Rashid Akmaev

First forecast of a sudden stratospheric warming with a coupled whole‐atmosphere/ionosphere model IDEA

We present the first “weather forecast” with a coupled whole-atmosphere/ionosphere model of Integrated Dynamics in Earths Atmosphere (IDEA) for the January 2009 Sudden Stratospheric Warming (SSW). IDEA consists of the Whole Atmosphere Model and Global Ionosphere-Plasmasphere model. A 30 day forecast is performed using the IDEA model initialized at 0000 UT on 13 January 2009, 10 days prior to the peak of the SSW. IDEA successfully predicts both the time and amplitude of the peak warming in the polar cap. This is about 2 days earlier than the National Centers for Environmental Prediction operational Global Forecast System terrestrial weather model forecast. The forecast of the semidiurnal, westward propagating, zonal wave number 2 (SW2) tide in zonal wind also shows an increase in the amplitude and a phase shift to earlier hours in the equatorial dynamo region during and after the peak warming, before recovering to their prior values about 15 days later. The SW2 amplitude and phase changes are shown to be likely due to the stratospheric ozone and/or circulation changes. The daytime upward plasma drift and total electron content in the equatorial American sector show a clear shift to earlier hours and enhancement during and after the peak warming, before returning to their prior conditions. These ionospheric responses compare well with other observational studies. Therefore, the predicted ionospheric response to the January 2009 SSW can be largely explained in simple terms of the amplitude and phase changes of the SW2 zonal wind in the equatorial E region.

Ionospheric response to sudden stratospheric warming events at low and high solar activity

The sensitivity of the ionospheric response to a sudden stratospheric warming (SSW) event has been examined under conditions of low and high solar activity through simulations using the whole atmosphere model (WAM) and the global ionosphere plasmasphere model (GIP). During non-SSW conditions, simulated daytime mean vertical drifts at the magnetic equator show similar solar activity dependence as an empirical model. Model results of ionospheric total electron content (TEC) and equatorial vertical drift for the January 2009 major SSW, which occurred at very low solar activity conditions, show reasonable agreement with observations. The simulations demonstrate that the E region dynamo is capable of creating the semidiurnal variation of vertical drift. WAM and GIP were also run at high solar activity conditions, using the same lower atmosphere conditions as present in the January 2009 SSW event. The simulations indicate that the amplitude and phase of migrating tides in the dynamo region during the event have similar magnitudes for both solar flux conditions. However, comparing the ionospheric responses to a major SSW under low and high solar activity periods, it was found that the changes in the ionospheric vertical drifts and relative changes in TEC decreased with increasing solar activity. The simulations indicate that the F region dynamo becomes more important throughout the daytime and contributes to the upward drift in the afternoon during the event when the solar activity is higher. Our test simulations also confirm that the increase of the ionospheric conductivity associated with increasing solar activity is responsible for the decrease of vertical drift changes during an SSW. In particular, first, the increase in F region conductivity allows the closure of E region currents through the F region, reducing the polarization electric field before noon. Second, the F region dynamo contributes an upward drift postnoon, maintaining upward drifts till after sunset. The direct changes of the thermospheric wind at higher solar activity due to increased dissipation of the tides from the lower atmosphere are relatively minor and do not contribute greatly to the changes of ionospheric responses in the low-latitude region.

Diagnostics and simulation of an annual cycle in the middle atmosphere

A global spectral model extending approximately from 15 up to 120 km is used in both diagnostics and simulations of the observed monthly middle atmospheric climatology as represented in the CIRA-86 empirical model. First, the model in a two-dimensional mode is initialized with the empirical temperature and wind distributions for each month. An analog of the nudging data assimilation technique is then applied to retrieve zonal mean zonal momentum deposition rates necessary to keep the models state in the vicinity of the observed climatology. This is combined with a straightforward procedure for estimating the global mean vertical diffusion coefficient from the thermodynamic equation averaged globally for each month. These estimates agree surprisingly well with recent observations in the mesosphere and lower thermosphere. At the second stage the inferred eddy diffusivities and zonal accelerations that are commonly believed to be produced by dissipating and breaking atmospheric waves of different scales are used as input in annual cycle integrations of the model. For the first time quantitative comparisons of the simulated and initial diagnosed climatologies are presented. The simulation results obtained using the diagnosed zonal forcing and vertical diffusion coefficients compare favorably with the empirical model on the one hand and with simulations using a gravity wave parameterization on the other. In particular, the empirical temperature model is reproduced with an annual global rms temperature deviation of 3.2 K or 2% in the 15- to 110-km layer.

A new source of the midlatitude ionospheric peak density structure revealed by a new Ionosphere‐Plasmasphere model

The newly developed Ionosphere-Plasmasphere (IP) model has revealed neutral winds as a primary source of the “third-peak” density structure in the daytime global ionosphere that has been observed by the low-latitude ionospheric sensor network GPS total electron content measurements over South America. This third peak is located near −30° magnetic latitude and is clearly separate from the conventional twin equatorial ionization anomaly peaks. The IP model reproduces the global electron density structure as observed by the FORMOSAT-3/COSMIC mission. The model reveals that the third peak is mainly created by the prevailing neutral meridional wind, which flows from the summer hemisphere to the winter hemisphere lifting the plasma along magnetic field lines to higher altitudes where recombination is slower. The same prevailing wind that increases the midlatitude density decreases the low-latitude density in the summer hemisphere by counteracting the equatorial fountain flow. The longitudinal variation of the three-peak structure is explained by the displacement between the geographic and geomagnetic equators.

Tides in the mesopause region over Antarctica: Comparison of whole atmosphere model simulations with ground‐based observations

Almost a quarter century ago first optical and radar observations from the South Pole revealed rich dynamics unexpected from classical tidal theory. A strong semidiurnal wind oscillation was detected near the mesopause implying substantial deviations from the classical view that the semidiurnal variation is dominated by the migrating tide. Subsequent systematic observations exhibited large seasonal variations of both the diurnal and semidiurnal tide with dramatic reduction in amplitude from summer to winter. First numerical simulations with a realistic general circulation model extending into the lower thermosphere indicated the presence of nonmigrating tides with substantial amplitudes in the polar regions. However, direct model-data comparisons have been limited to idealized linear models. Here whole atmosphere model (WAM) simulations for January and July are compared with available wind climatologies based on multiyear radar observations at different locations in Antarctica as well as with first summertime lidar measurements of temperature. The diurnal tide simulation agrees well with most of the independent radar and satellite wind observations in both seasons. The strong semidiurnal tide comprised of migrating and nonmigrating components is well reproduced in summer, while in winter the model tends to overestimate the amplitudes over the continental edge. Besides model validation, a self-consistent numerical solution also enables cross validation of observations made with different instruments at different locales.