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Why Jamaica's Weather Models Cannot See the Blue Mountains

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Adrian Dunkley Climate Studies Group Mona, Department of Physics, UWI Mona
September 15, 2026 12 min read

Portland is the wettest parish in Jamaica. The north-eastern slopes of the Blue Mountains take 3,000 to 5,000 mm of rain a year. Drive four parishes west and south and the coastal plains of St Catherine and Clarendon take under 1,500 mm, which is why St Elizabeth is where we talk about drought and Portland is where we talk about landslides. Both numbers come from State of the Jamaican Climate, produced by the Climate Studies Group Mona at UWI.

The two places are about twenty kilometres apart. To the climate models Jamaica uses for planning, they are the same place.

One Island, Four Grid Cells

Regional climate projections for this region typically run on a 25 to 50 km grid. At 50 km, Jamaica occupies four to six cells depending on where the lattice happens to fall. We are, at least, present. Further east the picture is worse: between 12 and 20 degrees north and 64 and 56 degrees west there is no land grid point at all, so every island in the Lesser Antilles is represented as ocean, with the nearest land about 500 km away in Puerto Rico or Venezuela.

Jamaica keeps its existence and loses its shape. Blue Mountain Peak reaches 2,256 m. A 50 km cell carries the average of everything inside it, and along the line of the trade winds that average is about 851 m. The mountain becomes a bump, and once the mountain is a bump the rainfall pattern that the mountain creates disappears with it.

Grid resolution and a small islandThree panels showing Jamaica under a 50 km climate-model grid, a 6.3 km km-scale grid, and a 1 km field. At 50 km six cells are more than half land and four more contain the island but count as ocean, and its rainfall gradient disappears; at 1 km the four-to-one gradient between the north-eastern mountain slopes and the southern coastal plains is resolved.What grid spacing does to an islandJamaica, 10,991 km². Same island, three grids. Coastline simplified; rainfall field is a schematic (see caption).A50 km · CORDEX class56%76%71%65%57%52%50 km6 cells are more than half land. 4 contain the island but count as ocean.B6.3 km · km-scale global50 kmAbout 280 land cells. The main ranges appear, the valleys do not.C1 km · what decisions need50 kmAbout 11,000 land cells. The 4:1 rainfall gradient is resolved.3,000–5,000 mmunder 1,500 mm2,256 m850annual rainfall (mm)5,200counted as land (≥50% land)counted as ocean
Figure 1. Jamaica under three grids. At the 50 km spacing typical of CORDEX-class regional projections, six cells are more than half land and four more contain the island but are counted as ocean. Further east the same spacing is worse: between 12–20°N and 64–56°W there is no land grid point at all, so every island in the Lesser Antilles is sea to the model, and the nearest land is roughly 500 km away in Puerto Rico and Venezuela (Cantet, Déqué, Palany and Maridet, Tellus A, 2014). Panel C paints a rainfall field built from an orographic relation and scaled to the published climatology, which puts 3,000–5,000 mm a year on the north-eastern Blue Mountain slopes and under 1,500 mm on the southern coastal plains of St Catherine and Clarendon (State of the Jamaican Climate, Climate Studies Group Mona). It is a schematic, not model output.

The Mechanism Is Not Complicated

Anyone who has driven from Buff Bay over to Kingston has watched this happen.

Air comes off the sea on the north-east coast at around 28 °C with a dewpoint near 24 °C. The trade wind pushes it up the slope. It cools at 9.8 K for every kilometre it rises until temperature and dewpoint meet, which with a 4 K gap happens at about 500 m. That is the cloud base you can see sitting on the hills most mornings. Above it the air is saturated and cools more slowly, roughly 5 K per kilometre in air this warm, so it reaches the summit around 14 °C.

On the way up, each kilogram of that air drops from 19.0 to 13.3 grams of water. The 5.6 grams that went missing is the rain falling on Portland. The same air, now dry, comes down the southern side, warms at the full dry rate, and arrives over the Liguanea plain and the south coast near 36 °C with nothing left to give.

