model.

FIGURE 8
The atmospheric regions covered by the current operational version of the NOGAPS model and the newly developed NOGAPS-ALPHA model. Arrows represent wave-induced transport throughout the troposphere, stratosphere, and mesosphere.

Implementation and Innovations: NOGAPS-ALPHA is a spectral model with 54 vertical levels from the surface to approximately 85 km on a hybrid sigma-pressure grid. This new vertical grid is terrain-following near the surface, transitioning to constant pressure levels in the lower stratosphere. NOGAPS-ALPHA contains a new radiative transfer scheme and updated climatologies for radiatively active trace species such as ozone and water vapor. NOGAPS-ALPHA also uses new initialization routines for winds and temperatures in the upper stratosphere and mesosphere, beyond the range of current operational analyses. New physical packages have been added describing the effects of sub-grid scale gravity waves on the global atmospheric circulation. In addition, the model now combines global transport calculations of chemical constituents such as stratospheric ozone with an interactive photochemical scheme to describe the time evolution of these constituents over periods of days, weeks, or months. Multi-year model runs have also been performed to verify the performance of the new high-altitude components of the NOGAPS model. These simulations extend to 5 years and were performed on a suite of massively parallel supercomputers made available through the DOD High Performance Computing Modernization Program.

Simulations of the 2002 Ozone Hole: The unprecedented southern hemisphere stratospheric major warming event during September 2002, provides an excellent opportunity to showcase the new capabilities of NOGAPS-ALPHA. On September 23, 2002, the Antarctic polar vortex began to split in two following a series of large planetary-scale wave disturbances in the southern hemisphere winter stratosphere (see Fig. 9) that brought unusually warm air over the South Pole. As a result, the size and depth of the springtime ozone hole over Antarctica decreased to its smallest value in over 20 years. This was the first time a stratospheric major warming has occurred over the South Pole since observations began. Figure 9 shows that NOGAPS-ALPHA hindcasts of 10 hPa geopotential height and total ozone column amounts initialized on September 23, 2002, compare well with NASA GEOS 4 meteorological analyses and NASA TOMS total ozone measurements during the period of September 23 to September 27, 2002.

FIGURE 9
NOGAPS-ALPHAS hindcasts of the September 2002, stratospheric major sudden warming and early break-up of the antarctic ozone hole. Also shown are NASA GEOS4 meteorological analyses and TOMS total ozone observatons. Total ozone is measured in Dobson Units (DU).

Tracer Transport: With the new NOGAPS-ALPHA model, it is now possible to diagnose global-scale constituent transport throughout the stratosphere. As planetary wave disturbances propagate upwards from the troposphere into the stratosphere and mesosphere, they grow in amplitude until they "break," resulting in an irreversible transfer of heat and momentum. The resulting motions transport chemical constituents such as ozone and nitrous oxide (N₂O) across regions where the mixing of air is otherwise minimal under quiescent conditions. Figure 10 shows a NOGAPS-ALPHA simulation of the effect of a planetary wave-breaking event on the distribution of N₂O near 32 km altitude, where a large "tongue" of air with high N₂O mixing ratio is stripped out of the lower latitudes and transported poleward into a region of relatively low N₂O mixing ratio.

FIGURE 10
NOGAPS-ALPHA simulations of stratospheric N₂O mixing ratio in parts per billion by volume illustrating complex poleward transport of air with high N₂O mixing ratios during a planetary wave-breaking event.

Summary: NOGAPS-ALPHA, a high-altitude version of the Navy's global numerical weather prediction model, now makes it possible to generate both medium-range forecasts (1 to 2 weeks) and multi-year climate simulations from the surface to 85 km altitude. With its state-of-the-art treatment of the physical and dynamical processes coupling the tropospheric, stratospheric, and mesospheric regions of the atmosphere, NOGAPS-ALPHA opens the door to the next generation in Navy forecasting and analysis capabilities. Linking NOGAPS-ALPHA with the new NRL Atmospheric Variational Data Assimilation Scheme (NAVDAS) is the next step in fully developing and exploiting these new capabilities.

[Sponsored by ONR]