microhh · MennoVeerman · Feb 6, 2026 · Feb 6, 2026 · Feb 6, 2026 · Feb 6, 2026
diff --git a/CLAUDE.md b/CLAUDE.md
@@ -0,0 +1,93 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+C++ implementation of RTE+RRTMGP (Radiative Transfer for Energetics + Rapid Radiative Transfer Model for GCM applications Parallel), including a CUDA Monte Carlo ray tracer for 3D radiative transfer. Original Fortran version lives in the `rte-rrtmgp/` submodule.
+
+## Build Commands
+
+```bash
+# CPU-only build (from repo root)
+mkdir build && cd build
+cmake -DSYST=macbook_brew ..
+make
+
+# CUDA build
+cmake -DSYST=<config> -DUSECUDA=ON ..
+
+# Single precision build
+cmake -DSYST=<config> -DUSESP=ON ..
+
+# Debug build
+cmake -DSYST=<config> -DCMAKE_BUILD_TYPE=DEBUG ..
+```
+
+Config files are in `config/` (e.g., `macbook_brew`, `macbook_brew_gcc`, `ubuntu_22lts`).
+
+## Running Tests
+
+The build produces `test_rte_rrtmgp` (CPU) and optionally `test_rte_rrtmgp_gpu`, `test_rte_rrtmgp_rt_gpu`, `test_rte_rrtmgp_bw_gpu` (CUDA).
+
+Test cases use Python scripts for setup, execution, and validation:
+
+```bash
+# RFMIP test (clear-sky reference case)
+cd rfmip && ./make_links.sh && ln -sf ../build/test_rte_rrtmgp .
+python3 rfmip_init.py && python3 rfmip_run.py
+python3 compare-to-reference.py --ref_dir ../rrtmgp-data/examples/rfmip-clear-sky/reference --tst_dir . --var rld rlu rsd rsu --file 'r??_Efx_RTE-RRTMGP-181204_rad-irf_r1i1p1f1_gn.nc' --failure_threshold=5.8e-2
+
+# All-sky test (cloudy conditions)
+cd allsky && ./make_links.sh && ln -sf ../build/test_rte_rrtmgp .
+python3 allsky_init.py && python3 allsky_run.py
+python3 allsky_check.py --failure_threshold=5.8e-2
+```
+
+Required data files (linked via `make_links.sh`): `coefficients_lw.nc`, `coefficients_sw.nc`, `cloud_coefficients_lw.nc`, `cloud_coefficients_sw.nc`, and `rte_rrtmgp_input.nc`.
+
+## Dependencies
+
+- C++ and Fortran compilers (Fortran needed for `src_kernels/`)
+- NetCDF (with HDF5/SZIP)
+- Boost
+- CUDA toolkit + cuRAND (optional, for GPU builds)
+- Python 3 with numpy, netcdf4, xarray (for test scripts)
+- Git submodules: `rte-rrtmgp/` (Fortran reference) and `rrtmgp-data/` (coefficient data)
+
+## Architecture
+
+### Dual CPU/GPU implementation pattern
+
+Core physics classes exist in both CPU (`src/`, `include/`) and CUDA (`src_cuda/`, `include/`) variants. The CUDA ray tracer lives in `src_cuda_rt/` and `include_rt/`. Each CPU class has a GPU mirror with device-side memory management.
+
+### Key classes
+
+- **`Array<T,N>`** (`include/array.h`): Template N-dimensional array, used throughout for all data storage
+- **`Gas_optics_rrtmgp`**: Computes gas optical properties from k-distributions (largest source file)
+- **`Optical_props`** / `Optical_props_1scl` / `Optical_props_2str`: Optical properties with 1-scalar or 2-stream representations
+- **`Rte_lw`** / **`Rte_sw`**: Longwave and shortwave RTE solvers
+- **`Cloud_optics`** / **`Aerosol_optics`**: Cloud and aerosol optical property parameterizations
+- **`Source_functions`**: Planck source function calculations
+- **`Gas_concs`**: Gas concentration container (maps gas names to arrays)
+- **`Radiation_solver`** (`src_test/`): Top-level solver that orchestrates the full radiation calculation
+
+### Precision and type abstraction
+
+`include/types.h` defines `Float` (double or float via `-DRTE_USE_SP`), `Bool` (int or signed char via `-DRTE_USE_CBOOL`), and `Int` (unsigned long long). All physics code uses these type aliases.
+
+### Fortran kernel bridge
+
+`src_kernels/` contains Fortran kernels from the original RTE-RRTMGP, compiled and linked into the C++ library. Headers in `include/` declare the C-compatible interfaces (e.g., `rrtmgp_kernels.h`).
+
+### Ray tracer (CUDA only)
+
+Monte Carlo ray tracer in `src_cuda_rt/` and `include_rt/` supports forward (`Raytracer`), longwave (`Raytracer_lw`), and backward/adjoint (`Raytracer_bw`) modes. Uses cuRAND for random number generation. Helper functions for scattering (Rayleigh, Henyey-Greenstein) are in `raytracer_functions.h`.
+
+### Test executables
+
+`src_test/` contains the test driver code. `radiation_solver.cpp` (CPU) and `radiation_solver.cu` (GPU) implement the full solver pipeline used by all test cases. The different `.cu` test files correspond to different GPU executables (two-stream, ray tracer, backward).
+
+## Licensing
+
+Library code (`src/`, `include/`): BSD 3-clause. Test code (`src_test/`, `include_test/`): GPLv3.
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -35,6 +35,7 @@ else()
 endif()
 