Orographic rainfall across the Blue MountainsA 64 km cross-section along the trade-wind flow through Blue Mountain Peak. Air rises over the John Crow and Blue Mountain ranges, saturates at 500 m, cools at the saturated lapse rate to 14 degrees at the 2,256 m summit, sheds 5.6 grams of water per kilogram of air, then descends and warms at the dry rate. One 50 km cell replaces the whole profile with a single mean elevation of 851 m and a single mean rainfall.The mechanism a 50 km cell cannot holdCross-section along the trade-wind flow through Blue Mountain Peak, Jamaica. Idealised single-parcel trajectory.05001,0001,5002,0002,500metres0102030405060distance along flow (km)lifting condensation level, 500 mone 50 km cell sees mean elevation 851 mtrade wind from ENEsea level: T 28 °C, dewpoint 24 °Cdry ascentΓd 9.8 K/kmsaturated ascent, Γs ≈ 5 K/kmmixing ratio 19.0 → 13.3 g/kg, so 5.6 g/kg rains outdry descent, Γd 9.8 K/kmnothing left to condense, so the lee runs dryJohn Crow MtnsBlue Mountainssummit 2,256 m · 14.3 °Cleeward plain · 35.9 °CSchematic rainfall along the same transect, scaled to the published climatology2,5005,0000mm/yrcell mean 3,096 mmlee 850 mm, 3.6 times below the cell mean
Figure 2. A section along the trade-wind flow through Blue Mountain Peak. A parcel leaving the sea at 28 °C with a dewpoint of 24 °C saturates at 500 m, cools at about 5 K per kilometre to 14.3 °C at the 2,256 m summit, and sheds 5.6 grams of water per kilogram of air on the way up. Descending the far side it warms at the dry rate and reaches the leeward plain near 36 °C. One 50 km cell replaces that whole profile with a mean elevation of 851 m and a single rainfall number 3.6 times what the leeward plain actually gets. Saturation vapour pressure from Bolton (1980); the rainfall strip uses the same schematic field. A real parcel is a blend of air that crossed the summit and air that went around it, so the problem needs three dimensions.

That single mechanism explains Jamaica's rainfall map. It operates over about twenty kilometres of horizontal distance. A 50 km cell has one elevation, so it has one path through that calculation, and everything from the cloud base to the rain shadow collapses into one number.

Nine Minutes

Here is where the resolution question stops being academic.

When rain falls on a steep Jamaican catchment, the water does not soak in. It runs. Peak discharge from a small catchment follows the rational method, Q = CiA, and channel velocity follows Manning's equation. Take a 4 km² catchment, already saturated, under 80 mm of rain an hour. That gives 62 m³ s−1 of water moving down a rocky gully at about 5.3 m s−1.

A community 2.8 km down that gully has nine minutes.

Nine minutes is a number a parish disaster coordinator can do something with. It is a phone call, a siren, a decision about a bridge. "Heavy rainfall expected over eastern Jamaica" is not. The gap between those two statements is the gap between the resolution we model at and the resolution we live at.

A three-kilometre scene rendered from the governing relationsAn isometric view of a ridge and a coastal settlement. Wind vectors accelerate over the crest according to the standard topographic speed-up, so buildings on the ridge carry about twice the load of buildings on the plain in the same storm. The gully is routed with Manning's equation and the rational method, giving a nine-minute travel time from the crest to the settlement, and the floodplain is inundated to the computed water surface.Where a downscaled field becomes a damage estimateA 3 km × 3 km scene. Every number below is computed from the relation printed beside it, at V₀ = 45 m s⁻¹ upstream and 80 mm h⁻¹ of rain on a 4 km² catchment. Vertical scale exaggerated about 8×.ridge crestV = 64 m/s, q = 2.38 kPacoastal plainV = 46 m/s, q = 1.22 kPasame storm, 2.0× the load on the crestgullyQ = C i A = 62 m³ s⁻¹v = (1/n) R²ᐟ³ S¹ᐟ² = 5.3 m s⁻¹2.8 km of channel, so about9 minutes of warning belowLoad relative to a building on the plainunder 1.4× · light exposure1.4 to 2.2× · moderate exposureover 2.2× · severe exposureExposure ratios, not fragility curves. A fragility model needs the local building stock.What the scene is forA 50 km cell gives one wind speed and one rainfall for this whole view.At 1 km the crest and the plain separate, and so do their loads.The gully then tells you how long the settlement below has, in minutes.That is the difference between a forecast and an instruction.
Figure 3. A three-kilometre scene, rendered from the relations printed on it. Topographic speed-up puts 64 m s−1 on the ridge crest where the plain sees 46 m s−1, so a house on the crest carries twice the load of an identical house below it in the same storm. The gully carries 62 m³ s−1 under 80 mm of rain an hour on a 4 km² catchment and moves it at 5.3 m s−1, which leaves the settlement below about nine minutes. The colours are exposure ratios. Turning them into expected damage needs a fragility model fitted to the local building stock, and that is separate work that has not been done.

