Regular Latitude Longitude Research Articles

Daily Rainfall for the Indian Monsoon Region from Merged Satellite and Rain Gauge Values: Large-Scale Analysis from Real-Time Data

Abstract A system for objectively producing daily large-scale analysis of rainfall for the Indian region has been developed and tested by using only available real-time rain gauge data and quantitative precipitation estimates from INSAT-1D IR data. The system uses a successive correction method to produce the analysis on a regular latitude–longitude grid. Quantitative precipitation estimates from the Indian National Satellite System (INSAT) operational geostationary satellite, INSAT-1D, IR data are used as the initial guess in the objective analysis method. Accumulated 24-h (daily) rainfall analyses are prepared each day by merging satellite and rain gauge data. The characteristics of the output from this analysis system have been examined by comparing the accumulated monthly observed rainfall with other available independent widely used datasets from the Global Precipitation Climatology Project (GPCP) and Climate Prediction Center Merged Analysis of Precipitation (CMAP) analyses. The monthly data prepare...

Vorticity-Divergence Semi-Lagrangian Shallow-Water Model of the Sphere Based on Compact Finite Differences

The semi-Lagrangian representation of advection allows circumventing of the CFL restriction on time steps, which is especially severe for finite-difference models on the regular latitude–longitude grid. The distinct features of the presented semi-Lagrangian model are the use of vorticity and divergence as prognostic variables in conjunction with the fourth-order compact finite differences on the unstaggered regular latitude–longitude grid. The key point of this approach is the solution of the Poisson equations on the sphere, which is necessary for reconstructing the velocity field from vorticity and divergence. The accurate and efficient direct solver for this problem is described. The results of the standard test set for shallow-water equations on the sphere demonstrate the accuracy and computational efficiency of the model with the time steps several times greater than in the Eulerian model.

The Mass-Conservative Cell-Integrated Semi-Lagrangian Advection Scheme on the Sphere

A mass-conservative cell-integrated semi-Lagrangian (CISL) scheme is presented and tested for 2D transport on the sphere. The total mass is conserved exactly and the mass of each individual grid cell is conserved in general. The scheme is based on a general scheme developed by Machenhauer and Olk that has increased cost effectiveness without loss of accuracy, compared to the CISL scheme of Rančić. A regular latitude–longitude grid is used on the sphere and upstream trajectories from the corner points of the regular grid cells (the Eulerian cells) define the corner points of the departure cells. The sides in these so-called Lagrangian cells are generally defined as straight lines in a (λ, μ) plane, where λ is the longitude and μ is the sine of the latitude. The mass distribution within each Eulerian grid cell is defined by quasi-biparabolic functions, which are used to integrate analytically the mass in each Lagrangian computational cell. The auxiliary computational cells are polygons with each side parallel to the coordinate axis. Also, the computational cells have the same area as the Lagrangian cells they approximate. They were introduced in order to simplify the analytical integrals of mass. Near the poles, the east and west sides of certain Lagrangian cells cannot be approximated by straight lines in the (λ, μ) plane, and are instead represented by straight lines in polar tangent plane coordinates. Each of the latitudinal belts of Lagrangian cells in the polar caps are split up into several latitudinal belts of subcells, which can be approximated by computational cells as in the case of cells closer to the equator. One latitudinal belt in each hemisphere, which encloses the Eulerian pole (singular belt), is treated in a special way. First the total mass in the singular belt is determined and then it is redistributed among the cells in the belt using weights determined by a traditional SL scheme at the midpoints of the cells. By this procedure the total mass is still conserved while the conservation is only approximately maintained for the individual cells in the singular belt. These special treatments in the polar caps fit well into the general structure of the code and can be implemented with minor modifications in the code used for the rest of the sphere. Compared to two other conservative advection schemes implemented on the sphere the CISL scheme used here was found to be competitive in terms of accuracy for the same resolution. In addition the CISL scheme has the advantage over these schemes that it is applicable for Courant numbers larger than one. In plane geometry the scheme of Rančić had an overhead factor of 2.5 in CPU time compared to a traditional bicubic semi-Lagrangian scheme. This factor is reduced to 1.1 for the Machenhauer and Olk scheme on the plane while on the sphere the factor is found to be 1.28 for the present scheme. This overhead seems to be a reasonable price to pay for increased accuracy and exact mass conservation.

Stratospheric Analyses Produced by the United Kingdom Meteorological Office

Abstract Since late 1978, the U.K. Meteorological Office (UKMO) has processed accurate radiance measurements of the stratosphere from a set of three radiometers (Statospheric Sounding Unit, Microwave Sounding Unit, High-Resolution Infrared Sounder) flown on satellites in the TIROS series. The radiometers scan both sides of the subsatellite track giving almost global coverage. UKMO uses its own retrieval algorithm to derive thickness from radiances. The retrieval is a linear, physiral–statistical method that utilizes a reference set of temperature profiles as a priori data. Thickness are added to a contemporaneous field of geopotential height at 100 hPa to give geopotential heights at standard pressure levels in the stratosphere up to 1 hPa. Global synoptic fields for 1200 UTC are produced every day on a regular latitude–longitude grid by linear interpolation in space and time. The paper describes the retrieval method and objective analysis in detail. It outlines the availability of data in the 14-year arc...