 # Start the project only after the system specific settings are loaded.
+# If CUDA is enabled, don't compile the Fortran kernels as they are not needed.
 if(USECUDA)
   project(rte-rrtmgp-cpp C CXX Fortran CUDA)
 else()
@@ -106,12 +107,13 @@ if(USECUDA)
   endif()
 endif()
 
-add_subdirectory(src_kernels)
-add_subdirectory(src)
 if(USECUDA)
   add_subdirectory(src_kernels_cuda)
   add_subdirectory(src_cuda)
   add_subdirectory(src_kernels_cuda_rt)
   add_subdirectory(src_cuda_rt)
 endif()
+
+add_subdirectory(src)
+add_subdirectory(src_kernels)
 add_subdirectory(src_test)
diff --git a/include/Aerosol_optics.h → include/aerosol_optics.h b/include/Aerosol_optics.h → include/aerosol_optics.h
@@ -4,9 +4,9 @@
 
 #ifndef AEROSOL_OPTICS_H
 #define AEROSOL_OPTICS_H
-#include "Array.h"
-#include "Optical_props.h"
-#include "Gas_concs.h"
+#include "array.h"
+#include "optical_props.h"
+#include "gas_concs.h"
 #include "types.h"
 
 

diff --git a/include/Array.h → include/array.h b/include/Array.h → include/array.h
diff --git a/include/Cloud_optics.h → include/cloud_optics.h b/include/Cloud_optics.h → include/cloud_optics.h
@@ -25,8 +25,8 @@
 #ifndef CLOUD_OPTICS_H
 #define CLOUD_OPTICS_H
 
-#include "Array.h"
-#include "Optical_props.h"
+#include "array.h"
+#include "optical_props.h"
 #include "types.h"
 
 

diff --git a/include/Fluxes.h → include/fluxes.h b/include/Fluxes.h → include/fluxes.h
@@ -28,7 +28,7 @@
 #include <memory>
 #include <stdexcept>
 
-#include "Array.h"
+#include "array.h"
 #include "types.h"
 
 // Forward declarations.

diff --git a/include/Gas_concs.h → include/gas_concs.h b/include/Gas_concs.h → include/gas_concs.h
diff --git a/include/Gas_optics.h → include/gas_optics.h b/include/Gas_optics.h → include/gas_optics.h
@@ -27,8 +27,8 @@
 
 #include <string>
 
-#include "Array.h"
-#include "Optical_props.h"
+#include "array.h"
+#include "optical_props.h"
 
 // Forward declarations.
 class Gas_concs_gpu;

diff --git a/include/Gas_optics_rrtmgp.h → include/gas_optics_rrtmgp.h b/include/Gas_optics_rrtmgp.h → include/gas_optics_rrtmgp.h
@@ -27,8 +27,8 @@
 
 #include <string>
 
-#include "Array.h"
-#include "Gas_optics.h"
+#include "array.h"
+#include "gas_optics.h"
 #include "types.h"
 
 

diff --git a/include/Optical_props.h → include/optical_props.h b/include/Optical_props.h → include/optical_props.h
@@ -26,7 +26,7 @@
 #define OPTICAL_PROPS_H
 
 #include <memory>
-#include "Array.h"
+#include "array.h"
 #include "types.h"
 
 

diff --git a/include/Rte_lw.h → include/rte_lw.h b/include/Rte_lw.h → include/rte_lw.h
diff --git a/include/Rte_sw.h → include/rte_sw.h b/include/Rte_sw.h → include/rte_sw.h
diff --git a/include/Source_functions.h → include/source_functions.h b/include/Source_functions.h → include/source_functions.h
@@ -25,7 +25,7 @@
 #ifndef SOURCE_FUNCTIONS_H
 #define SOURCE_FUNCTIONS_H
 