The same figure shows the other half of it. Wind load is dynamic pressure, q = ½ρV², so it goes as the square of wind speed. Air accelerating over a ridge gains around 40 per cent in speed at the crest, which by the square law roughly doubles the load. A house on a ridge and an identical house on the flat a kilometre away are in two materially different storms, and a model that reports one wind speed for the whole parish has described neither.

What Melissa Actually Cost

Hurricane Melissa made landfall in western Jamaica on 28 October 2025 with sustained winds near 185 mph. The Planning Institute of Jamaica put total damage and loss at J$1.952 trillion, about US$12.2 billion, equal to 56.7 per cent of the country's 2024 GDP. The PIOJ projects three to five years before national output returns to where it was.

CCRIF paid US$70.8 million under Jamaica's tropical cyclone policy, the largest single payout in the facility's history, and US$21.1 million under the excess rainfall policy. US$91.9 million in total, against US$12.2 billion of loss.

That gap has several causes and modelling is only one of them. But part of it is structural: a parametric policy pays on a modelled index, and an index built on a coarse hazard field cannot distinguish the parish that was destroyed from the parish next door that was spared. The finer the hazard model, the closer the payout tracks the actual damage.

And the Droughts

Hurricanes get the attention. The slower hazard has been running all year. As at 10 August 2026, Hermitage Dam was at 40.7 per cent of capacity and the Mona Reservoir at 54.5 per cent. The National Water Commission issued a prohibition notice effective 17 August. The Met Service has been forecasting hotter and drier than normal conditions into September and October.

Drought is the same physics running the other way. The rain shadow that makes the south coast dry in a normal year makes it critical in a dry one, and the soil moisture that determines whether a St Elizabeth farmer loses a crop varies over hundreds of metres, not tens of kilometres. A model that cannot resolve the hillside cannot tell a farmer anything a calendar could not.

What I Am Building

The work is at the Climate Studies Group Mona, in the Department of Physics at UWI Mona, where Caribbean climate science has been done since 1994.

Running a conventional model at one kilometre is unaffordable, and the reason is arithmetic rather than ambition. Halving the grid spacing quarters the cell area, so cell count grows as the inverse square. A dynamical model also has to shorten its timestep to keep the Courant condition, so the step count grows on top of that. Moving from 50 km to 1 km is a factor of 125,000 in two dimensions and 6.25 million in three, then multiplied again by the ensemble size you need to say anything honest about uncertainty.

An AI emulator has no timestep to shorten. Splitting the domain into subdomains that are evaluated independently means the memory requirement is set by the size of a subdomain rather than the size of the country, and the subdomains do not wait for each other. That changes compute from a physical ceiling into a budget line.

The specific technical contribution is narrower. Physics-constrained downscaling models already enforce a conservation law: the average of the fine field should equal the coarse input. That law assumes the coarse driver is unbiased, which is true in a laboratory experiment and false in operation, and global models have documented rainfall biases over this basin. There is a second law nobody has enforced, which is that what leaves one subdomain through a face must equal what enters its neighbour through the same face. That one binds the fine field to itself, so it does not inherit the driver's bias. Whether that distinction survives contact with real data is the experiment.

Where this could fail. The new constraint is necessary and not sufficient: a field can be perfectly consistent across every boundary and still be wrong about the weather. Jamaica's rain-gauge network is sparse and clustered, so validating against stations here will always be weaker than it would be in a country with a dense network. There are no results yet. The figures above are computed directly from the equations printed on them, or labelled as schematics where they are.

Why It Matters Here Specifically

Jamaica has a hard version of this problem and a good position from which to solve it. Hard, because the island has 2,256 m of relief packed into 235 km of length, which means the gradients are steep and the observations are thin. Good, because the Climate Studies Group Mona has thirty years of Caribbean climate work behind it and because the country has just been through an event that makes the case without any argument from me.

The goal is a model any Jamaican can run for the ground they are standing on. Not a regional summary. The hillside.

Adrian Dunkley is a physicist and AI researcher in the Department of Physics at UWI Mona, with degrees in mathematics, physics and financial engineering and fifteen years in applied AI. His UWI profile is here, and the technical version of this argument is at climatephysicsai.com.