TY  - JOUR
A1  - Gottwald, Georg A.
A1  - Mitchell, Lewis
A1  - Reich, Sebastian
T1  - Controlling overestimation of error covariance in ensemble kalman filters with sparse observations a variance-limiting kalman filter
JF  - Monthly weather review
N2  - The problem of an ensemble Kalman filter when only partial observations are available is considered. In particular, the situation is investigated where the observational space consists of variables that are directly observable with known observational error, and of variables of which only their climatic variance and mean are given. To limit the variance of the latter poorly resolved variables a variance-limiting Kalman filter (VLKF) is derived in a variational setting. The VLKF for a simple linear toy model is analyzed and its range of optimal performance is determined. The VLKF is explored in an ensemble transform setting for the Lorenz-96 system, and it is shown that incorporating the information of the variance of some unobservable variables can improve the skill and also increase the stability of the data assimilation procedure.
Y1  - 2011
U6  - https://doi.org/10.1175/2011MWR3557.1
SN  - 0027-0644
VL  - 139
IS  - 8
SP  - 2650
EP  - 2667
PB  - American Meteorological Soc.
CY  - Boston
ER  - 
TY  - JOUR
A1  - Jones, Geraint H.
A1  - Arridge, Christopher S.
A1  - Coates, Andrew J.
A1  - Lewis, Gethyn R.
A1  - Kanani, Sheila
A1  - Wellbrock, Anne
A1  - Young, David T.
A1  - Crary, Frank J.
A1  - Tokar, Robert L.
A1  - Wilson, R. J.
A1  - Hill, Thomas W.
A1  - Johnson, Robert E.
A1  - Mitchell, Donald G.
A1  - Schmidt, Jürgen
A1  - Kempf, Sascha
A1  - Beckmann, Uwe
A1  - Russell, Christopher T.
A1  - Jia, Y. D.
A1  - Dougherty, Michele K.
A1  - Waite, J. Hunter
A1  - Magee, Brian A.
T1  - Fine jet structure of electrically charged grains in Enceladus' plume
N2  - By traversing the plume erupting from high southern latitudes on Saturn's moon Enceladus, Cassini orbiter instruments can directly sample the material therein. Cassini Plasma Spectrometer, CAPS, data show that a major plume component comprises previously-undetected particles of nanometer scales and larger that bridge the mass gap between previously observed gaseous species and solid icy grains. This population is electrically charged both negative and positive, indicating that subsurface triboelectric charging, i.e., contact electrification of condensed plume material may occur through mutual collisions within vents. The electric field of Saturn's magnetosphere controls the jets' morphologies, separating particles according to mass and charge. Fine-scale structuring of these particles' spatial distribution correlates with discrete plume jets' sources, and reveals locations of other possible active regions. The observed plume population likely forms a major component of high velocity nanometer particle streams detected outside Saturn's magnetosphere.
Y1  - 2009
UR  - http://www.agu.org/journals/gl/
U6  - https://doi.org/10.1029/2009gl038284
SN  - 0094-8276
ER  -