## Overview

Parallel Analog Ensemble (PAnEn) generates accurate forecast ensembles relying on a single deterministic model simulation and the historical observations. The technique was introduced by Luca Delle Monache et al. in the paper Probabilistic Weather Prediction with an Analog Ensemble. Developed and maintained by GEOlab at Penn State, PAnEn aims to provide an efficient implementation for this technique and user-friendly interfaces in R and C++ for researchers who want to use this technique in their own research.

The easiest way to use this package is to install the R package, ‘RAnEn’. C++ libraries are also available but they are designed for intermediate users with requirement for performance. For installation guidance, please refer to the installation section.

## Citation

To cite this package, you have several options:

• Using LaTex: Please use this file for citation.
• Using R: Simply type citation('RAnEn') and the citation message will be printed.
• Using plain text: Please use the following citation format:
Weiming Hu, Guido Cervone, Laura Clemente-Harding, and Martina Calovi. (2019). Parallel Analog Ensemble. Zenodo. http://doi.org/10.5281/zenodo.3384321

## Installation

RAnEn is very easy to install if you are already using R. This is the recommended way to start.

### RAnEn

The command is the same for RAnEn installation and update.

To install RAnEn, please install the following packages first:

• BH: install.packages('BH')
• Rcpp: install.packages('Rcpp')
• If you are using Windows, please also install the latest version of Rtools.

The following R command install the latest RAnEn.

install.packages("https://github.com/Weiming-Hu/AnalogsEnsemble/raw/master/RAnalogs/releases/RAnEn_latest.tar.gz", repos = NULL)

That’s it. You are good to go. Please refer to tutorials or the R documentation to learn more about using RAnEn. You might also want to install RAnEnExtra package with functions for visualization and verification. After RAnEn installation, you can simply run devtools::install_github("Weiming-Hu/RAnEnExtra").

Mac users: if the package shows that OpenMP is not supported. You can do one of the followings:

1. Avoid using Clang compilers and convert to GNU compilers. To change the compilers used by R, create a file ~/.R/Makevars if you do not have it already and add the following content to it. Of course, change the compilers to what you have. If you do not have any alternative compilers other than Clang, HomeBrew is your friend.
CC=gcc-8
CXX=g++-8
CXX1X=g++-8
CXX11=g++-8
1. You can also follow the instructions here provided by data.table. They provide similar solutions but stick with Clang compilers.

After the installation, you can always revert back to your original setup and RAnEn will stay supported by OpenMP.

### CAnEn

To install the C++ libraries, please check the following dependencies:

• Required CMake is the required build system generator.
• Required NetCDF provides the file I/O with NetCDF files.
• Required Eccodes provides the file I/O with Grib2 files.
• Optional Boost provides high-performance data structures. Boost is a very large library. If you don’t want to install the entire package, PAnEn is able to build the required ones automatically.
• Optional CppUnit provides test frameworks. If CppUnit is found in the system, test programs will be compiled.

Please use the following scripts to install the libraries:

# Download the source files (~10 Mb)
wget https://github.com/Weiming-Hu/AnalogsEnsemble/archive/master.zip

# Unzip
unzip master.zip

# Create a separate folder to store all intermediate files during the installation process
cd AnalogsEnsemble-master/
mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=~/AnalogEnsemble ..

# Compile
make -j 4

# Install
make install

### CMake Parameters

Below is a list of parameters you can change and customize.