-#include "Optical_props.h"
+#include "optical_props.h"
 
 
 template<typename, int> class Array;

diff --git a/include_rt/Aerosol_optics_rt.h → include_rt/aerosol_optics_rt.h b/include_rt/Aerosol_optics_rt.h → include_rt/aerosol_optics_rt.h
@@ -5,9 +5,9 @@
 #ifndef AEROSOL_OPTICS_RT_H
 #define AEROSOL_OPTICS_RT_H
 
-#include "Array.h"
-#include "Optical_props_rt.h"
-#include "Gas_concs.h"
+#include "array.h"
+#include "optical_props_rt.h"
+#include "gas_concs.h"
 #include "types.h"
 
 using Aerosol_concs_gpu = Gas_concs_gpu;

diff --git a/include_rt/Cloud_optics_rt.h → include_rt/cloud_optics_rt.h b/include_rt/Cloud_optics_rt.h → include_rt/cloud_optics_rt.h
@@ -25,8 +25,8 @@
 #ifndef CLOUD_OPTICS_RT_H
 #define CLOUD_OPTICS_RT_H
 
-#include "Array.h"
-#include "Optical_props_rt.h"
+#include "array.h"
+#include "optical_props_rt.h"
 #include "types.h"
 
 // Forward declarations.

diff --git a/include_rt/Fluxes_rt.h → include_rt/fluxes_rt.h b/include_rt/Fluxes_rt.h → include_rt/fluxes_rt.h
@@ -29,7 +29,7 @@
 #include <stdexcept>
 
 #include "types.h"
-#include "Array.h"
+#include "array.h"
 
 // Forward declarations.
 class Optical_props_arry_rt;

diff --git a/include_rt/Gas_concs_rt.h → include_rt/gas_concs_rt.h b/include_rt/Gas_concs_rt.h → include_rt/gas_concs_rt.h
@@ -28,7 +28,7 @@
 #include <map>
 #include <string>
 
-#include "Gas_concs.h"
+#include "gas_concs.h"
 #include "types.h"
 
 template<typename, int> class Array;

diff --git a/include_rt/Gas_optics_rrtmgp_rt.h → include_rt/gas_optics_rrtmgp_rt.h b/include_rt/Gas_optics_rrtmgp_rt.h → include_rt/gas_optics_rrtmgp_rt.h
@@ -27,8 +27,8 @@
 
 #include <string>
 
-#include "Array.h"
-#include "Gas_optics_rt.h"
+#include "array.h"
+#include "gas_optics_rt.h"
 #include "types.h"
 
 // Forward declarations.

diff --git a/include_rt/Gas_optics_rt.h → include_rt/gas_optics_rt.h b/include_rt/Gas_optics_rt.h → include_rt/gas_optics_rt.h
@@ -27,8 +27,8 @@
 
 #include <string>
 
-#include "Array.h"
-#include "Optical_props_rt.h"
+#include "array.h"
+#include "optical_props_rt.h"
 
 // Forward declarations.
 class Gas_concs_gpu;

diff --git a/include_rt/Optical_props_rt.h → include_rt/optical_props_rt.h b/include_rt/Optical_props_rt.h → include_rt/optical_props_rt.h
@@ -27,7 +27,7 @@
 
 
 #include <memory>
-#include "Array.h"
+#include "array.h"
 #include "types.h"
 
 // GPU version of optical props class

diff --git a/include_rt/Raytracer.h → include_rt/raytracer.h b/include_rt/Raytracer.h → include_rt/raytracer.h
@@ -7,7 +7,7 @@
 #endif
 
 #include "types.h"
-#include "Optical_props_rt.h"
+#include "optical_props_rt.h"
 #include "raytracer_definitions.h"
 
 // Forward declarations.

diff --git a/include_rt/Raytracer_bw.h → include_rt/raytracer_bw.h b/include_rt/Raytracer_bw.h → include_rt/raytracer_bw.h
@@ -5,7 +5,7 @@
 #include "types.h"
 #include <curand_kernel.h>
 #include "raytracer_kernels_bw.h"
-#include "Optical_props_rt.h"
+#include "optical_props_rt.h"
 
 // Forward declarations.
 template<typename, int> class Array_gpu;

diff --git a/include_rt/Raytracer_lw.h → include_rt/raytracer_lw.h b/include_rt/Raytracer_lw.h → include_rt/raytracer_lw.h
@@ -7,7 +7,7 @@
 #endif
 
 #include "types.h"
-#include "Optical_props_rt.h"
+#include "optical_props_rt.h"
 #include "raytracer_definitions.h"
 
 // Forward declarations.

diff --git a/include_rt/Rte_lw_rt.h → include_rt/rte_lw_rt.h b/include_rt/Rte_lw_rt.h → include_rt/rte_lw_rt.h
diff --git a/include_rt/Rte_sw_rt.h → include_rt/rte_sw_rt.h b/include_rt/Rte_sw_rt.h → include_rt/rte_sw_rt.h
diff --git a/include_rt/Source_functions_rt.h → include_rt/source_functions_rt.h b/include_rt/Source_functions_rt.h → include_rt/source_functions_rt.h
@@ -24,7 +24,7 @@
 
