From 2b1b9642dc4fc627ce73a40abede4c9577fade17 Mon Sep 17 00:00:00 2001
From: milankl <milankloewer@gmx.de>
Date: Mon, 6 Jan 2025 23:56:35 -0100
Subject: [PATCH 1/3] set! for time step

---
 src/dynamics/time_integration.jl | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/dynamics/time_integration.jl b/src/dynamics/time_integration.jl
index a30c3aa02..acd90f49a 100644
--- a/src/dynamics/time_integration.jl
+++ b/src/dynamics/time_integration.jl
@@ -114,7 +114,7 @@ function initialize!(L::Leapfrog, model::AbstractModel)
         L.Δt = L.Δt_sec/L.radius
     end
 
-    # check how time stepping time step and output time step align
+    # check how time steps from time integration and output align
     n = round(Int, Millisecond(output_dt).value/L.Δt_millisec.value)
     nΔt = n*L.Δt_millisec
     if nΔt != output_dt
@@ -122,6 +122,21 @@ function initialize!(L::Leapfrog, model::AbstractModel)
     end
 end
 
+"""$(TYPEDSIGNATURES)
+Change time step of timestepper `L` to `Δt`
+and disables adjustment to output frequency."""
+function set!(L::AbstractTimeStepper, Δt::Period)
+    L.Δt_millisec = Millisecond(Δt)
+    L.Δt_sec = L.Δt_millisec.value/1000
+    L.Δt = L.Δt_sec/L.radius
+
+    resolution_factor = (L.trunc+1)/(DEFAULT_TRUNC+1)
+    L.Δt_at_T31 = Second(round(Int, L.Δt_sec*resolution_factor))
+
+    L.adjust_with_output = false
+    return L
+end
+
 """
 $(TYPEDSIGNATURES)
 Performs one leapfrog time step with (`lf=2`) or without (`lf=1`) Robert+Williams filter

From 9ad7b07494cb793afadeeba52dfef2d9327f441e Mon Sep 17 00:00:00 2001
From: milankl <milankloewer@gmx.de>
Date: Tue, 7 Jan 2025 00:00:09 -0100
Subject: [PATCH 2/3] update changelog

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 74bc44421..bc28382da 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- set! for time step [#650](https://github.com/SpeedyWeather/SpeedyWeather.jl/pull/650)
 - Spectral Gradients are now differentiable, with correctness check in extended CI [#638](https://github.com/SpeedyWeather/SpeedyWeather.jl/pull/638)
 - Tracer advection [#579](https://github.com/SpeedyWeather/SpeedyWeather.jl/pull/579)
 - Buildkite CI with dummy pipeline [#646](https://github.com/SpeedyWeather/SpeedyWeather.jl/pull/646)

From cf8cb7bbac04a88bff9eac432f087f9820f7b144 Mon Sep 17 00:00:00 2001
From: milankl <milankloewer@gmx.de>
Date: Tue, 7 Jan 2025 09:44:33 -0100
Subject: [PATCH 3/3] Add keyword method, comments, tests

---
 src/dynamics/time_integration.jl | 14 +++++++++++---
 test/time_stepping.jl            | 12 ++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/src/dynamics/time_integration.jl b/src/dynamics/time_integration.jl
index acd90f49a..0f583b0a7 100644
--- a/src/dynamics/time_integration.jl
+++ b/src/dynamics/time_integration.jl
@@ -123,20 +123,28 @@ function initialize!(L::Leapfrog, model::AbstractModel)
 end
 
 """$(TYPEDSIGNATURES)
-Change time step of timestepper `L` to `Δt`
+Change time step of timestepper `L` to `Δt` (unscaled)
 and disables adjustment to output frequency."""
-function set!(L::AbstractTimeStepper, Δt::Period)
-    L.Δt_millisec = Millisecond(Δt)
+function set!(
+    L::AbstractTimeStepper,
+    Δt::Period,                 # unscaled time step in Second, Minute, ...
+)
+    L.Δt_millisec = Millisecond(Δt)         # recalculate all Δt fields
     L.Δt_sec = L.Δt_millisec.value/1000
     L.Δt = L.Δt_sec/L.radius
 
+    # recalculate the default time step at resolution T31 to be consistent
     resolution_factor = (L.trunc+1)/(DEFAULT_TRUNC+1)
     L.Δt_at_T31 = Second(round(Int, L.Δt_sec*resolution_factor))
 
+    # given Δt was manually set disallow adjustment to output frequency
     L.adjust_with_output = false
     return L
 end
 
+# also allow for keyword arguments
+set!(L::AbstractTimeStepper; Δt::Period) = set!(L, Δt)
+
 """
 $(TYPEDSIGNATURES)
 Performs one leapfrog time step with (`lf=2`) or without (`lf=1`) Robert+Williams filter
diff --git a/test/time_stepping.jl b/test/time_stepping.jl
index 63a924636..1f5c4324a 100644
--- a/test/time_stepping.jl
+++ b/test/time_stepping.jl
@@ -114,4 +114,16 @@ end
         @test M_Ronly < 1
         @test M_Ronly <= M_RAW
     end
+end
+
+@testset "Set timestep manually" begin
+    @testset for trunc in (31, 63, 127)
+        spectral_grid = SpectralGrid(; trunc)
+        time_stepping = Leapfrog(spectral_grid)
+        set!(time_stepping, Minute(10))
+        @test time_stepping.Δt_sec == 10*60
+
+        set!(time_stepping, Δt=Minute(20))
+        @test time_stepping.Δt_sec == 20*60
+    end
 end
\ No newline at end of file