Parameter Explanation Default
CMAKE_C_COMPILER The C compiler to use. [System dependent]
CMAKE_CXX_COMPILER The C++ compiler to use. [System dependent]
CMAKE_INSTALL_PREFIX The installation directory. [System dependent]
CMAKE_PREFIX_PATH Which folder(s) should cmake search for packages besides the default. Paths are surrounded by double quotes and separated with semicolons. [Empty]
CMAKE_INSTALL_RPATH The run-time library path. Paths are surrounded by double quotes and separated with semicolons. [Empty]
CMAKE_BUILD_TYPE Release for release mode; Debug for debug mode. Release
INSTALL_RAnEn Build and install the RAnEn library. OFF
BUILD_BOOST Build Boost regardless of whether it exists in the system. OFF
BOOST_URL The URL for downloading Boost. This is only used when BUILD_BOOST is ON. [From SourceForge]
ENABLE_MPI Build the MPI supported libraries and executables. This requires the MPI dependency. OFF
ENABLE_OPENMP Enable multi-threading with OpenMP ON
ENABLE_AI Enable PyTorch integration and the power of AI. OFF

You can change the default of the parameters, for example, cmake -DCMAKE_INSTALL_PREFIX=~/AnalogEnsemble ... Don’t forget the extra letter D when specifying argument names.

### High-Performance Computing and Supercomputers

Here is a list of instructions to build and install AnEn on supercomputers.

### MPI and OpenMP

TL;DR

Launching an MPI-OpenMP hybrid program can be tricky.

If the performance with MPI is acceptable,
disable OpenMP (cmake -DENABLE_OPENMP=OFF ..).

If the hybrid solution is desired,
make sure you have the proper setup.

When ENABLE_MPI is turned on, MPI programs will be built. These MPI programs are hybrid programs (unless you set -DENABLE_OPENMP=OFF for cmake) that use both MPI and OpenMP. Please check with your individual supercomputer platform to find out what the proper configuration for launching an MPI + OpenMP hybrid program is. Users are responsible not to launch too many process and threads at the same time which would overtask the machine and might lead to hanging problems (as what I have seen on XSEDE Stampede2).

On NCAR Cheyenne, the proper way to launch a hybrid program can be found here. If you use mpirun, instead of mpiexec_mpt, you will loose the multi-threading performance improvement.

To dive deeper into the hybrid parallelization design, MPI is used for computationally expensive portions of the code, e.g. file I/O and analog generation while OpenMP is used by the master process during bottleneck portion of the code, e.g. data reshaping and information queries.

When analogs with a long search and test periods are desired, MPI is used to distribute forecast files across processes. Each process reads a subset of the forecast files. This solves the problem where serial I/O can be very slow.

When a large number of stations/grids present, MPI is used to distribute analog generation for different stations across processes. Each process takes charge of generating analogs for a subset of stations.

Sitting between the file I/O and the analog generation is the bottleneck which is hard to parallelize with MPI, e.g. reshaping the data and querying test/search times. Therefore, they are parallelized with OpenMP on master process only.

So if the platform support heterogeneous task layout, users can theoretically allocate one core per worker process and more cores for the master process to facilitate its multi-threading scope. But again, only do this when you find the bottleneck is taking much longer time than file I/O and analog generation. Use --profile to have profiling information in standard message output.

## Tutorials

Tutorials can be accessed on binder or be found in this directory

Here are also some tips and caveats in this ticket.

## Feedbacks

We appreciate collaborations and feedbacks from users. Please contact the maintainer Weiming Hu through weiming@psu.edu or submit tickets if you have any problems.

Thank you!

# "-''-/").___..--''"-._
#  (6_ 6  )   -.  (     ).-.__.)   WE ARE ...
#  (_Y_.)'  ._   )  ._ . -..-'    PENN STATE!
#    _ ..--'_..-_/  /--'_.' ,'
#  (il),-''  (li),'  ((!.-'
#
# Authors:
#     Weiming Hu <weiming@psu.edu>
#     Guido Cervone <cervone@psu.edu>
#     Laura Clemente-Harding <lec170@psu.edu>
#     Martina Calovi <mcalovi@psu.edu>
#
# Contributors:
#     Luca Delle Monache
#
# Geoinformatics and Earth Observation Laboratory (http://geolab.psu.edu)
# Department of Geography and Institute for CyberScience
# The Pennsylvania State University`