 #ifndef SOURCE_FUNCTIONS_RT_H
 #define SOURCE_FUNCTIONS_RT_H
-#include "Optical_props_rt.h"
+#include "optical_props_rt.h"
 
 template<typename, int> class Array_gpu;
 

diff --git a/include_rt_kernels/gpt_combine_kernels_cuda_rt.h b/include_rt_kernels/gpt_combine_kernels_cuda_rt.h
@@ -26,7 +26,7 @@
 #define GPT_COMBINE_KERNELS_CUDA_RT_H
 
 #include "types.h"
-// #include "Gas_concs.h"
+// #include "gas_concs.h"
 
 
 namespace Gpt_combine_kernels_cuda_rt

diff --git a/include_test/Netcdf_interface.h → include_test/netcdf_interface.h b/include_test/Netcdf_interface.h → include_test/netcdf_interface.h
@@ -26,7 +26,7 @@
 #include <numeric>
 #include <netcdf.h>
 
-#include "Status.h"
+#include "status.h"
 
 enum class Netcdf_mode { Create, Read, Write };
 

diff --git a/include_test/Radiation_solver.h → include_test/radiation_solver.h b/include_test/Radiation_solver.h → include_test/radiation_solver.h
@@ -20,14 +20,14 @@
 #ifndef RADIATION_SOLVER_H
 #define RADIATION_SOLVER_H
 
-#include "Array.h"
-#include "Gas_concs.h"
-#include "Gas_optics_rrtmgp.h"
-#include "Cloud_optics.h"
-#include "Aerosol_optics.h"
-#include "Rte_lw.h"
-#include "Rte_sw.h"
-#include "Source_functions.h"
+#include "array.h"
+#include "gas_concs.h"
+#include "gas_optics_rrtmgp.h"
+#include "cloud_optics.h"
+#include "aerosol_optics.h"
+#include "rte_lw.h"
+#include "rte_sw.h"
+#include "source_functions.h"
 
 
 class Radiation_solver_longwave

diff --git a/include_test/Radiation_solver_bw.h → include_test/radiation_solver_bw.h b/include_test/Radiation_solver_bw.h → include_test/radiation_solver_bw.h
@@ -20,16 +20,16 @@
 #ifndef RADIATION_SOLVER_BW_H
 #define RADIATION_SOLVER_BW_H
 
-#include "Array.h"
-#include "Gas_concs.h"
-#include "Gas_optics_rrtmgp_rt.h"
-#include "Cloud_optics_rt.h"
-#include "Aerosol_optics_rt.h"
-#include "Rte_lw_rt.h"
-#include "Rte_sw_rt.h"
-#include "Raytracer_bw.h"
+#include "array.h"
+#include "gas_concs.h"
+#include "gas_optics_rrtmgp_rt.h"
+#include "cloud_optics_rt.h"
+#include "aerosol_optics_rt.h"
+#include "rte_lw_rt.h"
+#include "rte_sw_rt.h"
+#include "raytracer_bw.h"
 #include "raytracer_kernels_bw.h"
-#include "Source_functions_rt.h"
+#include "source_functions_rt.h"
 #include <curand_kernel.h>
 
 

diff --git a/include_test/Radiation_solver_rt.h → include_test/radiation_solver_rt.h b/include_test/Radiation_solver_rt.h → include_test/radiation_solver_rt.h
@@ -20,17 +20,17 @@
 #ifndef RADIATION_SOLVER_RT_H
 #define RADIATION_SOLVER_RT_H
 
-#include "Array.h"
-#include "Gas_concs.h"
-#include "Gas_optics_rrtmgp_rt.h"
-#include "Cloud_optics_rt.h"
-#include "Aerosol_optics_rt.h"
-#include "Rte_lw_rt.h"
-#include "Rte_sw_rt.h"
-#include "Raytracer.h"
-#include "Raytracer_lw.h"
+#include "array.h"
+#include "gas_concs.h"
+#include "gas_optics_rrtmgp_rt.h"
+#include "cloud_optics_rt.h"
+#include "aerosol_optics_rt.h"
+#include "rte_lw_rt.h"
+#include "rte_sw_rt.h"
+#include "raytracer.h"
+#include "raytracer_lw.h"
 #include "raytracer_kernels.h"
-#include "Source_functions_rt.h"
+#include "source_functions_rt.h"
 #include <curand_kernel.h>
 
 

diff --git a/include_test/Status.h → include_test/status.h b/include_test/Status.h → include_test/status.h